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+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