2017-10-29 18:30:14 +08:00
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=====================================
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Filesystem-level encryption (fscrypt)
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=====================================
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Introduction
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============
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fscrypt is a library which filesystems can hook into to support
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transparent encryption of files and directories.
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Note: "fscrypt" in this document refers to the kernel-level portion,
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implemented in ``fs/crypto/``, as opposed to the userspace tool
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`fscrypt <https://github.com/google/fscrypt>`_. This document only
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covers the kernel-level portion. For command-line examples of how to
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use encryption, see the documentation for the userspace tool `fscrypt
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<https://github.com/google/fscrypt>`_. Also, it is recommended to use
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the fscrypt userspace tool, or other existing userspace tools such as
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`fscryptctl <https://github.com/google/fscryptctl>`_ or `Android's key
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management system
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<https://source.android.com/security/encryption/file-based>`_, over
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using the kernel's API directly. Using existing tools reduces the
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chance of introducing your own security bugs. (Nevertheless, for
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completeness this documentation covers the kernel's API anyway.)
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Unlike dm-crypt, fscrypt operates at the filesystem level rather than
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at the block device level. This allows it to encrypt different files
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with different keys and to have unencrypted files on the same
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filesystem. This is useful for multi-user systems where each user's
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data-at-rest needs to be cryptographically isolated from the others.
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However, except for filenames, fscrypt does not encrypt filesystem
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metadata.
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Unlike eCryptfs, which is a stacked filesystem, fscrypt is integrated
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directly into supported filesystems --- currently ext4, F2FS, and
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UBIFS. This allows encrypted files to be read and written without
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caching both the decrypted and encrypted pages in the pagecache,
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thereby nearly halving the memory used and bringing it in line with
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unencrypted files. Similarly, half as many dentries and inodes are
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needed. eCryptfs also limits encrypted filenames to 143 bytes,
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causing application compatibility issues; fscrypt allows the full 255
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bytes (NAME_MAX). Finally, unlike eCryptfs, the fscrypt API can be
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used by unprivileged users, with no need to mount anything.
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fscrypt does not support encrypting files in-place. Instead, it
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supports marking an empty directory as encrypted. Then, after
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userspace provides the key, all regular files, directories, and
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symbolic links created in that directory tree are transparently
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encrypted.
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Threat model
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============
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Offline attacks
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---------------
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Provided that userspace chooses a strong encryption key, fscrypt
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protects the confidentiality of file contents and filenames in the
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event of a single point-in-time permanent offline compromise of the
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block device content. fscrypt does not protect the confidentiality of
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non-filename metadata, e.g. file sizes, file permissions, file
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timestamps, and extended attributes. Also, the existence and location
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of holes (unallocated blocks which logically contain all zeroes) in
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files is not protected.
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fscrypt is not guaranteed to protect confidentiality or authenticity
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if an attacker is able to manipulate the filesystem offline prior to
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an authorized user later accessing the filesystem.
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Online attacks
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--------------
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fscrypt (and storage encryption in general) can only provide limited
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protection, if any at all, against online attacks. In detail:
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2019-08-05 10:35:49 +08:00
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Side-channel attacks
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~~~~~~~~~~~~~~~~~~~~
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2017-10-29 18:30:14 +08:00
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fscrypt is only resistant to side-channel attacks, such as timing or
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electromagnetic attacks, to the extent that the underlying Linux
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2021-09-17 01:49:26 +08:00
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Cryptographic API algorithms or inline encryption hardware are. If a
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vulnerable algorithm is used, such as a table-based implementation of
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AES, it may be possible for an attacker to mount a side channel attack
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against the online system. Side channel attacks may also be mounted
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against applications consuming decrypted data.
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2017-10-29 18:30:14 +08:00
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2019-08-05 10:35:49 +08:00
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Unauthorized file access
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~~~~~~~~~~~~~~~~~~~~~~~~
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After an encryption key has been added, fscrypt does not hide the
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plaintext file contents or filenames from other users on the same
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system. Instead, existing access control mechanisms such as file mode
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bits, POSIX ACLs, LSMs, or namespaces should be used for this purpose.
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(For the reasoning behind this, understand that while the key is
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added, the confidentiality of the data, from the perspective of the
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system itself, is *not* protected by the mathematical properties of
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encryption but rather only by the correctness of the kernel.
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Therefore, any encryption-specific access control checks would merely
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be enforced by kernel *code* and therefore would be largely redundant
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with the wide variety of access control mechanisms already available.)
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Kernel memory compromise
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~~~~~~~~~~~~~~~~~~~~~~~~
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An attacker who compromises the system enough to read from arbitrary
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memory, e.g. by mounting a physical attack or by exploiting a kernel
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security vulnerability, can compromise all encryption keys that are
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currently in use.
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However, fscrypt allows encryption keys to be removed from the kernel,
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which may protect them from later compromise.
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In more detail, the FS_IOC_REMOVE_ENCRYPTION_KEY ioctl (or the
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FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS ioctl) can wipe a master
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encryption key from kernel memory. If it does so, it will also try to
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evict all cached inodes which had been "unlocked" using the key,
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thereby wiping their per-file keys and making them once again appear
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"locked", i.e. in ciphertext or encrypted form.
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However, these ioctls have some limitations:
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- Per-file keys for in-use files will *not* be removed or wiped.
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Therefore, for maximum effect, userspace should close the relevant
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encrypted files and directories before removing a master key, as
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well as kill any processes whose working directory is in an affected
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encrypted directory.
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- The kernel cannot magically wipe copies of the master key(s) that
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userspace might have as well. Therefore, userspace must wipe all
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copies of the master key(s) it makes as well; normally this should
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be done immediately after FS_IOC_ADD_ENCRYPTION_KEY, without waiting
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for FS_IOC_REMOVE_ENCRYPTION_KEY. Naturally, the same also applies
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to all higher levels in the key hierarchy. Userspace should also
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follow other security precautions such as mlock()ing memory
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containing keys to prevent it from being swapped out.
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- In general, decrypted contents and filenames in the kernel VFS
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caches are freed but not wiped. Therefore, portions thereof may be
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recoverable from freed memory, even after the corresponding key(s)
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were wiped. To partially solve this, you can set
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CONFIG_PAGE_POISONING=y in your kernel config and add page_poison=1
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to your kernel command line. However, this has a performance cost.
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- Secret keys might still exist in CPU registers, in crypto
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accelerator hardware (if used by the crypto API to implement any of
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the algorithms), or in other places not explicitly considered here.
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Limitations of v1 policies
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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v1 encryption policies have some weaknesses with respect to online
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attacks:
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- There is no verification that the provided master key is correct.
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Therefore, a malicious user can temporarily associate the wrong key
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with another user's encrypted files to which they have read-only
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access. Because of filesystem caching, the wrong key will then be
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used by the other user's accesses to those files, even if the other
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user has the correct key in their own keyring. This violates the
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meaning of "read-only access".
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- A compromise of a per-file key also compromises the master key from
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which it was derived.
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- Non-root users cannot securely remove encryption keys.
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All the above problems are fixed with v2 encryption policies. For
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this reason among others, it is recommended to use v2 encryption
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policies on all new encrypted directories.
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2017-10-29 18:30:14 +08:00
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Key hierarchy
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=============
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Master Keys
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-----------
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Each encrypted directory tree is protected by a *master key*. Master
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keys can be up to 64 bytes long, and must be at least as long as the
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fscrypt: allow 256-bit master keys with AES-256-XTS
fscrypt currently requires a 512-bit master key when AES-256-XTS is
used, since AES-256-XTS keys are 512-bit and fscrypt requires that the
master key be at least as long any key that will be derived from it.
However, this is overly strict because AES-256-XTS doesn't actually have
a 512-bit security strength, but rather 256-bit. The fact that XTS
takes twice the expected key size is a quirk of the XTS mode. It is
sufficient to use 256 bits of entropy for AES-256-XTS, provided that it
is first properly expanded into a 512-bit key, which HKDF-SHA512 does.
Therefore, relax the check of the master key size to use the security
strength of the derived key rather than the size of the derived key
(except for v1 encryption policies, which don't use HKDF).
Besides making things more flexible for userspace, this is needed in
order for the use of a KDF which only takes a 256-bit key to be
introduced into the fscrypt key hierarchy. This will happen with
hardware-wrapped keys support, as all known hardware which supports that
feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys
are wrapped 256-bit AES keys. Moreover, there is interest in fscrypt
supporting the same type of AES-256-CMAC based KDF in software as an
alternative to HKDF-SHA512. There is no security problem with such
features, so fix the key length check to work properly with them.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2021-09-21 11:03:03 +08:00
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greater of the security strength of the contents and filenames
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encryption modes being used. For example, if any AES-256 mode is
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used, the master key must be at least 256 bits, i.e. 32 bytes. A
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stricter requirement applies if the key is used by a v1 encryption
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policy and AES-256-XTS is used; such keys must be 64 bytes.
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2017-10-29 18:30:14 +08:00
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To "unlock" an encrypted directory tree, userspace must provide the
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appropriate master key. There can be any number of master keys, each
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of which protects any number of directory trees on any number of
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filesystems.
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2019-08-05 10:35:49 +08:00
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Master keys must be real cryptographic keys, i.e. indistinguishable
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from random bytestrings of the same length. This implies that users
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**must not** directly use a password as a master key, zero-pad a
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shorter key, or repeat a shorter key. Security cannot be guaranteed
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if userspace makes any such error, as the cryptographic proofs and
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analysis would no longer apply.
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Instead, users should generate master keys either using a
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cryptographically secure random number generator, or by using a KDF
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(Key Derivation Function). The kernel does not do any key stretching;
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therefore, if userspace derives the key from a low-entropy secret such
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as a passphrase, it is critical that a KDF designed for this purpose
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be used, such as scrypt, PBKDF2, or Argon2.
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Key derivation function
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-----------------------
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With one exception, fscrypt never uses the master key(s) for
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encryption directly. Instead, they are only used as input to a KDF
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(Key Derivation Function) to derive the actual keys.
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The KDF used for a particular master key differs depending on whether
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the key is used for v1 encryption policies or for v2 encryption
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policies. Users **must not** use the same key for both v1 and v2
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encryption policies. (No real-world attack is currently known on this
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specific case of key reuse, but its security cannot be guaranteed
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since the cryptographic proofs and analysis would no longer apply.)
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For v1 encryption policies, the KDF only supports deriving per-file
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encryption keys. It works by encrypting the master key with
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AES-128-ECB, using the file's 16-byte nonce as the AES key. The
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resulting ciphertext is used as the derived key. If the ciphertext is
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longer than needed, then it is truncated to the needed length.
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For v2 encryption policies, the KDF is HKDF-SHA512. The master key is
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passed as the "input keying material", no salt is used, and a distinct
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"application-specific information string" is used for each distinct
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key to be derived. For example, when a per-file encryption key is
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derived, the application-specific information string is the file's
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nonce prefixed with "fscrypt\\0" and a context byte. Different
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context bytes are used for other types of derived keys.
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HKDF-SHA512 is preferred to the original AES-128-ECB based KDF because
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HKDF is more flexible, is nonreversible, and evenly distributes
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entropy from the master key. HKDF is also standardized and widely
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used by other software, whereas the AES-128-ECB based KDF is ad-hoc.
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2017-10-29 18:30:14 +08:00
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2020-01-21 06:31:58 +08:00
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Per-file encryption keys
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------------------------
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2017-10-29 18:30:14 +08:00
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fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
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Since each master key can protect many files, it is necessary to
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"tweak" the encryption of each file so that the same plaintext in two
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files doesn't map to the same ciphertext, or vice versa. In most
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cases, fscrypt does this by deriving per-file keys. When a new
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encrypted inode (regular file, directory, or symlink) is created,
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fscrypt randomly generates a 16-byte nonce and stores it in the
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2019-08-05 10:35:49 +08:00
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inode's encryption xattr. Then, it uses a KDF (as described in `Key
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derivation function`_) to derive the file's key from the master key
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and nonce.
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2017-10-29 18:30:14 +08:00
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fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
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Key derivation was chosen over key wrapping because wrapped keys would
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require larger xattrs which would be less likely to fit in-line in the
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filesystem's inode table, and there didn't appear to be any
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significant advantages to key wrapping. In particular, currently
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there is no requirement to support unlocking a file with multiple
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alternative master keys or to support rotating master keys. Instead,
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the master keys may be wrapped in userspace, e.g. as is done by the
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`fscrypt <https://github.com/google/fscrypt>`_ tool.
