Documentation/device-mapper/verity.txt

   1 dm-verity
   2 ==========
   3
   4 Device-Mapper's "verity" target provides transparent integrity checking of
   5 block devices using a cryptographic digest provided by the kernel crypto API.
   6 This target is read-only.
   7
   8 Construction 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     [<#opt_params> <opt_params>]
  15
  16 <version>
  17     This is the type of the on-disk hash format.
  18
  19     0 is the original format used in the Chromium OS.
  20       The salt is appended when hashing, digests are stored continuously and
  21       the rest of the block is padded with zeroes.
  22
  23     1 is the current format that should be used for new devices.
  24       The salt is prepended when hashing and each digest is
  25       padded with zeroes to the power of two.
  26
  27 <dev>
  28     This is the device containing data, the integrity of which needs to be
  29     checked.  It may be specified as a path, like /dev/sdaX, or a device number,
  30     <major>:<minor>.
  31
  32 <hash_dev>
  33     This is the device that supplies the hash tree data.  It may be
  34     specified similarly to the device path and may be the same device.  If the
  35     same device is used, the hash_start should be outside the configured
  36     dm-verity device.
  37
  38 <data_block_size>
  39     The block size on a data device in bytes.
  40     Each block corresponds to one digest on the hash device.
  41
  42 <hash_block_size>
  43     The size of a hash block in bytes.
  44
  45 <num_data_blocks>
  46     The number of data blocks on the data device.  Additional blocks are
  47     inaccessible.  You can place hashes to the same partition as data, in this
  48     case hashes are placed after <num_data_blocks>.
  49
  50 <hash_start_block>
  51     This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
  52     to the root block of the hash tree.
  53
  54 <algorithm>
  55     The cryptographic hash algorithm used for this device.  This should
  56     be the name of the algorithm, like "sha1".
  57
  58 <digest>
  59     The hexadecimal encoding of the cryptographic hash of the root hash block
  60     and the salt.  This hash should be trusted as there is no other authenticity
  61     beyond this point.
  62
  63 <salt>
  64     The hexadecimal encoding of the salt value.
  65
  66 <#opt_params>
  67     Number of optional parameters. If there are no optional parameters,
  68     the optional paramaters section can be skipped or #opt_params can be zero.
  69     Otherwise #opt_params is the number of following arguments.
  70
  71     Example of optional parameters section:
  72         1 ignore_corruption
  73
  74 ignore_corruption
  75     Log corrupted blocks, but allow read operations to proceed normally.
  76
  77 restart_on_corruption
  78     Restart the system when a corrupted block is discovered. This option is
  79     not compatible with ignore_corruption and requires user space support to
  80     avoid restart loops.
  81
  82 ignore_zero_blocks
  83     Do not verify blocks that are expected to contain zeroes and always return
  84     zeroes instead. This may be useful if the partition contains unused blocks
  85     that are not guaranteed to contain zeroes.
  86
  87 use_fec_from_device <fec_dev>
  88     Use forward error correction (FEC) to recover from corruption if hash
  89     verification fails. Use encoding data from the specified device. This
  90     may be the same device where data and hash blocks reside, in which case
  91     fec_start must be outside data and hash areas.
  92
  93     If the encoding data covers additional metadata, it must be accessible
  94     on the hash device after the hash blocks.
  95
  96     Note: block sizes for data and hash devices must match. Also, if the
  97     verity <dev> is encrypted the <fec_dev> should be too.
  98
  99 fec_roots <num>
 100     Number of generator roots. This equals to the number of parity bytes in
 101     the encoding data. For example, in RS(M, N) encoding, the number of roots
 102     is M-N.
 103
 104 fec_blocks <num>
 105     The number of encoding data blocks on the FEC device. The block size for
 106     the FEC device is <data_block_size>.
 107
 108 fec_start <offset>
 109     This is the offset, in <data_block_size> blocks, from the start of the
 110     FEC device to the beginning of the encoding data.
 111
 112 check_at_most_once
 113     Verify data blocks only the first time they are read from the data device,
 114     rather than every time.  This reduces the overhead of dm-verity so that it
 115     can be used on systems that are memory and/or CPU constrained.  However, it
 116     provides a reduced level of security because only offline tampering of the
 117     data device's content will be detected, not online tampering.
