1========= 2dm-verity 3========= 4 5Device-Mapper's "verity" target provides transparent integrity checking of 6block devices using a cryptographic digest provided by the kernel crypto API. 7This target is read-only. 8 9Construction Parameters 10======================= 11 12:: 13 14 <version> <dev> <hash_dev> 15 <data_block_size> <hash_block_size> 16 <num_data_blocks> <hash_start_block> 17 <algorithm> <digest> <salt> 18 [<#opt_params> <opt_params>] 19 20<version> 21 This is the type of the on-disk hash format. 22 23 0 is the original format used in the Chromium OS. 24 The salt is appended when hashing, digests are stored continuously and 25 the rest of the block is padded with zeroes. 26 27 1 is the current format that should be used for new devices. 28 The salt is prepended when hashing and each digest is 29 padded with zeroes to the power of two. 30 31<dev> 32 This is the device containing data, the integrity of which needs to be 33 checked. It may be specified as a path, like /dev/sdaX, or a device number, 34 <major>:<minor>. 35 36<hash_dev> 37 This is the device that supplies the hash tree data. It may be 38 specified similarly to the device path and may be the same device. If the 39 same device is used, the hash_start should be outside the configured 40 dm-verity device. 41 42<data_block_size> 43 The block size on a data device in bytes. 44 Each block corresponds to one digest on the hash device. 45 46<hash_block_size> 47 The size of a hash block in bytes. 48 49<num_data_blocks> 50 The number of data blocks on the data device. Additional blocks are 51 inaccessible. You can place hashes to the same partition as data, in this 52 case hashes are placed after <num_data_blocks>. 53 54<hash_start_block> 55 This is the offset, in <hash_block_size>-blocks, from the start of hash_dev 56 to the root block of the hash tree. 57 58<algorithm> 59 The cryptographic hash algorithm used for this device. This should 60 be the name of the algorithm, like "sha1". 61 62<digest> 63 The hexadecimal encoding of the cryptographic hash of the root hash block 64 and the salt. This hash should be trusted as there is no other authenticity 65 beyond this point. 66 67<salt> 68 The hexadecimal encoding of the salt value. 69 70<#opt_params> 71 Number of optional parameters. If there are no optional parameters, 72 the optional parameters section can be skipped or #opt_params can be zero. 73 Otherwise #opt_params is the number of following arguments. 74 75 Example of optional parameters section: 76 1 ignore_corruption 77 78ignore_corruption 79 Log corrupted blocks, but allow read operations to proceed normally. 80 81restart_on_corruption 82 Restart the system when a corrupted block is discovered. This option is 83 not compatible with ignore_corruption and requires user space support to 84 avoid restart loops. 85 86panic_on_corruption 87 Panic the device when a corrupted block is discovered. This option is 88 not compatible with ignore_corruption and restart_on_corruption. 89 90restart_on_error 91 Restart the system when an I/O error is detected. 92 This option can be combined with the restart_on_corruption option. 93 94panic_on_error 95 Panic the device when an I/O error is detected. This option is 96 not compatible with the restart_on_error option but can be combined 97 with the panic_on_corruption option. 98 99ignore_zero_blocks 100 Do not verify blocks that are expected to contain zeroes and always return 101 zeroes instead. This may be useful if the partition contains unused blocks 102 that are not guaranteed to contain zeroes. 103 104use_fec_from_device <fec_dev> 105 Use forward error correction (FEC) parity data from the specified device to 106 try to automatically recover from corruption and I/O errors. 107 108 If this option is given, then <fec_roots> and <fec_blocks> must also be 109 given. <hash_block_size> must also be equal to <data_block_size>. 110 111 <fec_dev> can be the same as <dev>, in which case <fec_start> must be 112 outside the data area. It can also be the same as <hash_dev>, in which case 113 <fec_start> must be outside the hash and optional additional metadata areas. 114 115 If the data <dev> is encrypted, the <fec_dev> should be too. 116 117 For more information, see `Forward error correction`_. 118 119fec_roots <num> 120 The number of parity bytes in each 255-byte Reed-Solomon codeword. The 121 Reed-Solomon code used will be an RS(255, k) code where k = 255 - fec_roots. 122 123 The supported values are 2 through 24 inclusive. Higher values provide 124 stronger error correction. However, the minimum value of 2 already provides 125 strong error correction due to the use of interleaving, so 2 is the 126 recommended value for most users. fec_roots=2 corresponds to an 127 RS(255, 253) code, which has a space overhead of about 0.8%. 128 129fec_blocks <num> 130 The total number of <data_block_size> blocks that are error-checked using 131 FEC. This must be at least the sum of <num_data_blocks> and the number of 132 blocks needed by the hash tree. It can include additional metadata blocks, 133 which are assumed to be accessible on <hash_dev> following the hash blocks. 