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fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
DIRECT_KEY policies
|
|
|
|
-------------------
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
The Adiantum encryption mode (see `Encryption modes and usage`_) is
|
|
|
|
suitable for both contents and filenames encryption, and it accepts
|
|
|
|
long IVs --- long enough to hold both an 8-byte logical block number
|
|
|
|
and a 16-byte per-file nonce. Also, the overhead of each Adiantum key
|
|
|
|
is greater than that of an AES-256-XTS key.
|
|
|
|
|
|
|
|
Therefore, to improve performance and save memory, for Adiantum a
|
|
|
|
"direct key" configuration is supported. When the user has enabled
|
|
|
|
this by setting FSCRYPT_POLICY_FLAG_DIRECT_KEY in the fscrypt policy,
|
2020-01-21 06:31:58 +08:00
|
|
|
per-file encryption keys are not used. Instead, whenever any data
|
|
|
|
(contents or filenames) is encrypted, the file's 16-byte nonce is
|
|
|
|
included in the IV. Moreover:
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
- For v1 encryption policies, the encryption is done directly with the
|
|
|
|
master key. Because of this, users **must not** use the same master
|
|
|
|
key for any other purpose, even for other v1 policies.
|
|
|
|
|
|
|
|
- For v2 encryption policies, the encryption is done with a per-mode
|
|
|
|
key derived using the KDF. Users may use the same master key for
|
|
|
|
other v2 encryption policies.
|
|
|
|
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
IV_INO_LBLK_64 policies
|
|
|
|
-----------------------
|
|
|
|
|
|
|
|
When FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64 is set in the fscrypt policy,
|
|
|
|
the encryption keys are derived from the master key, encryption mode
|
|
|
|
number, and filesystem UUID. This normally results in all files
|
|
|
|
protected by the same master key sharing a single contents encryption
|
|
|
|
key and a single filenames encryption key. To still encrypt different
|
|
|
|
files' data differently, inode numbers are included in the IVs.
|
|
|
|
Consequently, shrinking the filesystem may not be allowed.
|
|
|
|
|
|
|
|
This format is optimized for use with inline encryption hardware
|
2020-05-16 04:41:41 +08:00
|
|
|
compliant with the UFS standard, which supports only 64 IV bits per
|
|
|
|
I/O request and may have only a small number of keyslots.
|
|
|
|
|
|
|
|
IV_INO_LBLK_32 policies
|
|
|
|
-----------------------
|
|
|
|
|
|
|
|
IV_INO_LBLK_32 policies work like IV_INO_LBLK_64, except that for
|
|
|
|
IV_INO_LBLK_32, the inode number is hashed with SipHash-2-4 (where the
|
|
|
|
SipHash key is derived from the master key) and added to the file
|
|
|
|
logical block number mod 2^32 to produce a 32-bit IV.
|
|
|
|
|
|
|
|
This format is optimized for use with inline encryption hardware
|
|
|
|
compliant with the eMMC v5.2 standard, which supports only 32 IV bits
|
|
|
|
per I/O request and may have only a small number of keyslots. This
|
|
|
|
format results in some level of IV reuse, so it should only be used
|
|
|
|
when necessary due to hardware limitations.
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
Key identifiers
|
|
|
|
---------------
|
|
|
|
|
|
|
|
For master keys used for v2 encryption policies, a unique 16-byte "key
|
|
|
|
identifier" is also derived using the KDF. This value is stored in
|
|
|
|
the clear, since it is needed to reliably identify the key itself.
|
|
|
|
|
fscrypt: derive dirhash key for casefolded directories
When we allow indexed directories to use both encryption and
casefolding, for the dirhash we can't just hash the ciphertext filenames
that are stored on-disk (as is done currently) because the dirhash must
be case insensitive, but the stored names are case-preserving. Nor can
we hash the plaintext names with an unkeyed hash (or a hash keyed with a
value stored on-disk like ext4's s_hash_seed), since that would leak
information about the names that encryption is meant to protect.
Instead, if we can accept a dirhash that's only computable when the
fscrypt key is available, we can hash the plaintext names with a keyed
hash using a secret key derived from the directory's fscrypt master key.
We'll use SipHash-2-4 for this purpose.
Prepare for this by deriving a SipHash key for each casefolded encrypted
directory. Make sure to handle deriving the key not only when setting
up the directory's fscrypt_info, but also in the case where the casefold
flag is enabled after the fscrypt_info was already set up. (We could
just always derive the key regardless of casefolding, but that would
introduce unnecessary overhead for people not using casefolding.)
Signed-off-by: Daniel Rosenberg <drosen@google.com>
[EB: improved commit message, updated fscrypt.rst, squashed with change
that avoids unnecessarily deriving the key, and many other cleanups]
Link: https://lore.kernel.org/r/20200120223201.241390-3-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2020-01-21 06:31:57 +08:00
|
|
|
Dirhash keys
|
|
|
|
------------
|
|
|
|
|
|
|
|
For directories that are indexed using a secret-keyed dirhash over the
|
|
|
|
plaintext filenames, the KDF is also used to derive a 128-bit
|
|
|
|
SipHash-2-4 key per directory in order to hash filenames. This works
|
|
|
|
just like deriving a per-file encryption key, except that a different
|
|
|
|
KDF context is used. Currently, only casefolded ("case-insensitive")
|
|
|
|
encrypted directories use this style of hashing.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
Encryption modes and usage
|
|
|
|
==========================
|
|
|
|
|
|
|
|
fscrypt allows one encryption mode to be specified for file contents
|
|
|
|
and one encryption mode to be specified for filenames. Different
|
|
|
|
directory trees are permitted to use different encryption modes.
|
|
|
|
Currently, the following pairs of encryption modes are supported:
|
|
|
|
|
|
|
|
- AES-256-XTS for contents and AES-256-CTS-CBC for filenames
|
|
|
|
- AES-128-CBC for contents and AES-128-CTS-CBC for filenames
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
- Adiantum for both contents and filenames
|
2022-05-21 02:15:01 +08:00
|
|
|
- AES-256-XTS for contents and AES-256-HCTR2 for filenames (v2 policies only)
|
2022-12-01 20:58:19 +08:00
|
|
|
- SM4-XTS for contents and SM4-CTS-CBC for filenames (v2 policies only)
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
|
|
|
|
If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
AES-128-CBC was added only for low-powered embedded devices with
|
2019-06-21 02:15:05 +08:00
|
|
|
crypto accelerators such as CAAM or CESA that do not support XTS. To
|
2019-10-10 07:34:16 +08:00
|
|
|
use AES-128-CBC, CONFIG_CRYPTO_ESSIV and CONFIG_CRYPTO_SHA256 (or
|
|
|
|
another SHA-256 implementation) must be enabled so that ESSIV can be
|
|
|
|
used.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
Adiantum is a (primarily) stream cipher-based mode that is fast even
|
|
|
|
on CPUs without dedicated crypto instructions. It's also a true
|
|
|
|
wide-block mode, unlike XTS. It can also eliminate the need to derive
|
2020-01-21 06:31:58 +08:00
|
|
|
per-file encryption keys. However, it depends on the security of two
|
|
|
|
primitives, XChaCha12 and AES-256, rather than just one. See the
|
|
|
|
paper "Adiantum: length-preserving encryption for entry-level
|
|
|
|
processors" (https://eprint.iacr.org/2018/720.pdf) for more details.
|
|
|
|
To use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled. Also, fast
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
implementations of ChaCha and NHPoly1305 should be enabled, e.g.
|
|
|
|
CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM.
|
|
|
|
|
2022-05-21 02:15:01 +08:00
|
|
|
AES-256-HCTR2 is another true wide-block encryption mode that is intended for
|
|
|
|
use on CPUs with dedicated crypto instructions. AES-256-HCTR2 has the property
|
|
|
|
that a bitflip in the plaintext changes the entire ciphertext. This property
|
|
|
|
makes it desirable for filename encryption since initialization vectors are
|
|
|
|
reused within a directory. For more details on AES-256-HCTR2, see the paper
|
|
|
|
"Length-preserving encryption with HCTR2"
|
|
|
|
(https://eprint.iacr.org/2021/1441.pdf). To use AES-256-HCTR2,
|
|
|
|
CONFIG_CRYPTO_HCTR2 must be enabled. Also, fast implementations of XCTR and
|
|
|
|
POLYVAL should be enabled, e.g. CRYPTO_POLYVAL_ARM64_CE and
|
|
|
|
CRYPTO_AES_ARM64_CE_BLK for ARM64.
|
|
|
|
|
2022-12-02 03:14:52 +08:00
|
|
|
SM4 is a Chinese block cipher that is an alternative to AES. It has
|
|
|
|
not seen as much security review as AES, and it only has a 128-bit key
|
|
|
|
size. It may be useful in cases where its use is mandated.
|
|
|
|
Otherwise, it should not be used. For SM4 support to be available, it
|
|
|
|
also needs to be enabled in the kernel crypto API.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
New encryption modes can be added relatively easily, without changes
|
|
|
|
to individual filesystems. However, authenticated encryption (AE)
|
|
|
|
modes are not currently supported because of the difficulty of dealing
|
|
|
|
with ciphertext expansion.
|
|
|
|
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
Contents encryption
|
|
|
|
-------------------
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
For file contents, each filesystem block is encrypted independently.
|
2019-10-23 11:33:12 +08:00
|
|
|
Starting from Linux kernel 5.5, encryption of filesystems with block
|
|
|
|
size less than system's page size is supported.
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
|
|
|
|
Each block's IV is set to the logical block number within the file as
|
|
|
|
a little endian number, except that:
|
|
|
|
|
|
|
|
- With CBC mode encryption, ESSIV is also used. Specifically, each IV
|
|
|
|
is encrypted with AES-256 where the AES-256 key is the SHA-256 hash
|
|
|
|
of the file's data encryption key.
|
|
|
|
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
- With `DIRECT_KEY policies`_, the file's nonce is appended to the IV.
|
|
|
|
Currently this is only allowed with the Adiantum encryption mode.
|
|
|
|
|
|
|
|
- With `IV_INO_LBLK_64 policies`_, the logical block number is limited
|
|
|
|
to 32 bits and is placed in bits 0-31 of the IV. The inode number
|
|
|
|
(which is also limited to 32 bits) is placed in bits 32-63.
|
|
|
|
|
2020-05-16 04:41:41 +08:00
|
|
|
- With `IV_INO_LBLK_32 policies`_, the logical block number is limited
|
|
|
|
to 32 bits and is placed in bits 0-31 of the IV. The inode number
|
|
|
|
is then hashed and added mod 2^32.
|
|
|
|
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
Note that because file logical block numbers are included in the IVs,
|
|
|
|
filesystems must enforce that blocks are never shifted around within
|
|
|
|
encrypted files, e.g. via "collapse range" or "insert range".
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
|
|
|
|
Filenames encryption
|
|
|
|
--------------------
|
|
|
|
|
|
|
|
For filenames, each full filename is encrypted at once. Because of
|
|
|
|
the requirements to retain support for efficient directory lookups and
|
|
|
|
filenames of up to 255 bytes, the same IV is used for every filename
|
|
|
|
in a directory.
|
|
|
|
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
However, each encrypted directory still uses a unique key, or
|
|
|
|
alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
|
|
|
|
inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
|
|
|
|
Thus, IV reuse is limited to within a single directory.
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
|
2022-05-21 02:15:01 +08:00
|
|
|
With CTS-CBC, the IV reuse means that when the plaintext filenames share a
|
|
|
|
common prefix at least as long as the cipher block size (16 bytes for AES), the
|
|
|
|
corresponding encrypted filenames will also share a common prefix. This is
|
|
|
|
undesirable. Adiantum and HCTR2 do not have this weakness, as they are
|
|
|
|
wide-block encryption modes.
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
|
|
|
|
All supported filenames encryption modes accept any plaintext length
|
|
|
|
>= 16 bytes; cipher block alignment is not required. However,
|
|
|
|
filenames shorter than 16 bytes are NUL-padded to 16 bytes before
|
|
|
|
being encrypted. In addition, to reduce leakage of filename lengths
|
|
|
|
via their ciphertexts, all filenames are NUL-padded to the next 4, 8,
|
|
|
|
16, or 32-byte boundary (configurable). 32 is recommended since this
|
|
|
|
provides the best confidentiality, at the cost of making directory
|
|
|
|
entries consume slightly more space. Note that since NUL (``\0``) is
|
|
|
|
not otherwise a valid character in filenames, the padding will never
|
|
|
|
produce duplicate plaintexts.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Symbolic link targets are considered a type of filename and are
|
fscrypt: add Adiantum support
Add support for the Adiantum encryption mode to fscrypt. Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS. See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details. Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").
On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function. These algorithms are fast even on processors
without dedicated crypto instructions. Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted. On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.
