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 90ignore_zero_blocks 91 Do not verify blocks that are expected to contain zeroes and always return 92 zeroes instead. This may be useful if the partition contains unused blocks 93 that are not guaranteed to contain zeroes. 94 95use_fec_from_device <fec_dev> 96 Use forward error correction (FEC) to recover from corruption if hash 97 verification fails. Use encoding data from the specified device. This 98 may be the same device where data and hash blocks reside, in which case 99 fec_start must be outside data and hash areas. 100 101 If the encoding data covers additional metadata, it must be accessible 102 on the hash device after the hash blocks. 103 104 Note: block sizes for data and hash devices must match. Also, if the 105 verity <dev> is encrypted the <fec_dev> should be too. 106 107fec_roots <num> 108 Number of generator roots. This equals to the number of parity bytes in 109 the encoding data. For example, in RS(M, N) encoding, the number of roots 110 is M-N. 111 112fec_blocks <num> 113 The number of encoding data blocks on the FEC device. The block size for 114 the FEC device is <data_block_size>. 115 116fec_start <offset> 117 This is the offset, in <data_block_size> blocks, from the start of the 118 FEC device to the beginning of the encoding data. 119 120check_at_most_once 121 Verify data blocks only the first time they are read from the data device, 122 rather than every time. This reduces the overhead of dm-verity so that it 123 can be used on systems that are memory and/or CPU constrained. However, it 124 provides a reduced level of security because only offline tampering of the 125 data device's content will be detected, not online tampering. 126 127 Hash blocks are still verified each time they are read from the hash device, 128 since verification of hash blocks is less performance critical than data 129 blocks, and a hash block will not be verified any more after all the data 130 blocks it covers have been verified anyway. 131 132root_hash_sig_key_desc <key_description> 133 This is the description of the USER_KEY that the kernel will lookup to get 134 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate 135 the root hash during the creation of the device mapper block device. 136 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG 137 being set in the kernel. The signatures are checked against the builtin 138 trusted keyring by default, or the secondary trusted keyring if 139 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING is set. The secondary 140 trusted keyring includes by default the builtin trusted keyring, and it can 141 also gain new certificates at run time if they are signed by a certificate 142 already in the secondary trusted keyring. 143 144Theory of operation 145=================== 146 147dm-verity is meant to be set up as part of a verified boot path. This 148may be anything ranging from a boot using tboot or trustedgrub to just 149booting from a known-good device (like a USB drive or CD). 150 151When a dm-verity device is configured, it is expected that the caller 152has been authenticated in some way (cryptographic signatures, etc). 153After instantiation, all hashes will be verified on-demand during 154disk access. If they cannot be verified up to the root node of the 155tree, the root hash, then the I/O will fail. This should detect 156tampering with any data on the device and the hash data. 157 158Cryptographic hashes are used to assert the integrity of the device on a 159per-block basis. This allows for a lightweight hash computation on first read 160into the page cache. Block hashes are stored linearly, aligned to the nearest 161block size. 162 163If forward error correction (FEC) support is enabled any recovery of 164corrupted data will be verified using the cryptographic hash of the 165corresponding data. This is why combining error correction with 166integrity checking is essential. 167 168Hash Tree 169--------- 170 171Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 172of some data block on disk is calculated. If it is an intermediary node, 173the hash of a number of child nodes is calculated. 174 175Each entry in the tree is a collection of neighboring nodes that fit in one 176block. The number is determined based on block_size and the size of the 177selected cryptographic digest algorithm. The hashes are linearly-ordered in 178this entry and any unaligned trailing space is ignored but included when 179calculating the parent node. 180 181The tree looks something like: 182 183 alg = sha256, num_blocks = 32768, block_size = 4096 184 185:: 186 187 [ root ] 188 / . . . \ 189 [entry_0] [entry_1] 190 / . . . \ . . . \ 191 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 192 / ... \ / . . . \ / \ 193 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 194 195 196On-disk format 197============== 198 199The verity kernel code does not read the verity metadata on-disk header. 200It only reads the hash blocks which directly follow the header. 201It is expected that a user-space tool will verify the integrity of the 202verity header. 203 204Alternatively, the header can be omitted and the dmsetup parameters can 205be passed via the kernel command-line in a rooted chain of trust where 206the command-line is verified. 207 208Directly following the header (and with sector number padded to the next hash 209block boundary) are the hash blocks which are stored a depth at a time 210(starting from the root), sorted in order of increasing index. 211 212The full specification of kernel parameters and on-disk metadata format 213is available at the cryptsetup project's wiki page 214 215 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 216 217Status 218====== 219V (for Valid) is returned if every check performed so far was valid. 220If any check failed, C (for Corruption) is returned. 221 222Example 223======= 224Set up a device:: 225 226 # dmsetup create vroot --readonly --table \ 227 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 228 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 229 "1234000000000000000000000000000000000000000000000000000000000000" 230 231A command line tool veritysetup is available to compute or verify 232the hash tree or activate the kernel device. This is available from 233the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 234(as a libcryptsetup extension). 235 236Create hash on the device:: 237 238 # veritysetup format /dev/sda1 /dev/sda2 239 ... 240 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 241 242Activate the device:: 243 244 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 245 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 246