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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> <hash_start>
+ <data_block_size> <hash_block_size>
+ <num_data_blocks> <hash_start_block>
+ <algorithm> <digest> <salt>
+
+<version>
+ This is the version number of the on-disk 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 the 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 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 of the dm-verity
+ configured device size.
+
+<data_block_size>
+ The block size on a data device. Each block corresponds to one digest on
+ the hash device.
+
+<hash_block_size>
+ The size of a hash block.
+
+<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 setup 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 identify
+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 the size of a page.
+
+Hash Tree
+---------
+
+Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
+is of some block data on disk. If it is an intermediary node, then the hash is
+of a number of child nodes.
+
+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
+==============
+
+Below is the recommended on-disk format. The verity kernel code does not
+read the 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 and then call dmsetup with the correct
+parameters. 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.
+
+The on-disk format is especially useful in cases where the hash blocks
+are on a separate partition. The magic number allows easy identification
+of the partition contents. Alternatively, the hash blocks can be stored
+in the same partition as the data to be verified. In such a configuration
+the filesystem on the partition would be sized a little smaller than
+the full-partition, leaving room for the hash blocks.
+
+struct superblock {
+ uint8_t signature[8]
+ "verity\0\0";
+
+ uint8_t version;
+ 1 - current format
+
+ uint8_t data_block_bits;
+ log2(data block size)
+
+ uint8_t hash_block_bits;
+ log2(hash block size)
+
+ uint8_t pad1[1];
+ zero padding
+
+ uint16_t salt_size;
+ big-endian salt size
+
+ uint8_t pad2[2];
+ zero padding
+
+ uint32_t data_blocks_hi;
+ big-endian high 32 bits of the 64-bit number of data blocks
+
+ uint32_t data_blocks_lo;
+ big-endian low 32 bits of the 64-bit number of data blocks
+
+ uint8_t algorithm[16];
+ cryptographic algorithm
+
+ uint8_t salt[384];
+ salt (the salt size is specified above)
+
+ uint8_t pad3[88];
+ zero padding to 512-byte boundary
+}
+
+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.
+
+Status
+======
+V (for Valid) is returned if every check performed so far was valid.
+If any check failed, C (for Corruption) is returned.
+
+Example
+=======
+
+Setup a device:
+ dmsetup create vroot --table \
+ "0 2097152 "\
+ "verity 1 /dev/sda1 /dev/sda2 4096 4096 2097152 1 "\
+ "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
+ "1234000000000000000000000000000000000000000000000000000000000000"
+
+A command line tool veritysetup is available to compute or verify
+the hash tree or activate the kernel driver. This is available from
+the LVM2 upstream repository and may be supplied as a package called
+device-mapper-verity-tools:
+ git://sources.redhat.com/git/lvm2
+ http://sourceware.org/git/?p=lvm2.git
+ http://sourceware.org/cgi-bin/cvsweb.cgi/LVM2/verity?cvsroot=lvm2
+
+veritysetup -a vroot /dev/sda1 /dev/sda2 \
+ 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076