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 paramaters 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 86ignore_zero_blocks 87 Do not verify blocks that are expected to contain zeroes and always return 88 zeroes instead. This may be useful if the partition contains unused blocks 89 that are not guaranteed to contain zeroes. 90 91use_fec_from_device <fec_dev> 92 Use forward error correction (FEC) to recover from corruption if hash 93 verification fails. Use encoding data from the specified device. This 94 may be the same device where data and hash blocks reside, in which case 95 fec_start must be outside data and hash areas. 96 97 If the encoding data covers additional metadata, it must be accessible 98 on the hash device after the hash blocks. 99 100 Note: block sizes for data and hash devices must match. Also, if the 101 verity <dev> is encrypted the <fec_dev> should be too. 102 103fec_roots <num> 104 Number of generator roots. This equals to the number of parity bytes in 105 the encoding data. For example, in RS(M, N) encoding, the number of roots 106 is M-N. 107 108fec_blocks <num> 109 The number of encoding data blocks on the FEC device. The block size for 110 the FEC device is <data_block_size>. 111 112fec_start <offset> 113 This is the offset, in <data_block_size> blocks, from the start of the 114 FEC device to the beginning of the encoding data. 115 116check_at_most_once 117 Verify data blocks only the first time they are read from the data device, 118 rather than every time. This reduces the overhead of dm-verity so that it 119 can be used on systems that are memory and/or CPU constrained. However, it 120 provides a reduced level of security because only offline tampering of the 121 data device's content will be detected, not online tampering. 122 123 Hash blocks are still verified each time they are read from the hash device, 124 since verification of hash blocks is less performance critical than data 125 blocks, and a hash block will not be verified any more after all the data 126 blocks it covers have been verified anyway. 127 128Theory of operation 129=================== 130 131dm-verity is meant to be set up as part of a verified boot path. This 132may be anything ranging from a boot using tboot or trustedgrub to just 133booting from a known-good device (like a USB drive or CD). 134 135When a dm-verity device is configured, it is expected that the caller 136has been authenticated in some way (cryptographic signatures, etc). 137After instantiation, all hashes will be verified on-demand during 138disk access. If they cannot be verified up to the root node of the 139tree, the root hash, then the I/O will fail. This should detect 140tampering with any data on the device and the hash data. 141 142Cryptographic hashes are used to assert the integrity of the device on a 143per-block basis. This allows for a lightweight hash computation on first read 144into the page cache. Block hashes are stored linearly, aligned to the nearest 145block size. 146 147If forward error correction (FEC) support is enabled any recovery of 148corrupted data will be verified using the cryptographic hash of the 149corresponding data. This is why combining error correction with 150integrity checking is essential. 151 152Hash Tree 153--------- 154 155Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 156of some data block on disk is calculated. If it is an intermediary node, 157the hash of a number of child nodes is calculated. 158 159Each entry in the tree is a collection of neighboring nodes that fit in one 160block. The number is determined based on block_size and the size of the 161selected cryptographic digest algorithm. The hashes are linearly-ordered in 162this entry and any unaligned trailing space is ignored but included when 163calculating the parent node. 164 165The tree looks something like: 166 167 alg = sha256, num_blocks = 32768, block_size = 4096 168 169:: 170 171 [ root ] 172 / . . . \ 173 [entry_0] [entry_1] 174 / . . . \ . . . \ 175 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 176 / ... \ / . . . \ / \ 177 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 178 179 180On-disk format 181============== 182 183The verity kernel code does not read the verity metadata on-disk header. 184It only reads the hash blocks which directly follow the header. 185It is expected that a user-space tool will verify the integrity of the 186verity header. 187 188Alternatively, the header can be omitted and the dmsetup parameters can 189be passed via the kernel command-line in a rooted chain of trust where 190the command-line is verified. 191 192Directly following the header (and with sector number padded to the next hash 193block boundary) are the hash blocks which are stored a depth at a time 194(starting from the root), sorted in order of increasing index. 195 196The full specification of kernel parameters and on-disk metadata format 197is available at the cryptsetup project's wiki page 198 199 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 200 201Status 202====== 203V (for Valid) is returned if every check performed so far was valid. 204If any check failed, C (for Corruption) is returned. 205 206Example 207======= 208Set up a device:: 209 210 # dmsetup create vroot --readonly --table \ 211 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 212 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 213 "1234000000000000000000000000000000000000000000000000000000000000" 214 215A command line tool veritysetup is available to compute or verify 216the hash tree or activate the kernel device. This is available from 217the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 218(as a libcryptsetup extension). 219 220Create hash on the device:: 221 222 # veritysetup format /dev/sda1 /dev/sda2 223 ... 224 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 225 226Activate the device:: 227 228 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 229 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 230