 118
 119     Hash blocks are still verified each time they are read from the hash device,
 120     since verification of hash blocks is less performance critical than data
 121     blocks, and a hash block will not be verified any more after all the data
 122     blocks it covers have been verified anyway.
 123
 124 Theory of operation
 125 ===================
 126
 127 dm-verity is meant to be set up as part of a verified boot path.  This
 128 may be anything ranging from a boot using tboot or trustedgrub to just
 129 booting from a known-good device (like a USB drive or CD).
 130
 131 When a dm-verity device is configured, it is expected that the caller
 132 has been authenticated in some way (cryptographic signatures, etc).
 133 After instantiation, all hashes will be verified on-demand during
 134 disk access.  If they cannot be verified up to the root node of the
 135 tree, the root hash, then the I/O will fail.  This should detect
 136 tampering with any data on the device and the hash data.
 137
 138 Cryptographic hashes are used to assert the integrity of the device on a
 139 per-block basis. This allows for a lightweight hash computation on first read
 140 into the page cache. Block hashes are stored linearly, aligned to the nearest
 141 block size.
 142
 143 If forward error correction (FEC) support is enabled any recovery of
 144 corrupted data will be verified using the cryptographic hash of the
 145 corresponding data. This is why combining error correction with
 146 integrity checking is essential.
 147
 148 Hash Tree
 149 ---------
 150
 151 Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
 152 of some data block on disk is calculated. If it is an intermediary node,
 153 the hash of a number of child nodes is calculated.
 154
 155 Each entry in the tree is a collection of neighboring nodes that fit in one
 156 block.  The number is determined based on block_size and the size of the
 157 selected cryptographic digest algorithm.  The hashes are linearly-ordered in
 158 this entry and any unaligned trailing space is ignored but included when
 159 calculating the parent node.
 160
 161 The tree looks something like:
 162
 163 alg = sha256, num_blocks = 32768, block_size = 4096
 164
 165                                  [   root    ]
 166                                 /    . . .    \
 167                      [entry_0]                 [entry_1]
 168                     /  . . .  \                 . . .   \
 169          [entry_0_0]   . . .  [entry_0_127]    . . . .  [entry_1_127]
 170            / ... \             /   . . .  \             /           \
 171      blk_0 ... blk_127  blk_16256   blk_16383      blk_32640 . . . blk_32767
 172
 173
 174 On-disk format
 175 ==============
 176
 177 The verity kernel code does not read the verity metadata on-disk header.
 178 It only reads the hash blocks which directly follow the header.
 179 It is expected that a user-space tool will verify the integrity of the
 180 verity header.
 181
 182 Alternatively, the header can be omitted and the dmsetup parameters can
 183 be passed via the kernel command-line in a rooted chain of trust where
 184 the command-line is verified.
 185
 186 Directly following the header (and with sector number padded to the next hash
 187 block boundary) are the hash blocks which are stored a depth at a time
 188 (starting from the root), sorted in order of increasing index.
 189
 190 The full specification of kernel parameters and on-disk metadata format
 191 is available at the cryptsetup project's wiki page
 192   https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
 193
 194 Status
 195 ======
 196 V (for Valid) is returned if every check performed so far was valid.
 197 If any check failed, C (for Corruption) is returned.
 198
 199 Example
 200 =======
 201 Set up a device:
 202   # dmsetup create vroot --readonly --table \
 203     "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
 204     "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
 205     "1234000000000000000000000000000000000000000000000000000000000000"
 206
 207 A command line tool veritysetup is available to compute or verify
 208 the hash tree or activate the kernel device. This is available from
 209 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
 210 (as a libcryptsetup extension).
 211
 212 Create hash on the device:
 213   # veritysetup format /dev/sda1 /dev/sda2
 214   ...
 215   Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
 216
 217 Activate the device:
 218   # veritysetup create vroot /dev/sda1 /dev/sda2 \
 219     4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076