156 lines
5.8 KiB
Plaintext
156 lines
5.8 KiB
Plaintext
dm-verity
|
|
==========
|
|
|
|
Device-Mapper's "verity" target provides transparent integrity checking of
|
|
block devices using a cryptographic digest provided by the kernel crypto API.
|
|
This target is read-only.
|
|
|
|
Construction Parameters
|
|
=======================
|
|
<version> <dev> <hash_dev>
|
|
<data_block_size> <hash_block_size>
|
|
<num_data_blocks> <hash_start_block>
|
|
<algorithm> <digest> <salt>
|
|
|
|
<version>
|
|
This is the type of the on-disk hash format.
|
|
|
|
0 is the original format used in the Chromium OS.
|
|
The salt is appended when hashing, digests are stored continuously and
|
|
the rest of the block is padded with zeros.
|
|
|
|
1 is the current format that should be used for new devices.
|
|
The salt is prepended when hashing and each digest is
|
|
padded with zeros to the power of two.
|
|
|
|
<dev>
|
|
This is the device containing data, the integrity of which needs to be
|
|
checked. It may be specified as a path, like /dev/sdaX, or a device number,
|
|
<major>:<minor>.
|
|
|
|
<hash_dev>
|
|
This is the device that supplies the hash tree data. It may be
|
|
specified similarly to the device path and may be the same device. If the
|
|
same device is used, the hash_start should be outside the configured
|
|
dm-verity device.
|
|
|
|
<data_block_size>
|
|
The block size on a data device in bytes.
|
|
Each block corresponds to one digest on the hash device.
|
|
|
|
<hash_block_size>
|
|
The size of a hash block in bytes.
|
|
|
|
<num_data_blocks>
|
|
The number of data blocks on the data device. Additional blocks are
|
|
inaccessible. You can place hashes to the same partition as data, in this
|
|
case hashes are placed after <num_data_blocks>.
|
|
|
|
<hash_start_block>
|
|
This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
|
|
to the root block of the hash tree.
|
|
|
|
<algorithm>
|
|
The cryptographic hash algorithm used for this device. This should
|
|
be the name of the algorithm, like "sha1".
|
|
|
|
<digest>
|
|
The hexadecimal encoding of the cryptographic hash of the root hash block
|
|
and the salt. This hash should be trusted as there is no other authenticity
|
|
beyond this point.
|
|
|
|
<salt>
|
|
The hexadecimal encoding of the salt value.
|
|
|
|
Theory of operation
|
|
===================
|
|
|
|
dm-verity is meant to be set up as part of a verified boot path. This
|
|
may be anything ranging from a boot using tboot or trustedgrub to just
|
|
booting from a known-good device (like a USB drive or CD).
|
|
|
|
When a dm-verity device is configured, it is expected that the caller
|
|
has been authenticated in some way (cryptographic signatures, etc).
|
|
After instantiation, all hashes will be verified on-demand during
|
|
disk access. If they cannot be verified up to the root node of the
|
|
tree, the root hash, then the I/O will fail. This should detect
|
|
tampering with any data on the device and the hash data.
|
|
|
|
Cryptographic hashes are used to assert the integrity of the device on a
|
|
per-block basis. This allows for a lightweight hash computation on first read
|
|
into the page cache. Block hashes are stored linearly, aligned to the nearest
|
|
block size.
|
|
|
|
Hash Tree
|
|
---------
|
|
|
|
Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
|
|
of some data block on disk is calculated. If it is an intermediary node,
|
|
the hash of a number of child nodes is calculated.
|
|
|
|
Each entry in the tree is a collection of neighboring nodes that fit in one
|
|
block. The number is determined based on block_size and the size of the
|
|
selected cryptographic digest algorithm. The hashes are linearly-ordered in
|
|
this entry and any unaligned trailing space is ignored but included when
|
|
calculating the parent node.
|
|
|
|
The tree looks something like:
|
|
|
|
alg = sha256, num_blocks = 32768, block_size = 4096
|
|
|
|
[ root ]
|
|
/ . . . \
|
|
[entry_0] [entry_1]
|
|
/ . . . \ . . . \
|
|
[entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
|
|
/ ... \ / . . . \ / \
|
|
blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
|
|
|
|
|
|
On-disk format
|
|
==============
|
|
|
|
The verity kernel code does not read the verity metadata on-disk header.
|
|
It only reads the hash blocks which directly follow the header.
|
|
It is expected that a user-space tool will verify the integrity of the
|
|
verity header.
|
|
|
|
Alternatively, the header can be omitted and the dmsetup parameters can
|
|
be passed via the kernel command-line in a rooted chain of trust where
|
|
the command-line is verified.
|
|
|
|
Directly following the header (and with sector number padded to the next hash
|
|
block boundary) are the hash blocks which are stored a depth at a time
|
|
(starting from the root), sorted in order of increasing index.
|
|
|
|
The full specification of kernel parameters and on-disk metadata format
|
|
is available at the cryptsetup project's wiki page
|
|
http://code.google.com/p/cryptsetup/wiki/DMVerity
|
|
|
|
Status
|
|
======
|
|
V (for Valid) is returned if every check performed so far was valid.
|
|
If any check failed, C (for Corruption) is returned.
|
|
|
|
Example
|
|
=======
|
|
Set up a device:
|
|
# dmsetup create vroot --readonly --table \
|
|
"0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
|
|
"4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
|
|
"1234000000000000000000000000000000000000000000000000000000000000"
|
|
|
|
A command line tool veritysetup is available to compute or verify
|
|
the hash tree or activate the kernel device. This is available from
|
|
the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
|
|
(as a libcryptsetup extension).
|
|
|
|
Create hash on the device:
|
|
# veritysetup format /dev/sda1 /dev/sda2
|
|
...
|
|
Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
|
|
|
|
Activate the device:
|
|
# veritysetup create vroot /dev/sda1 /dev/sda2 \
|
|
4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
|