In fscrypt, Adiantum is suitable for encrypting both file contents and
names. With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.
Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode. This
configuration saves memory and improves performance. A new fscrypt
policy flag is added to allow users to opt-in to this configuration.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-01-06 21:36:21 +08:00
|
|
|
encrypted in the same way as filenames in directory entries, except
|
|
|
|
that IV reuse is not a problem as each symlink has its own inode.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
User API
|
|
|
|
========
|
|
|
|
|
|
|
|
Setting an encryption policy
|
|
|
|
----------------------------
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
FS_IOC_SET_ENCRYPTION_POLICY
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
The FS_IOC_SET_ENCRYPTION_POLICY ioctl sets an encryption policy on an
|
|
|
|
empty directory or verifies that a directory or regular file already
|
2020-10-14 14:40:47 +08:00
|
|
|
has the specified encryption policy. It takes in a pointer to
|
|
|
|
struct fscrypt_policy_v1 or struct fscrypt_policy_v2, defined as
|
|
|
|
follows::
|
2017-10-29 18:30:14 +08:00
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
#define FSCRYPT_POLICY_V1 0
|
|
|
|
#define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
|
|
|
|
struct fscrypt_policy_v1 {
|
2017-10-29 18:30:14 +08:00
|
|
|
__u8 version;
|
|
|
|
__u8 contents_encryption_mode;
|
|
|
|
__u8 filenames_encryption_mode;
|
|
|
|
__u8 flags;
|
fscrypt: use FSCRYPT_ prefix for uapi constants
Prefix all filesystem encryption UAPI constants except the ioctl numbers
with "FSCRYPT_" rather than with "FS_". This namespaces the constants
more appropriately and makes it clear that they are related specifically
to the filesystem encryption feature, and to the 'fscrypt_*' structures.
With some of the old names like "FS_POLICY_FLAGS_VALID", it was not
immediately clear that the constant had anything to do with encryption.
This is also useful because we'll be adding more encryption-related
constants, e.g. for the policy version, and we'd otherwise have to
choose whether to use unclear names like FS_POLICY_V1 or inconsistent
names like FS_ENCRYPTION_POLICY_V1.
For source compatibility with existing userspace programs, keep the old
names defined as aliases to the new names.
Finally, as long as new names are being defined anyway, I skipped
defining new names for the fscrypt mode numbers that aren't actually
used: INVALID (0), AES_256_GCM (2), AES_256_CBC (3), SPECK128_256_XTS
(7), and SPECK128_256_CTS (8).
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-08-05 10:35:44 +08:00
|
|
|
__u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
|
2017-10-29 18:30:14 +08:00
|
|
|
};
|
2019-08-05 10:35:49 +08:00
|
|
|
#define fscrypt_policy fscrypt_policy_v1
|
|
|
|
|
|
|
|
#define FSCRYPT_POLICY_V2 2
|
|
|
|
#define FSCRYPT_KEY_IDENTIFIER_SIZE 16
|
|
|
|
struct fscrypt_policy_v2 {
|
|
|
|
__u8 version;
|
|
|
|
__u8 contents_encryption_mode;
|
|
|
|
__u8 filenames_encryption_mode;
|
|
|
|
__u8 flags;
|
|
|
|
__u8 __reserved[4];
|
|
|
|
__u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
|
|
|
|
};
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
This structure must be initialized as follows:
|
|
|
|
|
2020-10-14 14:40:47 +08:00
|
|
|
- ``version`` must be FSCRYPT_POLICY_V1 (0) if
|
|
|
|
struct fscrypt_policy_v1 is used or FSCRYPT_POLICY_V2 (2) if
|
|
|
|
struct fscrypt_policy_v2 is used. (Note: we refer to the original
|
|
|
|
policy version as "v1", though its version code is really 0.)
|
|
|
|
For new encrypted directories, use v2 policies.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
- ``contents_encryption_mode`` and ``filenames_encryption_mode`` must
|
fscrypt: use FSCRYPT_ prefix for uapi constants
Prefix all filesystem encryption UAPI constants except the ioctl numbers
with "FSCRYPT_" rather than with "FS_". This namespaces the constants
more appropriately and makes it clear that they are related specifically
to the filesystem encryption feature, and to the 'fscrypt_*' structures.
With some of the old names like "FS_POLICY_FLAGS_VALID", it was not
immediately clear that the constant had anything to do with encryption.
This is also useful because we'll be adding more encryption-related
constants, e.g. for the policy version, and we'd otherwise have to
choose whether to use unclear names like FS_POLICY_V1 or inconsistent
names like FS_ENCRYPTION_POLICY_V1.
For source compatibility with existing userspace programs, keep the old
names defined as aliases to the new names.
Finally, as long as new names are being defined anyway, I skipped
defining new names for the fscrypt mode numbers that aren't actually
used: INVALID (0), AES_256_GCM (2), AES_256_CBC (3), SPECK128_256_XTS
(7), and SPECK128_256_CTS (8).
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-08-05 10:35:44 +08:00
|
|
|
be set to constants from ``<linux/fscrypt.h>`` which identify the
|
|
|
|
encryption modes to use. If unsure, use FSCRYPT_MODE_AES_256_XTS
|
|
|
|
(1) for ``contents_encryption_mode`` and FSCRYPT_MODE_AES_256_CTS
|
|
|
|
(4) for ``filenames_encryption_mode``.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
fscrypt: add support for IV_INO_LBLK_64 policies
Inline encryption hardware compliant with the UFS v2.1 standard or with
the upcoming version of the eMMC standard has the following properties:
(1) Per I/O request, the encryption key is specified by a previously
loaded keyslot. There might be only a small number of keyslots.
(2) Per I/O request, the starting IV is specified by a 64-bit "data unit
number" (DUN). IV bits 64-127 are assumed to be 0. The hardware
automatically increments the DUN for each "data unit" of
configurable size in the request, e.g. for each filesystem block.
Property (1) makes it inefficient to use the traditional fscrypt
per-file keys. Property (2) precludes the use of the existing
DIRECT_KEY fscrypt policy flag, which needs at least 192 IV bits.
Therefore, add a new fscrypt policy flag IV_INO_LBLK_64 which causes the
encryption to modified as follows:
- The encryption keys are derived from the master key, encryption mode
number, and filesystem UUID.
- The IVs are chosen as (inode_number << 32) | file_logical_block_num.
For filenames encryption, file_logical_block_num is 0.
Since the file nonces aren't used in the key derivation, many files may
share the same encryption key. This is much more efficient on the
target hardware. Including the inode number in the IVs and mixing the
filesystem UUID into the keys ensures that data in different files is
nevertheless still encrypted differently.
Additionally, limiting the inode and block numbers to 32 bits and
placing the block number in the low bits maintains compatibility with
the 64-bit DUN convention (property (2) above).
Since this scheme assumes that inode numbers are stable (which may
preclude filesystem shrinking) and that inode and file logical block
numbers are at most 32-bit, IV_INO_LBLK_64 will only be allowed on
filesystems that meet these constraints. These are acceptable
limitations for the cases where this format would actually be used.
Note that IV_INO_LBLK_64 is an on-disk format, not an implementation.
This patch just adds support for it using the existing filesystem layer
encryption. A later patch will add support for inline encryption.
Reviewed-by: Paul Crowley <paulcrowley@google.com>
Co-developed-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Satya Tangirala <satyat@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-10-25 05:54:36 +08:00
|
|
|
- ``flags`` contains optional flags from ``<linux/fscrypt.h>``:
|
|
|
|
|
|
|
|
- FSCRYPT_POLICY_FLAGS_PAD_*: The amount of NUL padding to use when
|
|
|
|
encrypting filenames. If unsure, use FSCRYPT_POLICY_FLAGS_PAD_32
|
|
|
|
(0x3).
|
|
|
|
- FSCRYPT_POLICY_FLAG_DIRECT_KEY: See `DIRECT_KEY policies`_.
|
|
|
|
- FSCRYPT_POLICY_FLAG_IV_INO_LBLK_64: See `IV_INO_LBLK_64
|
2020-05-16 04:41:41 +08:00
|
|
|
policies`_.
|
|
|
|
- FSCRYPT_POLICY_FLAG_IV_INO_LBLK_32: See `IV_INO_LBLK_32
|
|
|
|
policies`_.
|
|
|
|
|
|
|
|
v1 encryption policies only support the PAD_* and DIRECT_KEY flags.
|
|
|
|
The other flags are only supported by v2 encryption policies.
|
|
|
|
|
|
|
|
The DIRECT_KEY, IV_INO_LBLK_64, and IV_INO_LBLK_32 flags are
|
|
|
|
mutually exclusive.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
- For v2 encryption policies, ``__reserved`` must be zeroed.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
- For v1 encryption policies, ``master_key_descriptor`` specifies how
|
|
|
|
to find the master key in a keyring; see `Adding keys`_. It is up
|
|
|
|
to userspace to choose a unique ``master_key_descriptor`` for each
|
|
|
|
master key. The e4crypt and fscrypt tools use the first 8 bytes of
|
2017-10-29 18:30:14 +08:00
|
|
|
``SHA-512(SHA-512(master_key))``, but this particular scheme is not
|
|
|
|
required. Also, the master key need not be in the keyring yet when
|
|
|
|
FS_IOC_SET_ENCRYPTION_POLICY is executed. However, it must be added
|
|
|
|
before any files can be created in the encrypted directory.
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
For v2 encryption policies, ``master_key_descriptor`` has been
|
|
|
|
replaced with ``master_key_identifier``, which is longer and cannot
|
|
|
|
be arbitrarily chosen. Instead, the key must first be added using
|
|
|
|
`FS_IOC_ADD_ENCRYPTION_KEY`_. Then, the ``key_spec.u.identifier``
|
2020-10-14 14:40:47 +08:00
|
|
|
the kernel returned in the struct fscrypt_add_key_arg must
|
|
|
|
be used as the ``master_key_identifier`` in
|
|
|
|
struct fscrypt_policy_v2.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
If the file is not yet encrypted, then FS_IOC_SET_ENCRYPTION_POLICY
|
|
|
|
verifies that the file is an empty directory. If so, the specified
|
|
|
|
encryption policy is assigned to the directory, turning it into an
|
|
|
|
encrypted directory. After that, and after providing the
|
|
|
|
corresponding master key as described in `Adding keys`_, all regular
|
|
|
|
files, directories (recursively), and symlinks created in the
|
|
|
|
directory will be encrypted, inheriting the same encryption policy.
|
|
|
|
The filenames in the directory's entries will be encrypted as well.
|
|
|
|
|
|
|
|
Alternatively, if the file is already encrypted, then
|
|
|
|
FS_IOC_SET_ENCRYPTION_POLICY validates that the specified encryption
|
|
|
|
policy exactly matches the actual one. If they match, then the ioctl
|
|
|
|
returns 0. Otherwise, it fails with EEXIST. This works on both
|
|
|
|
regular files and directories, including nonempty directories.
