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) to recover from corruption if hash 106 verification fails. Use encoding data from the specified device. This 107 may be the same device where data and hash blocks reside, in which case 108 fec_start must be outside data and hash areas. 109 110 If the encoding data covers additional metadata, it must be accessible 111 on the hash device after the hash blocks. 112 113 Note: block sizes for data and hash devices must match. Also, if the 114 verity <dev> is encrypted the <fec_dev> should be too. 115 116fec_roots <num> 117 Number of generator roots. This equals to the number of parity bytes in 118 the encoding data. For example, in RS(M, N) encoding, the number of roots 119 is M-N. 120 121fec_blocks <num> 122 The number of encoding data blocks on the FEC device. The block size for 123 the FEC device is <data_block_size>. 124 125fec_start <offset> 126 This is the offset, in <data_block_size> blocks, from the start of the 127 FEC device to the beginning of the encoding data. 128 129check_at_most_once 130 Verify data blocks only the first time they are read from the data device, 131 rather than every time. This reduces the overhead of dm-verity so that it 132 can be used on systems that are memory and/or CPU constrained. However, it 133 provides a reduced level of security because only offline tampering of the 134 data device's content will be detected, not online tampering. 135 136 Hash blocks are still verified each time they are read from the hash device, 137 since verification of hash blocks is less performance critical than data 138 blocks, and a hash block will not be verified any more after all the data 139 blocks it covers have been verified anyway. 140 141root_hash_sig_key_desc <key_description> 142 This is the description of the USER_KEY that the kernel will lookup to get 143 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate 144 the root hash during the creation of the device mapper block device. 145 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG 146 being set in the kernel. The signatures are checked against the builtin 147 trusted keyring by default, or the secondary trusted keyring if 148 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING is set. The secondary 149 trusted keyring includes by default the builtin trusted keyring, and it can 150 also gain new certificates at run time if they are signed by a certificate 151 already in the secondary trusted keyring. 152 153try_verify_in_tasklet 154 If verity hashes are in cache, verify data blocks in kernel tasklet instead 155 of workqueue. This option can reduce IO latency. 156 157Theory of operation 158=================== 159 160dm-verity is meant to be set up as part of a verified boot path. This 161may be anything ranging from a boot using tboot or trustedgrub to just 162booting from a known-good device (like a USB drive or CD). 163 164When a dm-verity device is configured, it is expected that the caller 165has been authenticated in some way (cryptographic signatures, etc). 166After instantiation, all hashes will be verified on-demand during 167disk access. If they cannot be verified up to the root node of the 168tree, the root hash, then the I/O will fail. This should detect 169tampering with any data on the device and the hash data. 170 171Cryptographic hashes are used to assert the integrity of the device on a 172per-block basis. This allows for a lightweight hash computation on first read 173into the page cache. Block hashes are stored linearly, aligned to the nearest 174block size. 175 176If forward error correction (FEC) support is enabled any recovery of 177corrupted data will be verified using the cryptographic hash of the 178corresponding data. This is why combining error correction with 179integrity checking is essential. 180 181Hash Tree 182--------- 183 184Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 185of some data block on disk is calculated. If it is an intermediary node, 186the hash of a number of child nodes is calculated. 187 188Each entry in the tree is a collection of neighboring nodes that fit in one 189block. The number is determined based on block_size and the size of the 190selected cryptographic digest algorithm. The hashes are linearly-ordered in 191this entry and any unaligned trailing space is ignored but included when 192calculating the parent node. 193 194The tree looks something like: 195 196 alg = sha256, num_blocks = 32768, block_size = 4096 197 198:: 199 200 [ root ] 201 / . . . \ 202 [entry_0] [entry_1] 203 / . . . \ . . . \ 204 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 205 / ... \ / . . . \ / \ 206 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 207 208 209On-disk format 210============== 211 212The verity kernel code does not read the verity metadata on-disk header. 213It only reads the hash blocks which directly follow the header. 214It is expected that a user-space tool will verify the integrity of the 215verity header. 216 217Alternatively, the header can be omitted and the dmsetup parameters can 218be passed via the kernel command-line in a rooted chain of trust where 219the command-line is verified. 220 221Directly following the header (and with sector number padded to the next hash 222block boundary) are the hash blocks which are stored a depth at a time 223(starting from the root), sorted in order of increasing index. 224 225The full specification of kernel parameters and on-disk metadata format 226is available at the cryptsetup project's wiki page 227 228 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 229 230Status 231====== 232V (for Valid) is returned if every check performed so far was valid. 233If any check failed, C (for Corruption) is returned. 234 235Example 236======= 237Set up a device:: 238 239 # dmsetup create vroot --readonly --table \ 240 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 241 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 242 "1234000000000000000000000000000000000000000000000000000000000000" 243 244A command line tool veritysetup is available to compute or verify 245the hash tree or activate the kernel device. This is available from 246the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 247(as a libcryptsetup extension). 248 249Create hash on the device:: 250 251 # veritysetup format /dev/sda1 /dev/sda2 252 ... 253 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 254 255Activate the device:: 256 257 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 258 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 259