forked from luck/tmp_suning_uos_patched
18068bdd5f
Veritysetup is now part of cryptsetup package. Remove on-disk header description (which is not parsed in kernel) and point users to cryptsetup where it the format is documented. Mention units for block size paramaters. Fix target line specification and dmsetup parameters. Signed-off-by: Milan Broz <mbroz@redhat.com> Cc: stable@kernel.org Signed-off-by: Alasdair G Kergon <agk@redhat.com>
156 lines
5.8 KiB
Plaintext
156 lines
5.8 KiB
Plaintext
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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<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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<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 zeros.
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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 zeros 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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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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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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[ 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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http://code.google.com/p/cryptsetup/wiki/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 http://code.google.com/p/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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