134 135 Note that this is *not* the number of parity blocks. The number of parity 136 blocks is inferred from <fec_blocks>, <fec_roots>, and <data_block_size>. 137 138fec_start <offset> 139 This is the offset, in <data_block_size> blocks, from the start of <fec_dev> 140 to the beginning of the parity data. 141 142check_at_most_once 143 Verify data blocks only the first time they are read from the data device, 144 rather than every time. This reduces the overhead of dm-verity so that it 145 can be used on systems that are memory and/or CPU constrained. However, it 146 provides a reduced level of security because only offline tampering of the 147 data device's content will be detected, not online tampering. 148 149 Hash blocks are still verified each time they are read from the hash device, 150 since verification of hash blocks is less performance critical than data 151 blocks, and a hash block will not be verified any more after all the data 152 blocks it covers have been verified anyway. 153 154root_hash_sig_key_desc <key_description> 155 This is the description of the USER_KEY that the kernel will lookup to get 156 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate 157 the root hash during the creation of the device mapper block device. 158 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG 159 being set in the kernel. The signatures are checked against the builtin 160 trusted keyring by default, or the secondary trusted keyring if 161 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING is set. The secondary 162 trusted keyring includes by default the builtin trusted keyring, and it can 163 also gain new certificates at run time if they are signed by a certificate 164 already in the secondary trusted keyring. 165 166try_verify_in_tasklet 167 If verity hashes are in cache and the IO size does not exceed the limit, 168 verify data blocks in bottom half instead of workqueue. This option can 169 reduce IO latency. The size limits can be configured via 170 /sys/module/dm_verity/parameters/use_bh_bytes. The four parameters 171 correspond to limits for IOPRIO_CLASS_NONE, IOPRIO_CLASS_RT, 172 IOPRIO_CLASS_BE and IOPRIO_CLASS_IDLE in turn. 173 For example: 174 <none>,<rt>,<be>,<idle> 175 4096,4096,4096,4096 176 177Theory of operation 178=================== 179 180dm-verity is meant to be set up as part of a verified boot path. This 181may be anything ranging from a boot using tboot or trustedgrub to just 182booting from a known-good device (like a USB drive or CD). 183 184When a dm-verity device is configured, it is expected that the caller 185has been authenticated in some way (cryptographic signatures, etc). 186After instantiation, all hashes will be verified on-demand during 187disk access. If they cannot be verified up to the root node of the 188tree, the root hash, then the I/O will fail. This should detect 189tampering with any data on the device and the hash data. 190 191Cryptographic hashes are used to assert the integrity of the device on a 192per-block basis. This allows for a lightweight hash computation on first read 193into the page cache. Block hashes are stored linearly, aligned to the nearest 194block size. 195 196Hash Tree 197--------- 198 199Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 200of some data block on disk is calculated. If it is an intermediary node, 201the hash of a number of child nodes is calculated. 202 203Each entry in the tree is a collection of neighboring nodes that fit in one 204block. The number is determined based on block_size and the size of the 205selected cryptographic digest algorithm. The hashes are linearly-ordered in 206this entry and any unaligned trailing space is ignored but included when 207calculating the parent node. 208 209The tree looks something like: 210 211 alg = sha256, num_blocks = 32768, block_size = 4096 212 213:: 214 215 [ root ] 216 / . . . \ 217 [entry_0] [entry_1] 218 / . . . \ . . . \ 219 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 220 / ... \ / . . . \ / \ 221 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 222 223Forward error correction 224------------------------ 225 226dm-verity's optional forward error correction (FEC) support adds strong error 227correction capabilities to dm-verity. It allows systems that would be rendered 228inoperable by errors to continue operating, albeit with reduced performance. 229 230FEC uses Reed-Solomon (RS) codes that are interleaved across the entire 231device(s), allowing long bursts of corrupt or unreadable blocks to be recovered. 232 233dm-verity validates any FEC-corrected block against the wanted hash before using 234it. Therefore, FEC doesn't affect the security properties of dm-verity. 235 236The integration of FEC with dm-verity provides significant benefits over a 237separate error correction layer: 238 239- dm-verity invokes FEC only when a block's hash doesn't match the wanted hash 240 or the block cannot be read at all. As a result, FEC doesn't add overhead to 241 the common case where no error occurs. 