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
When a v2 encryption policy is assigned to a directory, it is also
|
|
|
|
required that either the specified key has been added by the current
|
|
|
|
user or that the caller has CAP_FOWNER in the initial user namespace.
|
|
|
|
(This is needed to prevent a user from encrypting their data with
|
|
|
|
another user's key.) The key must remain added while
|
|
|
|
FS_IOC_SET_ENCRYPTION_POLICY is executing. However, if the new
|
|
|
|
encrypted directory does not need to be accessed immediately, then the
|
|
|
|
key can be removed right away afterwards.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
Note that the ext4 filesystem does not allow the root directory to be
|
|
|
|
encrypted, even if it is empty. Users who want to encrypt an entire
|
|
|
|
filesystem with one key should consider using dm-crypt instead.
|
|
|
|
|
|
|
|
FS_IOC_SET_ENCRYPTION_POLICY can fail with the following errors:
|
|
|
|
|
|
|
|
- ``EACCES``: the file is not owned by the process's uid, nor does the
|
|
|
|
process have the CAP_FOWNER capability in a namespace with the file
|
|
|
|
owner's uid mapped
|
|
|
|
- ``EEXIST``: the file is already encrypted with an encryption policy
|
|
|
|
different from the one specified
|
|
|
|
- ``EINVAL``: an invalid encryption policy was specified (invalid
|
2020-01-21 06:31:56 +08:00
|
|
|
version, mode(s), or flags; or reserved bits were set); or a v1
|
|
|
|
encryption policy was specified but the directory has the casefold
|
|
|
|
flag enabled (casefolding is incompatible with v1 policies).
|
2019-08-05 10:35:49 +08:00
|
|
|
- ``ENOKEY``: a v2 encryption policy was specified, but the key with
|
|
|
|
the specified ``master_key_identifier`` has not been added, nor does
|
|
|
|
the process have the CAP_FOWNER capability in the initial user
|
|
|
|
namespace
|
2017-10-29 18:30:14 +08:00
|
|
|
- ``ENOTDIR``: the file is unencrypted and is a regular file, not a
|
|
|
|
directory
|
|
|
|
- ``ENOTEMPTY``: the file is unencrypted and is a nonempty directory
|
|
|
|
- ``ENOTTY``: this type of filesystem does not implement encryption
|
|
|
|
- ``EOPNOTSUPP``: the kernel was not configured with encryption
|
2018-12-12 17:50:12 +08:00
|
|
|
support for filesystems, or the filesystem superblock has not
|
2017-10-29 18:30:14 +08:00
|
|
|
had encryption enabled on it. (For example, to use encryption on an
|
2018-12-12 17:50:12 +08:00
|
|
|
ext4 filesystem, CONFIG_FS_ENCRYPTION must be enabled in the
|
2017-10-29 18:30:14 +08:00
|
|
|
kernel config, and the superblock must have had the "encrypt"
|
|
|
|
feature flag enabled using ``tune2fs -O encrypt`` or ``mkfs.ext4 -O
|
|
|
|
encrypt``.)
|
|
|
|
- ``EPERM``: this directory may not be encrypted, e.g. because it is
|
|
|
|
the root directory of an ext4 filesystem
|
|
|
|
- ``EROFS``: the filesystem is readonly
|
|
|
|
|
|
|
|
Getting an encryption policy
|
|
|
|
----------------------------
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
Two ioctls are available to get a file's encryption policy:
|
|
|
|
|
|
|
|
- `FS_IOC_GET_ENCRYPTION_POLICY_EX`_
|
|
|
|
- `FS_IOC_GET_ENCRYPTION_POLICY`_
|
|
|
|
|
|
|
|
The extended (_EX) version of the ioctl is more general and is
|
|
|
|
recommended to use when possible. However, on older kernels only the
|
|
|
|
original ioctl is available. Applications should try the extended
|
|
|
|
version, and if it fails with ENOTTY fall back to the original
|
|
|
|
version.
|
|
|
|
|
|
|
|
FS_IOC_GET_ENCRYPTION_POLICY_EX
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
The FS_IOC_GET_ENCRYPTION_POLICY_EX ioctl retrieves the encryption
|
|
|
|
policy, if any, for a directory or regular file. No additional
|
|
|
|
permissions are required beyond the ability to open the file. It
|
2020-10-14 14:40:47 +08:00
|
|
|
takes in a pointer to struct fscrypt_get_policy_ex_arg,
|
2019-08-05 10:35:49 +08:00
|
|
|
defined as follows::
|
|
|
|
|
|
|
|
struct fscrypt_get_policy_ex_arg {
|
|
|
|
__u64 policy_size; /* input/output */
|
|
|
|
union {
|
|
|
|
__u8 version;
|
|
|
|
struct fscrypt_policy_v1 v1;
|
|
|
|
struct fscrypt_policy_v2 v2;
|
|
|
|
} policy; /* output */
|
|
|
|
};
|
|
|
|
|
|
|
|
The caller must initialize ``policy_size`` to the size available for
|
|
|
|
the policy struct, i.e. ``sizeof(arg.policy)``.
|
|
|
|
|
|
|
|
On success, the policy struct is returned in ``policy``, and its
|
|
|
|
actual size is returned in ``policy_size``. ``policy.version`` should
|
|
|
|
be checked to determine the version of policy returned. Note that the
|
|
|
|
version code for the "v1" policy is actually 0 (FSCRYPT_POLICY_V1).
|
2017-10-29 18:30:14 +08:00
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
- ``EINVAL``: the file is encrypted, but it uses an unrecognized
|
2019-08-05 10:35:49 +08:00
|
|
|
encryption policy version
|
2017-10-29 18:30:14 +08:00
|
|
|
- ``ENODATA``: the file is not encrypted
|
2019-08-05 10:35:49 +08:00
|
|
|
- ``ENOTTY``: this type of filesystem does not implement encryption,
|
|
|
|
or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
|
|
|
|
(try FS_IOC_GET_ENCRYPTION_POLICY instead)
|
2017-10-29 18:30:14 +08:00
|
|
|
- ``EOPNOTSUPP``: the kernel was not configured with encryption
|
2019-08-04 17:56:43 +08:00
|
|
|
support for this filesystem, or the filesystem superblock has not
|
|
|
|
had encryption enabled on it
|
2019-08-05 10:35:49 +08:00
|
|
|
- ``EOVERFLOW``: the file is encrypted and uses a recognized
|
|
|
|
encryption policy version, but the policy struct does not fit into
|
|
|
|
the provided buffer
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Note: if you only need to know whether a file is encrypted or not, on
|
|
|
|
most filesystems it is also possible to use the FS_IOC_GETFLAGS ioctl
|
|
|
|
and check for FS_ENCRYPT_FL, or to use the statx() system call and
|
|
|
|
check for STATX_ATTR_ENCRYPTED in stx_attributes.
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
FS_IOC_GET_ENCRYPTION_POLICY
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
The FS_IOC_GET_ENCRYPTION_POLICY ioctl can also retrieve the
|
|
|
|
encryption policy, if any, for a directory or regular file. However,
|
|
|
|
unlike `FS_IOC_GET_ENCRYPTION_POLICY_EX`_,
|
|
|
|
FS_IOC_GET_ENCRYPTION_POLICY only supports the original policy
|
2020-10-14 14:40:47 +08:00
|
|
|
version. It takes in a pointer directly to struct fscrypt_policy_v1
|
|
|
|
rather than struct fscrypt_get_policy_ex_arg.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
The error codes for FS_IOC_GET_ENCRYPTION_POLICY are the same as those
|
|
|
|
for FS_IOC_GET_ENCRYPTION_POLICY_EX, except that
|
|
|
|
FS_IOC_GET_ENCRYPTION_POLICY also returns ``EINVAL`` if the file is
|
|
|
|
encrypted using a newer encryption policy version.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
Getting the per-filesystem salt
|
|
|
|
-------------------------------
|
|
|
|
|
|
|
|
Some filesystems, such as ext4 and F2FS, also support the deprecated
|
|
|
|
ioctl FS_IOC_GET_ENCRYPTION_PWSALT. This ioctl retrieves a randomly
|
|
|
|
generated 16-byte value stored in the filesystem superblock. This
|
|
|
|
value is intended to used as a salt when deriving an encryption key
|
|
|
|
from a passphrase or other low-entropy user credential.
|
|
|
|
|
|
|
|
FS_IOC_GET_ENCRYPTION_PWSALT is deprecated. Instead, prefer to
|
|
|
|
generate and manage any needed salt(s) in userspace.
|
|
|
|
|
2020-03-15 04:50:49 +08:00
|
|
|
Getting a file's encryption nonce
|
|
|
|
---------------------------------
|
|
|
|
|
|
|
|
Since Linux v5.7, the ioctl FS_IOC_GET_ENCRYPTION_NONCE is supported.
|
|
|
|
On encrypted files and directories it gets the inode's 16-byte nonce.
|
|
|
|
On unencrypted files and directories, it fails with ENODATA.
|
|
|
|
|
|
|
|
This ioctl can be useful for automated tests which verify that the
|
|
|
|
encryption is being done correctly. It is not needed for normal use
|
|
|
|
of fscrypt.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
Adding keys
|
|
|
|
-----------
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
FS_IOC_ADD_ENCRYPTION_KEY
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
The FS_IOC_ADD_ENCRYPTION_KEY ioctl adds a master encryption key to
|
|
|
|
the filesystem, making all files on the filesystem which were
|
|
|
|
encrypted using that key appear "unlocked", i.e. in plaintext form.
|
|
|
|
It can be executed on any file or directory on the target filesystem,
|
|
|
|
but using the filesystem's root directory is recommended. It takes in
|
2020-10-14 14:40:47 +08:00
|
|
|
a pointer to struct fscrypt_add_key_arg, defined as follows::
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
struct fscrypt_add_key_arg {
|
|
|
|
struct fscrypt_key_specifier key_spec;
|
|
|
|
__u32 raw_size;
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
__u32 key_id;
|
|
|
|
__u32 __reserved[8];
|
2019-08-05 10:35:49 +08:00
|
|
|
__u8 raw[];
|
|
|
|
};
|
|
|
|
|
|
|
|
#define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR 1
|
|
|
|
#define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER 2
|
|
|
|
|
|
|
|
struct fscrypt_key_specifier {
|
|
|
|
__u32 type; /* one of FSCRYPT_KEY_SPEC_TYPE_* */
|
|
|
|
__u32 __reserved;
|
|
|
|
union {
|
|
|
|
__u8 __reserved[32]; /* reserve some extra space */
|
|
|
|
__u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
|
|
|
|
__u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
|
|
|
|
} u;
|
|
|
|
};
|
|
|
|
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
struct fscrypt_provisioning_key_payload {
|
|
|
|
__u32 type;
|
|
|
|
__u32 __reserved;
|
|
|
|
__u8 raw[];
|
|
|
|
};
|
|
|
|
|
2020-10-14 14:40:47 +08:00
|
|
|
struct fscrypt_add_key_arg must be zeroed, then initialized
|
2019-08-05 10:35:49 +08:00
|
|
|
as follows:
|
|
|
|
|
|
|
|
- If the key is being added for use by v1 encryption policies, then
|
|
|
|
``key_spec.type`` must contain FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR, and
|
|
|
|
``key_spec.u.descriptor`` must contain the descriptor of the key
|
|
|
|
being added, corresponding to the value in the
|
2020-10-14 14:40:47 +08:00
|
|
|
``master_key_descriptor`` field of struct fscrypt_policy_v1.
|
|
|
|
To add this type of key, the calling process must have the
|
|
|
|
CAP_SYS_ADMIN capability in the initial user namespace.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
Alternatively, if the key is being added for use by v2 encryption
|
|
|
|
policies, then ``key_spec.type`` must contain
|
|
|
|
FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER, and ``key_spec.u.identifier`` is
|
|
|
|
an *output* field which the kernel fills in with a cryptographic
|
|
|
|
hash of the key. To add this type of key, the calling process does
|
|
|
|
not need any privileges. However, the number of keys that can be
|
|
|
|
added is limited by the user's quota for the keyrings service (see
|
|
|
|
``Documentation/security/keys/core.rst``).
|
|
|
|
|
|
|
|
- ``raw_size`` must be the size of the ``raw`` key provided, in bytes.
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
Alternatively, if ``key_id`` is nonzero, this field must be 0, since
|
|
|
|
in that case the size is implied by the specified Linux keyring key.
|
|
|
|
|
|
|
|
- ``key_id`` is 0 if the raw key is given directly in the ``raw``
|
|
|
|
field. Otherwise ``key_id`` is the ID of a Linux keyring key of
|
2020-10-14 14:40:47 +08:00
|
|
|
type "fscrypt-provisioning" whose payload is
|
|
|
|
struct fscrypt_provisioning_key_payload whose ``raw`` field contains
|
|
|
|
the raw key and whose ``type`` field matches ``key_spec.type``.
|
|
|
|
Since ``raw`` is variable-length, the total size of this key's
|
|
|
|
payload must be ``sizeof(struct fscrypt_provisioning_key_payload)``
|
|
|
|
plus the raw key size. The process must have Search permission on
|
|
|
|
this key.
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
|
|
|
|
Most users should leave this 0 and specify the raw key directly.
|
|
|
|
The support for specifying a Linux keyring key is intended mainly to
|
|
|
|
allow re-adding keys after a filesystem is unmounted and re-mounted,
|
|
|
|
without having to store the raw keys in userspace memory.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
- ``raw`` is a variable-length field which must contain the actual
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
key, ``raw_size`` bytes long. Alternatively, if ``key_id`` is
|
|
|
|
nonzero, then this field is unused.
