linux/Documentation/device-mapper/verity.txt
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   1dm-verity
   2==========
   3
   4Device-Mapper's "verity" target provides transparent integrity checking of
   5block devices using a cryptographic digest provided by the kernel crypto API.
   6This target is read-only.
   7
   8Construction Parameters
   9=======================
  10    <version> <dev> <hash_dev>
  11    <data_block_size> <hash_block_size>
  12    <num_data_blocks> <hash_start_block>
  13    <algorithm> <digest> <salt>
  14
  15<version>
  16    This is the type of the on-disk hash format.
  17
  18    0 is the original format used in the Chromium OS.
  19      The salt is appended when hashing, digests are stored continuously and
  20      the rest of the block is padded with zeros.
  21
  22    1 is the current format that should be used for new devices.
  23      The salt is prepended when hashing and each digest is
  24      padded with zeros to the power of two.
  25
  26<dev>
  27    This is the device containing data, the integrity of which needs to be
  28    checked.  It may be specified as a path, like /dev/sdaX, or a device number,
  29    <major>:<minor>.
  30
  31<hash_dev>
  32    This is the device that supplies the hash tree data.  It may be
  33    specified similarly to the device path and may be the same device.  If the
  34    same device is used, the hash_start should be outside the configured
  35    dm-verity device.
  36
  37<data_block_size>
  38    The block size on a data device in bytes.
  39    Each block corresponds to one digest on the hash device.
  40
  41<hash_block_size>
  42    The size of a hash block in bytes.
  43
  44<num_data_blocks>
  45    The number of data blocks on the data device.  Additional blocks are
  46    inaccessible.  You can place hashes to the same partition as data, in this
  47    case hashes are placed after <num_data_blocks>.
  48
  49<hash_start_block>
  50    This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
  51    to the root block of the hash tree.
  52
  53<algorithm>
  54    The cryptographic hash algorithm used for this device.  This should
  55    be the name of the algorithm, like "sha1".
  56
  57<digest>
  58    The hexadecimal encoding of the cryptographic hash of the root hash block
  59    and the salt.  This hash should be trusted as there is no other authenticity
  60    beyond this point.
  61
  62<salt>
  63    The hexadecimal encoding of the salt value.
  64
  65Theory of operation
  66===================
  67
  68dm-verity is meant to be set up as part of a verified boot path.  This
  69may be anything ranging from a boot using tboot or trustedgrub to just
  70booting from a known-good device (like a USB drive or CD).
  71
  72When a dm-verity device is configured, it is expected that the caller
  73has been authenticated in some way (cryptographic signatures, etc).
  74After instantiation, all hashes will be verified on-demand during
  75disk access.  If they cannot be verified up to the root node of the
  76tree, the root hash, then the I/O will fail.  This should detect
  77tampering with any data on the device and the hash data.
  78
  79Cryptographic hashes are used to assert the integrity of the device on a
  80per-block basis. This allows for a lightweight hash computation on first read
  81into the page cache. Block hashes are stored linearly, aligned to the nearest
  82block size.
  83
  84Hash Tree
  85---------
  86
  87Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
  88of some data block on disk is calculated. If it is an intermediary node,
  89the hash of a number of child nodes is calculated.
  90
  91Each entry in the tree is a collection of neighboring nodes that fit in one
  92block.  The number is determined based on block_size and the size of the
  93selected cryptographic digest algorithm.  The hashes are linearly-ordered in
  94this entry and any unaligned trailing space is ignored but included when
  95calculating the parent node.
  96
  97The tree looks something like:
  98
  99alg = sha256, num_blocks = 32768, block_size = 4096
 100
 101                                 [   root    ]
 102                                /    . . .    \
 103                     [entry_0]                 [entry_1]
 104                    /  . . .  \                 . . .   \
 105         [entry_0_0]   . . .  [entry_0_127]    . . . .  [entry_1_127]
 106           / ... \             /   . . .  \             /           \
 107     blk_0 ... blk_127  blk_16256   blk_16383      blk_32640 . . . blk_32767
 108
 109
 110On-disk format
 111==============
 112
 113The verity kernel code does not read the verity metadata on-disk header.
 114It only reads the hash blocks which directly follow the header.
 115It is expected that a user-space tool will verify the integrity of the
 116verity header.
 117
 118Alternatively, the header can be omitted and the dmsetup parameters can
 119be passed via the kernel command-line in a rooted chain of trust where
 120the command-line is verified.
 121
 122Directly following the header (and with sector number padded to the next hash
 123block boundary) are the hash blocks which are stored a depth at a time
 124(starting from the root), sorted in order of increasing index.
 125
 126The full specification of kernel parameters and on-disk metadata format
 127is available at the cryptsetup project's wiki page
 128  http://code.google.com/p/cryptsetup/wiki/DMVerity
 129
 130Status
 131======
 132V (for Valid) is returned if every check performed so far was valid.
 133If any check failed, C (for Corruption) is returned.
 134
 135Example
 136=======
 137Set up a device:
 138  # dmsetup create vroot --readonly --table \
 139    "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
 140    "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
 141    "1234000000000000000000000000000000000000000000000000000000000000"
 142
 143A command line tool veritysetup is available to compute or verify
 144the hash tree or activate the kernel device. This is available from
 145the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
 146(as a libcryptsetup extension).
 147
 148Create hash on the device:
 149  # veritysetup format /dev/sda1 /dev/sda2
 150  ...
 151  Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
 152
 153Activate the device:
 154  # veritysetup create vroot /dev/sda1 /dev/sda2 \
 155    4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
 156
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