242 243- dm-verity hashes are also used to identify erasure locations for RS decoding. 244 This allows correcting twice as many errors. 245 246FEC uses an RS(255, k) code where k = 255 - fec_roots. fec_roots is usually 2. 247This means that each k (usually 253) message bytes have fec_roots (usually 2) 248bytes of parity data added to get a 255-byte codeword. (Many external sources 249call RS codewords "blocks". Since dm-verity already uses the term "block" to 250mean something else, we'll use the clearer term "RS codeword".) 251 252FEC checks fec_blocks blocks of message data in total, consisting of: 253 2541. The data blocks from the data device 2552. The hash blocks from the hash device 2563. Optional additional metadata that follows the hash blocks on the hash device 257 258dm-verity assumes that the FEC parity data was computed as if the following 259procedure were followed: 260 2611. Concatenate the message data from the above sources. 2622. Zero-pad to the next multiple of k blocks. Let msg be the resulting byte 263 array, and msglen its length in bytes. 2643. For 0 <= i < msglen / k (for each RS codeword): 265 a. Select msg[i + j * msglen / k] for 0 <= j < k. 266 Consider these to be the 'k' message bytes of an RS codeword. 267 b. Compute the corresponding 'fec_roots' parity bytes of the RS codeword, 268 and concatenate them to the FEC parity data. 269 270Step 3a interleaves the RS codewords across the entire device using an 271interleaving degree of data_block_size * ceil(fec_blocks / k). This is the 272maximal interleaving, such that the message data consists of a region containing 273byte 0 of all the RS codewords, then a region containing byte 1 of all the RS 274codewords, and so on up to the region for byte 'k - 1'. Note that the number of 275codewords is set to a multiple of data_block_size; thus, the regions are 276block-aligned, and there is an implicit zero padding of up to 'k - 1' blocks. 277 278This interleaving allows long bursts of errors to be corrected. It provides 279much stronger error correction than storage devices typically provide, while 280keeping the space overhead low. 281 282The cost is slow decoding: correcting a single block usually requires reading 283254 extra blocks spread evenly across the device(s). However, that is 284acceptable because dm-verity uses FEC only when there is actually an error. 285 286The list below contains additional details about the RS codes used by 287dm-verity's FEC. Userspace programs that generate the parity data need to use 288these parameters for the parity data to match exactly: 289 290- Field used is GF(256) 291- Bytes are mapped to/from GF(256) elements in the natural way, where bits 0 292 through 7 (low-order to high-order) map to the coefficients of x^0 through x^7 293- Field generator polynomial is x^8 + x^4 + x^3 + x^2 + 1 294- The codes used are systematic, BCH-view codes 295- Primitive element alpha is 'x' 296- First consecutive root of code generator polynomial is 'x^0' 297 298On-disk format 299============== 300 301The verity kernel code does not read the verity metadata on-disk header. 302It only reads the hash blocks which directly follow the header. 303It is expected that a user-space tool will verify the integrity of the 304verity header. 305 306Alternatively, the header can be omitted and the dmsetup parameters can 307be passed via the kernel command-line in a rooted chain of trust where 308the command-line is verified. 309 310Directly following the header (and with sector number padded to the next hash 311block boundary) are the hash blocks which are stored a depth at a time 312(starting from the root), sorted in order of increasing index. 313 314The full specification of kernel parameters and on-disk metadata format 315is available at the cryptsetup project's wiki page 316 317 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 318 319Status 320====== 3211. V (for Valid) is returned if every check performed so far was valid. 322 If any check failed, C (for Corruption) is returned. 3232. Number of corrected blocks by Forward Error Correction. 324 '-' if Forward Error Correction is not enabled. 325 326Example 327======= 328Set up a device:: 329 330 # dmsetup create vroot --readonly --table \ 331 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 332 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 333 "1234000000000000000000000000000000000000000000000000000000000000" 334 335A command line tool veritysetup is available to compute or verify 336the hash tree or activate the kernel device. This is available from 337the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 338(as a libcryptsetup extension). 339 340Create hash on the device:: 341 342 # veritysetup format /dev/sda1 /dev/sda2 343 ... 344 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 345 346Activate the device:: 347 348 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 349 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 350