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
For v2 policy keys, the kernel keeps track of which user (identified
|
|
|
|
by effective user ID) added the key, and only allows the key to be
|
|
|
|
removed by that user --- or by "root", if they use
|
|
|
|
`FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_.
|
|
|
|
|
|
|
|
However, if another user has added the key, it may be desirable to
|
|
|
|
prevent that other user from unexpectedly removing it. Therefore,
|
|
|
|
FS_IOC_ADD_ENCRYPTION_KEY may also be used to add a v2 policy key
|
|
|
|
*again*, even if it's already added by other user(s). In this case,
|
|
|
|
FS_IOC_ADD_ENCRYPTION_KEY will just install a claim to the key for the
|
|
|
|
current user, rather than actually add the key again (but the raw key
|
|
|
|
must still be provided, as a proof of knowledge).
|
|
|
|
|
|
|
|
FS_IOC_ADD_ENCRYPTION_KEY returns 0 if either the key or a claim to
|
|
|
|
the key was either added or already exists.
|
|
|
|
|
|
|
|
FS_IOC_ADD_ENCRYPTION_KEY can fail with the following errors:
|
|
|
|
|
|
|
|
- ``EACCES``: FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR was specified, but the
|
|
|
|
caller does not have the CAP_SYS_ADMIN capability in the initial
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
user namespace; or the raw key was specified by Linux key ID but the
|
|
|
|
process lacks Search permission on the key.
|
2019-08-05 10:35:49 +08:00
|
|
|
- ``EDQUOT``: the key quota for this user would be exceeded by adding
|
|
|
|
the key
|
|
|
|
- ``EINVAL``: invalid key size or key specifier type, or reserved bits
|
|
|
|
were set
|
fscrypt: support passing a keyring key to FS_IOC_ADD_ENCRYPTION_KEY
Extend the FS_IOC_ADD_ENCRYPTION_KEY ioctl to allow the raw key to be
specified by a Linux keyring key, rather than specified directly.
This is useful because fscrypt keys belong to a particular filesystem
instance, so they are destroyed when that filesystem is unmounted.
Usually this is desired. But in some cases, userspace may need to
unmount and re-mount the filesystem while keeping the keys, e.g. during
a system update. This requires keeping the keys somewhere else too.
The keys could be kept in memory in a userspace daemon. But depending
on the security architecture and assumptions, it can be preferable to
keep them only in kernel memory, where they are unreadable by userspace.
We also can't solve this by going back to the original fscrypt API
(where for each file, the master key was looked up in the process's
keyring hierarchy) because that caused lots of problems of its own.
Therefore, add the ability for FS_IOC_ADD_ENCRYPTION_KEY to accept a
Linux keyring key. This solves the problem by allowing userspace to (if
needed) save the keys securely in a Linux keyring for re-provisioning,
while still using the new fscrypt key management ioctls.
This is analogous to how dm-crypt accepts a Linux keyring key, but the
key is then stored internally in the dm-crypt data structures rather
than being looked up again each time the dm-crypt device is accessed.
Use a custom key type "fscrypt-provisioning" rather than one of the
existing key types such as "logon". This is strongly desired because it
enforces that these keys are only usable for a particular purpose: for
fscrypt as input to a particular KDF. Otherwise, the keys could also be
passed to any kernel API that accepts a "logon" key with any service
prefix, e.g. dm-crypt, UBIFS, or (recently proposed) AF_ALG. This would
risk leaking information about the raw key despite it ostensibly being
unreadable. Of course, this mistake has already been made for multiple
kernel APIs; but since this is a new API, let's do it right.
This patch has been tested using an xfstest which I wrote to test it.
Link: https://lore.kernel.org/r/20191119222447.226853-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-11-20 06:24:47 +08:00
|
|
|
- ``EKEYREJECTED``: the raw key was specified by Linux key ID, but the
|
|
|
|
key has the wrong type
|
|
|
|
- ``ENOKEY``: the raw key was specified by Linux key ID, but no key
|
|
|
|
exists with that ID
|
2019-08-05 10:35:49 +08:00
|
|
|
- ``ENOTTY``: this type of filesystem does not implement encryption
|
|
|
|
- ``EOPNOTSUPP``: the kernel was not configured with encryption
|
|
|
|
support for this filesystem, or the filesystem superblock has not
|
|
|
|
had encryption enabled on it
|
|
|
|
|
|
|
|
Legacy method
|
|
|
|
~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
For v1 encryption policies, a master encryption key can also be
|
|
|
|
provided by adding it to a process-subscribed keyring, e.g. to a
|
|
|
|
session keyring, or to a user keyring if the user keyring is linked
|
|
|
|
into the session keyring.
|
|
|
|
|
|
|
|
This method is deprecated (and not supported for v2 encryption
|
|
|
|
policies) for several reasons. First, it cannot be used in
|
|
|
|
combination with FS_IOC_REMOVE_ENCRYPTION_KEY (see `Removing keys`_),
|
|
|
|
so for removing a key a workaround such as keyctl_unlink() in
|
|
|
|
combination with ``sync; echo 2 > /proc/sys/vm/drop_caches`` would
|
|
|
|
have to be used. Second, it doesn't match the fact that the
|
|
|
|
locked/unlocked status of encrypted files (i.e. whether they appear to
|
|
|
|
be in plaintext form or in ciphertext form) is global. This mismatch
|
|
|
|
has caused much confusion as well as real problems when processes
|
|
|
|
running under different UIDs, such as a ``sudo`` command, need to
|
|
|
|
access encrypted files.
|
|
|
|
|
|
|
|
Nevertheless, to add a key to one of the process-subscribed keyrings,
|
|
|
|
the add_key() system call can be used (see:
|
2017-10-29 18:30:14 +08:00
|
|
|
``Documentation/security/keys/core.rst``). The key type must be
|
|
|
|
"logon"; keys of this type are kept in kernel memory and cannot be
|
|
|
|
read back by userspace. The key description must be "fscrypt:"
|
|
|
|
followed by the 16-character lower case hex representation of the
|
|
|
|
``master_key_descriptor`` that was set in the encryption policy. The
|
|
|
|
key payload must conform to the following structure::
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
#define FSCRYPT_MAX_KEY_SIZE 64
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
struct fscrypt_key {
|
2019-08-05 10:35:49 +08:00
|
|
|
__u32 mode;
|
|
|
|
__u8 raw[FSCRYPT_MAX_KEY_SIZE];
|
|
|
|
__u32 size;
|
2017-10-29 18:30:14 +08:00
|
|
|
};
|
|
|
|
|
|
|
|
``mode`` is ignored; just set it to 0. The actual key is provided in
|
|
|
|
``raw`` with ``size`` indicating its size in bytes. That is, the
|
|
|
|
bytes ``raw[0..size-1]`` (inclusive) are the actual key.
|
|
|
|
|
|
|
|
The key description prefix "fscrypt:" may alternatively be replaced
|
|
|
|
with a filesystem-specific prefix such as "ext4:". However, the
|
|
|
|
filesystem-specific prefixes are deprecated and should not be used in
|
|
|
|
new programs.
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
Removing keys
|
|
|
|
-------------
|
|
|
|
|
|
|
|
Two ioctls are available for removing a key that was added by
|
|
|
|
`FS_IOC_ADD_ENCRYPTION_KEY`_:
|
|
|
|
|
|
|
|
- `FS_IOC_REMOVE_ENCRYPTION_KEY`_
|
|
|
|
- `FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS`_
|
|
|
|
|
|
|
|
These two ioctls differ only in cases where v2 policy keys are added
|
|
|
|
or removed by non-root users.
|
|
|
|
|
|
|
|
These ioctls don't work on keys that were added via the legacy
|
|
|
|
process-subscribed keyrings mechanism.
|
|
|
|
|
|
|
|
Before using these ioctls, read the `Kernel memory compromise`_
|
|
|
|
section for a discussion of the security goals and limitations of
|
|
|
|
these ioctls.
|
|
|
|
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
The FS_IOC_REMOVE_ENCRYPTION_KEY ioctl removes a claim to a master
|
|
|
|
encryption key from the filesystem, and possibly removes the key
|
|
|
|
itself. It can be executed on any file or directory on the target
|
|
|
|
filesystem, but using the filesystem's root directory is recommended.
|
2020-10-14 14:40:47 +08:00
|
|
|
It takes in a pointer to struct fscrypt_remove_key_arg, defined
|
|
|
|
as follows::
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
struct fscrypt_remove_key_arg {
|
|
|
|
struct fscrypt_key_specifier key_spec;
|
|
|
|
#define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY 0x00000001
|
|
|
|
#define FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS 0x00000002
|
|
|
|
__u32 removal_status_flags; /* output */
|
|
|
|
__u32 __reserved[5];
|
|
|
|
};
|
|
|
|
|
|
|
|
This structure must be zeroed, then initialized as follows:
|
|
|
|
|
|
|
|
- The key to remove is specified by ``key_spec``:
|
|
|
|
|
|
|
|
- To remove a key used by v1 encryption policies, set
|
|
|
|
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
|
|
|
|
in ``key_spec.u.descriptor``. To remove this type of key, the
|
|
|
|
calling process must have the CAP_SYS_ADMIN capability in the
|
|
|
|
initial user namespace.
|
|
|
|
|
|
|
|
- To remove a key used by v2 encryption policies, set
|
|
|
|
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
|
|
|
|
in ``key_spec.u.identifier``.
|
|
|
|
|
|
|
|
For v2 policy keys, this ioctl is usable by non-root users. However,
|
|
|
|
to make this possible, it actually just removes the current user's
|
|
|
|
claim to the key, undoing a single call to FS_IOC_ADD_ENCRYPTION_KEY.
|
|
|
|
Only after all claims are removed is the key really removed.
|
|
|
|
|
|
|
|
For example, if FS_IOC_ADD_ENCRYPTION_KEY was called with uid 1000,
|
|
|
|
then the key will be "claimed" by uid 1000, and
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY will only succeed as uid 1000. Or, if
|
|
|
|
both uids 1000 and 2000 added the key, then for each uid
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY will only remove their own claim. Only
|
|
|
|
once *both* are removed is the key really removed. (Think of it like
|
|
|
|
unlinking a file that may have hard links.)
|
|
|
|
|
|
|
|
If FS_IOC_REMOVE_ENCRYPTION_KEY really removes the key, it will also
|
|
|
|
try to "lock" all files that had been unlocked with the key. It won't
|
|
|
|
lock files that are still in-use, so this ioctl is expected to be used
|
|
|
|
in cooperation with userspace ensuring that none of the files are
|
|
|
|
still open. However, if necessary, this ioctl can be executed again
|
|
|
|
later to retry locking any remaining files.
|
|
|
|
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY returns 0 if either the key was removed
|
|
|
|
(but may still have files remaining to be locked), the user's claim to
|
|
|
|
the key was removed, or the key was already removed but had files
|
|
|
|
remaining to be the locked so the ioctl retried locking them. In any
|
|
|
|
of these cases, ``removal_status_flags`` is filled in with the
|
|
|
|
following informational status flags:
|
|
|
|
|
|
|
|
- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY``: set if some file(s)
|
|
|
|
are still in-use. Not guaranteed to be set in the case where only
|
|
|
|
the user's claim to the key was removed.
|
|
|
|
- ``FSCRYPT_KEY_REMOVAL_STATUS_FLAG_OTHER_USERS``: set if only the
|
|
|
|
user's claim to the key was removed, not the key itself
|
|
|
|
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY can fail with the following errors:
|
|
|
|
|
|
|
|
- ``EACCES``: The FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR key specifier type
|
|
|
|
was specified, but the caller does not have the CAP_SYS_ADMIN
|
|
|
|
capability in the initial user namespace
|
|
|
|
- ``EINVAL``: invalid key specifier type, or reserved bits were set
|
|
|
|
- ``ENOKEY``: the key object was not found at all, i.e. it was never
|
|
|
|
added in the first place or was already fully removed including all
|
|
|
|
files locked; or, the user does not have a claim to the key (but
|
|
|
|
someone else does).
|
|
|
|
- ``ENOTTY``: this type of filesystem does not implement encryption
|
|
|
|
- ``EOPNOTSUPP``: the kernel was not configured with encryption
|
|
|
|
support for this filesystem, or the filesystem superblock has not
|
|
|
|
had encryption enabled on it
|
|
|
|
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS is exactly the same as
|
|
|
|
`FS_IOC_REMOVE_ENCRYPTION_KEY`_, except that for v2 policy keys, the
|
|
|
|
ALL_USERS version of the ioctl will remove all users' claims to the
|
|
|
|
key, not just the current user's. I.e., the key itself will always be
|
|
|
|
removed, no matter how many users have added it. This difference is
|
|
|
|
only meaningful if non-root users are adding and removing keys.
|
|
|
|
|
|
|
|
Because of this, FS_IOC_REMOVE_ENCRYPTION_KEY_ALL_USERS also requires
|
|
|
|
"root", namely the CAP_SYS_ADMIN capability in the initial user
|
|
|
|
namespace. Otherwise it will fail with EACCES.
