237 lines
8.9 KiB
ReStructuredText
237 lines
8.9 KiB
ReStructuredText
=========
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dm-verity
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=========
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Device-Mapper's "verity" target provides transparent integrity checking of
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block devices using a cryptographic digest provided by the kernel crypto API.
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This target is read-only.
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Construction Parameters
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=======================
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::
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<version> <dev> <hash_dev>
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<data_block_size> <hash_block_size>
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<num_data_blocks> <hash_start_block>
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<algorithm> <digest> <salt>
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[<#opt_params> <opt_params>]
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<version>
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This is the type of the on-disk hash format.
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0 is the original format used in the Chromium OS.
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The salt is appended when hashing, digests are stored continuously and
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the rest of the block is padded with zeroes.
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1 is the current format that should be used for new devices.
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The salt is prepended when hashing and each digest is
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padded with zeroes to the power of two.
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<dev>
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This is the device containing data, the integrity of which needs to be
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checked. It may be specified as a path, like /dev/sdaX, or a device number,
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<major>:<minor>.
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<hash_dev>
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This is the device that supplies the hash tree data. It may be
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specified similarly to the device path and may be the same device. If the
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same device is used, the hash_start should be outside the configured
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dm-verity device.
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<data_block_size>
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The block size on a data device in bytes.
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Each block corresponds to one digest on the hash device.
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<hash_block_size>
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The size of a hash block in bytes.
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<num_data_blocks>
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The number of data blocks on the data device. Additional blocks are
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inaccessible. You can place hashes to the same partition as data, in this
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case hashes are placed after <num_data_blocks>.
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<hash_start_block>
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This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
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to the root block of the hash tree.
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<algorithm>
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The cryptographic hash algorithm used for this device. This should
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be the name of the algorithm, like "sha1".
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<digest>
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The hexadecimal encoding of the cryptographic hash of the root hash block
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and the salt. This hash should be trusted as there is no other authenticity
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beyond this point.
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<salt>
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The hexadecimal encoding of the salt value.
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<#opt_params>
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Number of optional parameters. If there are no optional parameters,
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the optional paramaters section can be skipped or #opt_params can be zero.
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Otherwise #opt_params is the number of following arguments.
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Example of optional parameters section:
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1 ignore_corruption
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ignore_corruption
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Log corrupted blocks, but allow read operations to proceed normally.
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restart_on_corruption
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Restart the system when a corrupted block is discovered. This option is
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not compatible with ignore_corruption and requires user space support to
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avoid restart loops.
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ignore_zero_blocks
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Do not verify blocks that are expected to contain zeroes and always return
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zeroes instead. This may be useful if the partition contains unused blocks
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that are not guaranteed to contain zeroes.
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use_fec_from_device <fec_dev>
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Use forward error correction (FEC) to recover from corruption if hash
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verification fails. Use encoding data from the specified device. This
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may be the same device where data and hash blocks reside, in which case
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fec_start must be outside data and hash areas.
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If the encoding data covers additional metadata, it must be accessible
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on the hash device after the hash blocks.
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Note: block sizes for data and hash devices must match. Also, if the
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verity <dev> is encrypted the <fec_dev> should be too.
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fec_roots <num>
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Number of generator roots. This equals to the number of parity bytes in
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the encoding data. For example, in RS(M, N) encoding, the number of roots
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is M-N.
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fec_blocks <num>
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The number of encoding data blocks on the FEC device. The block size for
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the FEC device is <data_block_size>.
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fec_start <offset>
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This is the offset, in <data_block_size> blocks, from the start of the
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FEC device to the beginning of the encoding data.
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check_at_most_once
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Verify data blocks only the first time they are read from the data device,
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rather than every time. This reduces the overhead of dm-verity so that it
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can be used on systems that are memory and/or CPU constrained. However, it
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provides a reduced level of security because only offline tampering of the
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data device's content will be detected, not online tampering.
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Hash blocks are still verified each time they are read from the hash device,
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since verification of hash blocks is less performance critical than data
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blocks, and a hash block will not be verified any more after all the data
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blocks it covers have been verified anyway.
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root_hash_sig_key_desc <key_description>
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This is the description of the USER_KEY that the kernel will lookup to get
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the pkcs7 signature of the roothash. The pkcs7 signature is used to validate
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the root hash during the creation of the device mapper block device.
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Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG
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being set in the kernel.
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Theory of operation
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===================
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dm-verity is meant to be set up as part of a verified boot path. This
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may be anything ranging from a boot using tboot or trustedgrub to just
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booting from a known-good device (like a USB drive or CD).
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When a dm-verity device is configured, it is expected that the caller
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has been authenticated in some way (cryptographic signatures, etc).
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After instantiation, all hashes will be verified on-demand during
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disk access. If they cannot be verified up to the root node of the
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tree, the root hash, then the I/O will fail. This should detect
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tampering with any data on the device and the hash data.
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Cryptographic hashes are used to assert the integrity of the device on a
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per-block basis. This allows for a lightweight hash computation on first read
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into the page cache. Block hashes are stored linearly, aligned to the nearest
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block size.
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If forward error correction (FEC) support is enabled any recovery of
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corrupted data will be verified using the cryptographic hash of the
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corresponding data. This is why combining error correction with
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integrity checking is essential.
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Hash Tree
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---------
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Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
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of some data block on disk is calculated. If it is an intermediary node,
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the hash of a number of child nodes is calculated.
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Each entry in the tree is a collection of neighboring nodes that fit in one
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block. The number is determined based on block_size and the size of the
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selected cryptographic digest algorithm. The hashes are linearly-ordered in
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this entry and any unaligned trailing space is ignored but included when
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calculating the parent node.
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The tree looks something like:
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alg = sha256, num_blocks = 32768, block_size = 4096
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::
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[ root ]
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/ . . . \
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[entry_0] [entry_1]
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/ . . . \ . . . \
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[entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
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/ ... \ / . . . \ / \
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blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
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On-disk format
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==============
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The verity kernel code does not read the verity metadata on-disk header.
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It only reads the hash blocks which directly follow the header.
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It is expected that a user-space tool will verify the integrity of the
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verity header.
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Alternatively, the header can be omitted and the dmsetup parameters can
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be passed via the kernel command-line in a rooted chain of trust where
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the command-line is verified.
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Directly following the header (and with sector number padded to the next hash
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block boundary) are the hash blocks which are stored a depth at a time
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(starting from the root), sorted in order of increasing index.
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The full specification of kernel parameters and on-disk metadata format
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is available at the cryptsetup project's wiki page
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https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
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Status
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======
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V (for Valid) is returned if every check performed so far was valid.
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If any check failed, C (for Corruption) is returned.
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Example
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=======
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Set up a device::
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# dmsetup create vroot --readonly --table \
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"0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
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"4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
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"1234000000000000000000000000000000000000000000000000000000000000"
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A command line tool veritysetup is available to compute or verify
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the hash tree or activate the kernel device. This is available from
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the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
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(as a libcryptsetup extension).
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Create hash on the device::
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# veritysetup format /dev/sda1 /dev/sda2
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...
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Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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Activate the device::
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# veritysetup create vroot /dev/sda1 /dev/sda2 \
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4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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