|
|
|
|
|
|
|
|
Getting key status
|
|
|
|
------------------
|
|
|
|
|
|
|
|
FS_IOC_GET_ENCRYPTION_KEY_STATUS
|
|
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
|
|
The FS_IOC_GET_ENCRYPTION_KEY_STATUS ioctl retrieves the status of a
|
|
|
|
master encryption key. It can be executed on any file or directory on
|
|
|
|
the target filesystem, but using the filesystem's root directory is
|
2020-10-14 14:40:47 +08:00
|
|
|
recommended. It takes in a pointer to
|
|
|
|
struct fscrypt_get_key_status_arg, defined as follows::
|
2019-08-05 10:35:49 +08:00
|
|
|
|
|
|
|
struct fscrypt_get_key_status_arg {
|
|
|
|
/* input */
|
|
|
|
struct fscrypt_key_specifier key_spec;
|
|
|
|
__u32 __reserved[6];
|
|
|
|
|
|
|
|
/* output */
|
|
|
|
#define FSCRYPT_KEY_STATUS_ABSENT 1
|
|
|
|
#define FSCRYPT_KEY_STATUS_PRESENT 2
|
|
|
|
#define FSCRYPT_KEY_STATUS_INCOMPLETELY_REMOVED 3
|
|
|
|
__u32 status;
|
|
|
|
#define FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF 0x00000001
|
|
|
|
__u32 status_flags;
|
|
|
|
__u32 user_count;
|
|
|
|
__u32 __out_reserved[13];
|
|
|
|
};
|
|
|
|
|
|
|
|
The caller must zero all input fields, then fill in ``key_spec``:
|
|
|
|
|
|
|
|
- To get the status of a key for v1 encryption policies, set
|
|
|
|
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR and fill
|
|
|
|
in ``key_spec.u.descriptor``.
|
|
|
|
|
|
|
|
- To get the status of a key for v2 encryption policies, set
|
|
|
|
``key_spec.type`` to FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER and fill
|
|
|
|
in ``key_spec.u.identifier``.
|
|
|
|
|
|
|
|
On success, 0 is returned and the kernel fills in the output fields:
|
|
|
|
|
|
|
|
- ``status`` indicates whether the key is absent, present, or
|
|
|
|
incompletely removed. Incompletely removed means that the master
|
|
|
|
secret has been removed, but some files are still in use; i.e.,
|
|
|
|
`FS_IOC_REMOVE_ENCRYPTION_KEY`_ returned 0 but set the informational
|
|
|
|
status flag FSCRYPT_KEY_REMOVAL_STATUS_FLAG_FILES_BUSY.
|
|
|
|
|
|
|
|
- ``status_flags`` can contain the following flags:
|
|
|
|
|
|
|
|
- ``FSCRYPT_KEY_STATUS_FLAG_ADDED_BY_SELF`` indicates that the key
|
|
|
|
has added by the current user. This is only set for keys
|
|
|
|
identified by ``identifier`` rather than by ``descriptor``.
|
|
|
|
|
|
|
|
- ``user_count`` specifies the number of users who have added the key.
|
|
|
|
This is only set for keys identified by ``identifier`` rather than
|
|
|
|
by ``descriptor``.
|
|
|
|
|
|
|
|
FS_IOC_GET_ENCRYPTION_KEY_STATUS can fail with the following errors:
|
|
|
|
|
|
|
|
- ``EINVAL``: invalid key specifier type, or reserved bits were set
|
|
|
|
- ``ENOTTY``: this type of filesystem does not implement encryption
|
|
|
|
- ``EOPNOTSUPP``: the kernel was not configured with encryption
|
|
|
|
support for this filesystem, or the filesystem superblock has not
|
|
|
|
had encryption enabled on it
|
|
|
|
|
|
|
|
Among other use cases, FS_IOC_GET_ENCRYPTION_KEY_STATUS can be useful
|
|
|
|
for determining whether the key for a given encrypted directory needs
|
|
|
|
to be added before prompting the user for the passphrase needed to
|
|
|
|
derive the key.
|
|
|
|
|
|
|
|
FS_IOC_GET_ENCRYPTION_KEY_STATUS can only get the status of keys in
|
|
|
|
the filesystem-level keyring, i.e. the keyring managed by
|
|
|
|
`FS_IOC_ADD_ENCRYPTION_KEY`_ and `FS_IOC_REMOVE_ENCRYPTION_KEY`_. It
|
|
|
|
cannot get the status of a key that has only been added for use by v1
|
|
|
|
encryption policies using the legacy mechanism involving
|
|
|
|
process-subscribed keyrings.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Access semantics
|
|
|
|
================
|
|
|
|
|
|
|
|
With the key
|
|
|
|
------------
|
|
|
|
|
|
|
|
With the encryption key, encrypted regular files, directories, and
|
|
|
|
symlinks behave very similarly to their unencrypted counterparts ---
|
|
|
|
after all, the encryption is intended to be transparent. However,
|
|
|
|
astute users may notice some differences in behavior:
|
|
|
|
|
|
|
|
- Unencrypted files, or files encrypted with a different encryption
|
|
|
|
policy (i.e. different key, modes, or flags), cannot be renamed or
|
|
|
|
linked into an encrypted directory; see `Encryption policy
|
fscrypt: return -EXDEV for incompatible rename or link into encrypted dir
Currently, trying to rename or link a regular file, directory, or
symlink into an encrypted directory fails with EPERM when the source
file is unencrypted or is encrypted with a different encryption policy,
and is on the same mountpoint. It is correct for the operation to fail,
but the choice of EPERM breaks tools like 'mv' that know to copy rather
than rename if they see EXDEV, but don't know what to do with EPERM.
Our original motivation for EPERM was to encourage users to securely
handle their data. Encrypting files by "moving" them into an encrypted
directory can be insecure because the unencrypted data may remain in
free space on disk, where it can later be recovered by an attacker.
It's much better to encrypt the data from the start, or at least try to
securely delete the source data e.g. using the 'shred' program.
However, the current behavior hasn't been effective at achieving its
goal because users tend to be confused, hack around it, and complain;
see e.g. https://github.com/google/fscrypt/issues/76. And in some cases
it's actually inconsistent or unnecessary. For example, 'mv'-ing files
between differently encrypted directories doesn't work even in cases
where it can be secure, such as when in userspace the same passphrase
protects both directories. Yet, you *can* already 'mv' unencrypted
files into an encrypted directory if the source files are on a different
mountpoint, even though doing so is often insecure.
There are probably better ways to teach users to securely handle their
files. For example, the 'fscrypt' userspace tool could provide a
command that migrates unencrypted files into an encrypted directory,
acting like 'shred' on the source files and providing appropriate
warnings depending on the type of the source filesystem and disk.
Receiving errors on unimportant files might also force some users to
disable encryption, thus making the behavior counterproductive. It's
desirable to make encryption as unobtrusive as possible.
Therefore, change the error code from EPERM to EXDEV so that tools
looking for EXDEV will fall back to a copy.
This, of course, doesn't prevent users from still doing the right things
to securely manage their files. Note that this also matches the
behavior when a file is renamed between two project quota hierarchies;
so there's precedent for using EXDEV for things other than mountpoints.
xfstests generic/398 will require an update with this change.
[Rewritten from an earlier patch series by Michael Halcrow.]
Cc: Michael Halcrow <mhalcrow@google.com>
Cc: Joe Richey <joerichey@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-01-23 08:20:21 +08:00
|
|
|
enforcement`_. Attempts to do so will fail with EXDEV. However,
|
2017-10-29 18:30:14 +08:00
|
|
|
encrypted files can be renamed within an encrypted directory, or
|
|
|
|
into an unencrypted directory.
|
|
|
|
|
fscrypt: return -EXDEV for incompatible rename or link into encrypted dir
Currently, trying to rename or link a regular file, directory, or
symlink into an encrypted directory fails with EPERM when the source
file is unencrypted or is encrypted with a different encryption policy,
and is on the same mountpoint. It is correct for the operation to fail,
but the choice of EPERM breaks tools like 'mv' that know to copy rather
than rename if they see EXDEV, but don't know what to do with EPERM.
Our original motivation for EPERM was to encourage users to securely
handle their data. Encrypting files by "moving" them into an encrypted
directory can be insecure because the unencrypted data may remain in
free space on disk, where it can later be recovered by an attacker.
It's much better to encrypt the data from the start, or at least try to
securely delete the source data e.g. using the 'shred' program.
However, the current behavior hasn't been effective at achieving its
goal because users tend to be confused, hack around it, and complain;
see e.g. https://github.com/google/fscrypt/issues/76. And in some cases
it's actually inconsistent or unnecessary. For example, 'mv'-ing files
between differently encrypted directories doesn't work even in cases
where it can be secure, such as when in userspace the same passphrase
protects both directories. Yet, you *can* already 'mv' unencrypted
files into an encrypted directory if the source files are on a different
mountpoint, even though doing so is often insecure.
There are probably better ways to teach users to securely handle their
files. For example, the 'fscrypt' userspace tool could provide a
command that migrates unencrypted files into an encrypted directory,
acting like 'shred' on the source files and providing appropriate
warnings depending on the type of the source filesystem and disk.
Receiving errors on unimportant files might also force some users to
disable encryption, thus making the behavior counterproductive. It's
desirable to make encryption as unobtrusive as possible.
Therefore, change the error code from EPERM to EXDEV so that tools
looking for EXDEV will fall back to a copy.
This, of course, doesn't prevent users from still doing the right things
to securely manage their files. Note that this also matches the
behavior when a file is renamed between two project quota hierarchies;
so there's precedent for using EXDEV for things other than mountpoints.
xfstests generic/398 will require an update with this change.
[Rewritten from an earlier patch series by Michael Halcrow.]
Cc: Michael Halcrow <mhalcrow@google.com>
Cc: Joe Richey <joerichey@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-01-23 08:20:21 +08:00
|
|
|
Note: "moving" an unencrypted file into an encrypted directory, e.g.
|
|
|
|
with the `mv` program, is implemented in userspace by a copy
|
|
|
|
followed by a delete. Be aware that the original unencrypted data
|
|
|
|
may remain recoverable from free space on the disk; prefer to keep
|
|
|
|
all files encrypted from the very beginning. The `shred` program
|
|
|
|
may be used to overwrite the source files but isn't guaranteed to be
|
|
|
|
effective on all filesystems and storage devices.
|
|
|
|
|
2022-01-29 07:39:40 +08:00
|
|
|
- Direct I/O is supported on encrypted files only under some
|
|
|
|
circumstances. For details, see `Direct I/O support`_.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
ext4: allow ZERO_RANGE on encrypted files
When ext4 encryption support was first added, ZERO_RANGE was disallowed,
supposedly because test failures (e.g. ext4/001) were seen when enabling
it, and at the time there wasn't enough time/interest to debug it.
However, there's actually no reason why ZERO_RANGE can't work on
encrypted files. And it fact it *does* work now. Whole blocks in the
zeroed range are converted to unwritten extents, as usual; encryption
makes no difference for that part. Partial blocks are zeroed in the
pagecache and then ->writepages() encrypts those blocks as usual.
ext4_block_zero_page_range() handles reading and decrypting the block if
needed before actually doing the pagecache write.
Also, f2fs has always supported ZERO_RANGE on encrypted files.
As far as I can tell, the reason that ext4/001 was failing in v4.1 was
actually because of one of the bugs fixed by commit 36086d43f657 ("ext4
crypto: fix bugs in ext4_encrypted_zeroout()"). The bug made
ext4_encrypted_zeroout() always return a positive value, which caused
unwritten extents in encrypted files to sometimes not be marked as
initialized after being written to. This bug was not actually in
ZERO_RANGE; it just happened to trigger during the extents manipulation
done in ext4/001 (and probably other tests too).
So, let's enable ZERO_RANGE on encrypted files on ext4.
Tested with:
gce-xfstests -c ext4/encrypt -g auto
gce-xfstests -c ext4/encrypt_1k -g auto
Got the same set of test failures both with and without this patch.
But with this patch 6 fewer tests are skipped: ext4/001, generic/008,
generic/009, generic/033, generic/096, and generic/511.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191226154216.4808-1-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
2019-12-26 23:42:16 +08:00
|
|
|
- The fallocate operations FALLOC_FL_COLLAPSE_RANGE and
|
|
|
|
FALLOC_FL_INSERT_RANGE are not supported on encrypted files and will
|
|
|
|
fail with EOPNOTSUPP.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
- Online defragmentation of encrypted files is not supported. The
|
|
|
|
EXT4_IOC_MOVE_EXT and F2FS_IOC_MOVE_RANGE ioctls will fail with
|
|
|
|
EOPNOTSUPP.
|
|
|
|
|
|
|
|
- The ext4 filesystem does not support data journaling with encrypted
|
|
|
|
regular files. It will fall back to ordered data mode instead.
|
|
|
|
|
|
|
|
- DAX (Direct Access) is not supported on encrypted files.
|
|
|
|
|
2018-01-12 12:30:09 +08:00
|
|
|
- The maximum length of an encrypted symlink is 2 bytes shorter than
|
|
|
|
the maximum length of an unencrypted symlink. For example, on an
|
|
|
|
EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
|
|
|
|
to 4095 bytes long, while encrypted symlinks can only be up to 4093
|
|
|
|
bytes long (both lengths excluding the terminating null).
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Note that mmap *is* supported. This is possible because the pagecache
|
|
|
|
for an encrypted file contains the plaintext, not the ciphertext.
|
|
|
|
|
|
|
|
Without the key
|
|
|
|
---------------
|
|
|
|
|
|
|
|
Some filesystem operations may be performed on encrypted regular
|
|
|
|
files, directories, and symlinks even before their encryption key has
|
2019-08-05 10:35:49 +08:00
|
|
|
been added, or after their encryption key has been removed:
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
- File metadata may be read, e.g. using stat().
|
|
|
|
|
|
|
|
- Directories may be listed, in which case the filenames will be
|
|
|
|
listed in an encoded form derived from their ciphertext. The
|
|
|
|
current encoding algorithm is described in `Filename hashing and
|
|
|
|
encoding`_. The algorithm is subject to change, but it is
|
|
|
|
guaranteed that the presented filenames will be no longer than
|
|
|
|
NAME_MAX bytes, will not contain the ``/`` or ``\0`` characters, and
|
|
|
|
will uniquely identify directory entries.
|
|
|
|
|
|
|
|
The ``.`` and ``..`` directory entries are special. They are always
|
|
|
|
present and are not encrypted or encoded.
|
|
|
|
|
|
|
|
- Files may be deleted. That is, nondirectory files may be deleted
|
|
|
|
with unlink() as usual, and empty directories may be deleted with
|
|
|
|
rmdir() as usual. Therefore, ``rm`` and ``rm -r`` will work as
|
|
|
|
expected.
|
|
|
|
|
|
|
|
- Symlink targets may be read and followed, but they will be presented
|
|
|
|
in encrypted form, similar to filenames in directories. Hence, they
|
|
|
|
are unlikely to point to anywhere useful.
|
|
|
|
|
|
|
|
Without the key, regular files cannot be opened or truncated.
|
|
|
|
Attempts to do so will fail with ENOKEY. This implies that any
|
|
|
|
regular file operations that require a file descriptor, such as
|
|
|
|
read(), write(), mmap(), fallocate(), and ioctl(), are also forbidden.
|
|
|
|
|
|
|
|
Also without the key, files of any type (including directories) cannot
|
|
|
|
be created or linked into an encrypted directory, nor can a name in an
|
|
|
|
encrypted directory be the source or target of a rename, nor can an
|
|
|
|
O_TMPFILE temporary file be created in an encrypted directory. All
|
|
|
|
such operations will fail with ENOKEY.
|
|
|
|
|
|
|
|
It is not currently possible to backup and restore encrypted files
|
|
|
|
without the encryption key. This would require special APIs which
|
|
|
|
have not yet been implemented.
|
|
|
|
|
|
|
|
Encryption policy enforcement
|
|
|
|
=============================
|
|
|
|
|
|
|
|
After an encryption policy has been set on a directory, all regular
|
|
|
|
files, directories, and symbolic links created in that directory
|
|
|
|
(recursively) will inherit that encryption policy. Special files ---
|
|
|
|
that is, named pipes, device nodes, and UNIX domain sockets --- will
|
|
|
|
not be encrypted.
|
|
|
|
|
|
|
|
Except for those special files, it is forbidden to have unencrypted
|
|
|
|
files, or files encrypted with a different encryption policy, in an
|
|
|
|
encrypted directory tree. Attempts to link or rename such a file into
|
fscrypt: return -EXDEV for incompatible rename or link into encrypted dir
Currently, trying to rename or link a regular file, directory, or
symlink into an encrypted directory fails with EPERM when the source
file is unencrypted or is encrypted with a different encryption policy,
and is on the same mountpoint. It is correct for the operation to fail,
but the choice of EPERM breaks tools like 'mv' that know to copy rather
than rename if they see EXDEV, but don't know what to do with EPERM.
Our original motivation for EPERM was to encourage users to securely
handle their data. Encrypting files by "moving" them into an encrypted
directory can be insecure because the unencrypted data may remain in
free space on disk, where it can later be recovered by an attacker.
It's much better to encrypt the data from the start, or at least try to
securely delete the source data e.g. using the 'shred' program.
However, the current behavior hasn't been effective at achieving its
goal because users tend to be confused, hack around it, and complain;
see e.g. https://github.com/google/fscrypt/issues/76. And in some cases
it's actually inconsistent or unnecessary. For example, 'mv'-ing files
between differently encrypted directories doesn't work even in cases
where it can be secure, such as when in userspace the same passphrase
protects both directories. Yet, you *can* already 'mv' unencrypted
files into an encrypted directory if the source files are on a different
mountpoint, even though doing so is often insecure.
There are probably better ways to teach users to securely handle their
files. For example, the 'fscrypt' userspace tool could provide a
command that migrates unencrypted files into an encrypted directory,
acting like 'shred' on the source files and providing appropriate
warnings depending on the type of the source filesystem and disk.
Receiving errors on unimportant files might also force some users to
disable encryption, thus making the behavior counterproductive. It's
desirable to make encryption as unobtrusive as possible.
Therefore, change the error code from EPERM to EXDEV so that tools
looking for EXDEV will fall back to a copy.
This, of course, doesn't prevent users from still doing the right things
to securely manage their files. Note that this also matches the
behavior when a file is renamed between two project quota hierarchies;
so there's precedent for using EXDEV for things other than mountpoints.
xfstests generic/398 will require an update with this change.
[Rewritten from an earlier patch series by Michael Halcrow.]
Cc: Michael Halcrow <mhalcrow@google.com>
Cc: Joe Richey <joerichey@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-01-23 08:20:21 +08:00
|
|
|
an encrypted directory will fail with EXDEV. This is also enforced
|
2017-10-29 18:30:14 +08:00
|
|
|
during ->lookup() to provide limited protection against offline
|
|
|
|
attacks that try to disable or downgrade encryption in known locations
|
|
|
|
where applications may later write sensitive data. It is recommended
|
|
|
|
that systems implementing a form of "verified boot" take advantage of
|
|
|
|
this by validating all top-level encryption policies prior to access.
|
|
|
|
|
2021-09-17 01:49:26 +08:00
|
|
|
Inline encryption support
|
|
|
|
=========================
|
|
|
|
|
|
|
|
By default, fscrypt uses the kernel crypto API for all cryptographic
|
|
|
|
operations (other than HKDF, which fscrypt partially implements
|
|
|
|
itself). The kernel crypto API supports hardware crypto accelerators,
|
|
|
|
but only ones that work in the traditional way where all inputs and
|
|
|
|
outputs (e.g. plaintexts and ciphertexts) are in memory. fscrypt can
|
|
|
|
take advantage of such hardware, but the traditional acceleration
|
|
|
|
model isn't particularly efficient and fscrypt hasn't been optimized
|
|
|
|
for it.
|
|
|
|
|
|
|
|
Instead, many newer systems (especially mobile SoCs) have *inline
|
|
|
|
encryption hardware* that can encrypt/decrypt data while it is on its
|
|
|
|
way to/from the storage device. Linux supports inline encryption
|
|
|
|
through a set of extensions to the block layer called *blk-crypto*.
|
|
|
|
blk-crypto allows filesystems to attach encryption contexts to bios
|
|
|
|
(I/O requests) to specify how the data will be encrypted or decrypted
|
|
|
|
in-line. For more information about blk-crypto, see
|
|
|
|
:ref:`Documentation/block/inline-encryption.rst <inline_encryption>`.
|
|
|
|
|
|
|
|
On supported filesystems (currently ext4 and f2fs), fscrypt can use
|
|
|
|
blk-crypto instead of the kernel crypto API to encrypt/decrypt file
|
|
|
|
contents. To enable this, set CONFIG_FS_ENCRYPTION_INLINE_CRYPT=y in
|
|
|
|
the kernel configuration, and specify the "inlinecrypt" mount option
|
|
|
|
when mounting the filesystem.
|
|
|
|
|
|
|
|
Note that the "inlinecrypt" mount option just specifies to use inline
|
|
|
|
encryption when possible; it doesn't force its use. fscrypt will
|
|
|
|
still fall back to using the kernel crypto API on files where the
|
|
|
|
inline encryption hardware doesn't have the needed crypto capabilities
|
|
|
|
(e.g. support for the needed encryption algorithm and data unit size)
|
|
|
|
and where blk-crypto-fallback is unusable. (For blk-crypto-fallback
|
|
|
|
to be usable, it must be enabled in the kernel configuration with
|
|
|
|
CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK=y.)
|
|
|
|
|
|
|
|
Currently fscrypt always uses the filesystem block size (which is
|
|
|
|
usually 4096 bytes) as the data unit size. Therefore, it can only use
|
|
|
|
inline encryption hardware that supports that data unit size.
|
|
|
|
|
|
|
|
Inline encryption doesn't affect the ciphertext or other aspects of
|
|
|
|
the on-disk format, so users may freely switch back and forth between
|
|
|
|
using "inlinecrypt" and not using "inlinecrypt".
|
|
|
|
|
2022-01-29 07:39:40 +08:00
|
|
|
Direct I/O support
|
|
|
|
==================
|
|
|
|
|
|
|
|
For direct I/O on an encrypted file to work, the following conditions
|
|
|
|
must be met (in addition to the conditions for direct I/O on an
|
|
|
|
unencrypted file):
|
|
|
|
|
|
|
|
* The file must be using inline encryption. Usually this means that
|
|
|
|
the filesystem must be mounted with ``-o inlinecrypt`` and inline
|
|
|
|
encryption hardware must be present. However, a software fallback
|
|
|
|
is also available. For details, see `Inline encryption support`_.
|
|
|
|
|
|
|
|
* The I/O request must be fully aligned to the filesystem block size.
|
|
|
|
This means that the file position the I/O is targeting, the lengths
|
|
|
|
of all I/O segments, and the memory addresses of all I/O buffers
|
|
|
|
must be multiples of this value. Note that the filesystem block
|
|
|
|
size may be greater than the logical block size of the block device.
|
|
|
|
|
|
|
|
If either of the above conditions is not met, then direct I/O on the
|
|
|
|
encrypted file will fall back to buffered I/O.
|
|
|
|
|
2017-10-29 18:30:14 +08:00
|
|
|
Implementation details
|
|
|
|
======================
|
|
|
|
|
|
|
|
Encryption context
|
|
|
|
------------------
|
|
|
|
|
2020-10-14 14:40:47 +08:00
|
|
|
An encryption policy is represented on-disk by
|
|
|
|
struct fscrypt_context_v1 or struct fscrypt_context_v2. It is up to
|
|
|
|
individual filesystems to decide where to store it, but normally it
|
|
|
|
would be stored in a hidden extended attribute. It should *not* be
|
2019-08-05 10:35:49 +08:00
|
|
|
exposed by the xattr-related system calls such as getxattr() and
|
|
|
|
setxattr() because of the special semantics of the encryption xattr.
|
|
|
|
(In particular, there would be much confusion if an encryption policy
|
|
|
|
were to be added to or removed from anything other than an empty
|
|
|
|
directory.) These structs are defined as follows::
|
2017-10-29 18:30:14 +08:00
|
|
|
|
2020-07-09 05:57:22 +08:00
|
|
|
#define FSCRYPT_FILE_NONCE_SIZE 16
|
2017-10-29 18:30:14 +08:00
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
#define FSCRYPT_KEY_DESCRIPTOR_SIZE 8
|
|
|
|
struct fscrypt_context_v1 {
|
|
|
|
u8 version;
|
2017-10-29 18:30:14 +08:00
|
|
|
u8 contents_encryption_mode;
|
|
|
|
u8 filenames_encryption_mode;
|
|
|
|
u8 flags;
|
fscrypt: use FSCRYPT_ prefix for uapi constants
Prefix all filesystem encryption UAPI constants except the ioctl numbers
with "FSCRYPT_" rather than with "FS_". This namespaces the constants
more appropriately and makes it clear that they are related specifically
to the filesystem encryption feature, and to the 'fscrypt_*' structures.
With some of the old names like "FS_POLICY_FLAGS_VALID", it was not
immediately clear that the constant had anything to do with encryption.
This is also useful because we'll be adding more encryption-related
constants, e.g. for the policy version, and we'd otherwise have to
choose whether to use unclear names like FS_POLICY_V1 or inconsistent
names like FS_ENCRYPTION_POLICY_V1.
For source compatibility with existing userspace programs, keep the old
names defined as aliases to the new names.
Finally, as long as new names are being defined anyway, I skipped
defining new names for the fscrypt mode numbers that aren't actually
used: INVALID (0), AES_256_GCM (2), AES_256_CBC (3), SPECK128_256_XTS
(7), and SPECK128_256_CTS (8).
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2019-08-05 10:35:44 +08:00
|
|
|
u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
|
2020-07-09 05:57:22 +08:00
|
|
|
u8 nonce[FSCRYPT_FILE_NONCE_SIZE];
|
2017-10-29 18:30:14 +08:00
|
|
|
};
|
|
|
|
|
2019-08-05 10:35:49 +08:00
|
|
|
#define FSCRYPT_KEY_IDENTIFIER_SIZE 16
|
|
|
|
struct fscrypt_context_v2 {
|
|
|
|
u8 version;
|
|
|
|
u8 contents_encryption_mode;
|
|
|
|
u8 filenames_encryption_mode;
|
|
|
|
u8 flags;
|
|
|
|
u8 __reserved[4];
|
|
|
|
u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
|
2020-07-09 05:57:22 +08:00
|
|
|
u8 nonce[FSCRYPT_FILE_NONCE_SIZE];
|
2019-08-05 10:35:49 +08:00
|
|
|
};
|
|
|
|
|
|
|
|
The context structs contain the same information as the corresponding
|
|
|
|
policy structs (see `Setting an encryption policy`_), except that the
|
|
|
|
context structs also contain a nonce. The nonce is randomly generated
|
|
|
|
by the kernel and is used as KDF input or as a tweak to cause
|
2020-01-21 06:31:58 +08:00
|
|
|
different files to be encrypted differently; see `Per-file encryption
|
|
|
|
keys`_ and `DIRECT_KEY policies`_.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Data path changes
|
|
|
|
-----------------
|
|
|
|
|
2021-09-17 01:49:26 +08:00
|
|
|
When inline encryption is used, filesystems just need to associate
|
|
|
|
encryption contexts with bios to specify how the block layer or the
|
|
|
|
inline encryption hardware will encrypt/decrypt the file contents.
|
|
|
|
|
|
|
|
When inline encryption isn't used, filesystems must encrypt/decrypt
|
|
|
|
the file contents themselves, as described below:
|
|
|
|
|
2022-04-29 20:45:43 +08:00
|
|
|
For the read path (->read_folio()) of regular files, filesystems can
|
2017-10-29 18:30:14 +08:00
|
|
|
read the ciphertext into the page cache and decrypt it in-place. The
|
2023-01-28 06:25:14 +08:00
|
|
|
folio lock must be held until decryption has finished, to prevent the
|
|
|
|
folio from becoming visible to userspace prematurely.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
For the write path (->writepage()) of regular files, filesystems
|
|
|
|
cannot encrypt data in-place in the page cache, since the cached
|
|
|
|
plaintext must be preserved. Instead, filesystems must encrypt into a
|
|
|
|
temporary buffer or "bounce page", then write out the temporary
|
|
|
|
buffer. Some filesystems, such as UBIFS, already use temporary
|
|
|
|
buffers regardless of encryption. Other filesystems, such as ext4 and
|
|
|
|
F2FS, have to allocate bounce pages specially for encryption.
|
|
|
|
|
|
|
|
Filename hashing and encoding
|
|
|
|
-----------------------------
|
|
|
|
|
|
|
|
Modern filesystems accelerate directory lookups by using indexed
|
|
|
|
directories. An indexed directory is organized as a tree keyed by
|
|
|
|
filename hashes. When a ->lookup() is requested, the filesystem
|
|
|
|
normally hashes the filename being looked up so that it can quickly
|
|
|
|
find the corresponding directory entry, if any.
|
|
|
|
|
|
|
|
With encryption, lookups must be supported and efficient both with and
|
|
|
|
without the encryption key. Clearly, it would not work to hash the
|
|
|
|
plaintext filenames, since the plaintext filenames are unavailable
|
|
|
|
without the key. (Hashing the plaintext filenames would also make it
|
|
|
|
impossible for the filesystem's fsck tool to optimize encrypted
|
|
|
|
directories.) Instead, filesystems hash the ciphertext filenames,
|
|
|
|
i.e. the bytes actually stored on-disk in the directory entries. When
|
|
|
|
asked to do a ->lookup() with the key, the filesystem just encrypts
|
|
|
|
the user-supplied name to get the ciphertext.
|
|
|
|
|
|
|
|
Lookups without the key are more complicated. The raw ciphertext may
|
|
|
|
contain the ``\0`` and ``/`` characters, which are illegal in
|
fscrypt: align Base64 encoding with RFC 4648 base64url
fscrypt uses a Base64 encoding to encode no-key filenames (the filenames
that are presented to userspace when a directory is listed without its
encryption key). There are many variants of Base64, but the most common
ones are specified by RFC 4648. fscrypt can't use the regular RFC 4648
"base64" variant because "base64" uses the '/' character, which isn't
allowed in filenames. However, RFC 4648 also specifies a "base64url"
variant for use in URLs and filenames. "base64url" is less common than
"base64", but it's still implemented in many programming libraries.
Unfortunately, what fscrypt actually uses is a custom Base64 variant
that differs from "base64url" in several ways:
- The binary data is divided into 6-bit chunks differently.
- Values 62 and 63 are encoded with '+' and ',' instead of '-' and '_'.
- '='-padding isn't used. This isn't a problem per se, as the padding
isn't technically necessary, and RFC 4648 doesn't strictly require it.
But it needs to be properly documented.
There have been two attempts to copy the fscrypt Base64 code into lib/
(https://lkml.kernel.org/r/20200821182813.52570-6-jlayton@kernel.org and
https://lkml.kernel.org/r/20210716110428.9727-5-hare@suse.de), and both
have been caught up by the fscrypt Base64 variant being nonstandard and
not properly documented. Also, the planned use of the fscrypt Base64
code in the CephFS storage back-end will prevent it from being changed
later (whereas currently it can still be changed), so we need to choose
an encoding that we're happy with before it's too late.
Therefore, switch the fscrypt Base64 variant to base64url, in order to
align more closely with RFC 4648 and other implementations and uses of
Base64. However, I opted not to implement '='-padding, as '='-padding
adds complexity, is unnecessary, and isn't required by the RFC.
Link: https://lore.kernel.org/r/20210718000125.59701-1-ebiggers@kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2021-07-18 08:01:25 +08:00
|
|
|
filenames. Therefore, readdir() must base64url-encode the ciphertext
|
|
|
|
for presentation. For most filenames, this works fine; on ->lookup(),
|
|
|
|
the filesystem just base64url-decodes the user-supplied name to get
|
|
|
|
back to the raw ciphertext.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
fscrypt: align Base64 encoding with RFC 4648 base64url
fscrypt uses a Base64 encoding to encode no-key filenames (the filenames
that are presented to userspace when a directory is listed without its
encryption key). There are many variants of Base64, but the most common
ones are specified by RFC 4648. fscrypt can't use the regular RFC 4648
"base64" variant because "base64" uses the '/' character, which isn't
allowed in filenames. However, RFC 4648 also specifies a "base64url"
variant for use in URLs and filenames. "base64url" is less common than
"base64", but it's still implemented in many programming libraries.
Unfortunately, what fscrypt actually uses is a custom Base64 variant
that differs from "base64url" in several ways:
- The binary data is divided into 6-bit chunks differently.
- Values 62 and 63 are encoded with '+' and ',' instead of '-' and '_'.
- '='-padding isn't used. This isn't a problem per se, as the padding
isn't technically necessary, and RFC 4648 doesn't strictly require it.
But it needs to be properly documented.
There have been two attempts to copy the fscrypt Base64 code into lib/
(https://lkml.kernel.org/r/20200821182813.52570-6-jlayton@kernel.org and
https://lkml.kernel.org/r/20210716110428.9727-5-hare@suse.de), and both
have been caught up by the fscrypt Base64 variant being nonstandard and
not properly documented. Also, the planned use of the fscrypt Base64
code in the CephFS storage back-end will prevent it from being changed
later (whereas currently it can still be changed), so we need to choose
an encoding that we're happy with before it's too late.
Therefore, switch the fscrypt Base64 variant to base64url, in order to
align more closely with RFC 4648 and other implementations and uses of
Base64. However, I opted not to implement '='-padding, as '='-padding
adds complexity, is unnecessary, and isn't required by the RFC.
Link: https://lore.kernel.org/r/20210718000125.59701-1-ebiggers@kernel.org
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Eric Biggers <ebiggers@google.com>
2021-07-18 08:01:25 +08:00
|
|
|
However, for very long filenames, base64url encoding would cause the
|
2017-10-29 18:30:14 +08:00
|
|
|
filename length to exceed NAME_MAX. To prevent this, readdir()
|
|
|
|
actually presents long filenames in an abbreviated form which encodes
|
|
|
|
a strong "hash" of the ciphertext filename, along with the optional
|
|
|
|
filesystem-specific hash(es) needed for directory lookups. This
|
|
|
|
allows the filesystem to still, with a high degree of confidence, map
|
|
|
|
the filename given in ->lookup() back to a particular directory entry
|
2020-10-14 14:40:47 +08:00
|
|
|
that was previously listed by readdir(). See
|
|
|
|
struct fscrypt_nokey_name in the source for more details.
|
2017-10-29 18:30:14 +08:00
|
|
|
|
|
|
|
Note that the precise way that filenames are presented to userspace
|
|
|
|
without the key is subject to change in the future. It is only meant
|
|
|
|
as a way to temporarily present valid filenames so that commands like
|
|
|
|
``rm -r`` work as expected on encrypted directories.
|
2019-06-21 02:16:58 +08:00
|
|
|
|
|
|
|
Tests
|
|
|
|
=====
|
|
|
|
|
|
|
|
To test fscrypt, use xfstests, which is Linux's de facto standard
|
|
|
|
filesystem test suite. First, run all the tests in the "encrypt"
|
2020-07-25 02:45:00 +08:00
|
|
|
group on the relevant filesystem(s). One can also run the tests
|
|
|
|
with the 'inlinecrypt' mount option to test the implementation for
|
|
|
|
inline encryption support. For example, to test ext4 and
|
2019-06-21 02:16:58 +08:00
|
|
|
f2fs encryption using `kvm-xfstests
|
|
|
|
<https://github.com/tytso/xfstests-bld/blob/master/Documentation/kvm-quickstart.md>`_::
|
|
|
|
|
|
|
|
kvm-xfstests -c ext4,f2fs -g encrypt
|
2020-07-02 09:56:05 +08:00
|
|
|
kvm-xfstests -c ext4,f2fs -g encrypt -m inlinecrypt
|
2019-06-21 02:16:58 +08:00
|
|
|
|
|
|
|
UBIFS encryption can also be tested this way, but it should be done in
|
|
|
|
a separate command, and it takes some time for kvm-xfstests to set up
|
|
|
|
emulated UBI volumes::
|
|
|
|
|
|
|
|
kvm-xfstests -c ubifs -g encrypt
|
|
|
|
|
|
|
|
No tests should fail. However, tests that use non-default encryption
|
|
|
|
modes (e.g. generic/549 and generic/550) will be skipped if the needed
|
|
|
|
algorithms were not built into the kernel's crypto API. Also, tests
|
|
|
|
that access the raw block device (e.g. generic/399, generic/548,
|
|
|
|
generic/549, generic/550) will be skipped on UBIFS.
|
|
|
|
|
|
|
|
Besides running the "encrypt" group tests, for ext4 and f2fs it's also
|
|
|
|
possible to run most xfstests with the "test_dummy_encryption" mount
|
|
|
|
option. This option causes all new files to be automatically
|
|
|
|
encrypted with a dummy key, without having to make any API calls.
|
|
|
|
This tests the encrypted I/O paths more thoroughly. To do this with
|
|
|
|
kvm-xfstests, use the "encrypt" filesystem configuration::
|
|
|
|
|
|
|
|
kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
|
2020-07-02 09:56:05 +08:00
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kvm-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt
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2019-06-21 02:16:58 +08:00
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Because this runs many more tests than "-g encrypt" does, it takes
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much longer to run; so also consider using `gce-xfstests
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<https://github.com/tytso/xfstests-bld/blob/master/Documentation/gce-xfstests.md>`_
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instead of kvm-xfstests::
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gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto
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2020-07-02 09:56:05 +08:00
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gce-xfstests -c ext4/encrypt,f2fs/encrypt -g auto -m inlinecrypt
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