1dm-verity 2========== 3 4Device-Mapper's "verity" target provides transparent integrity checking of 5block devices using a cryptographic digest provided by the kernel crypto API. 6This target is read-only. 7 8Construction 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 74ignore_corruption 75 Log corrupted blocks, but allow read operations to proceed normally. 76 77restart_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 82ignore_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 87use_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 99fec_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 104fec_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 108fec_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 112check_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 124Theory of operation 125=================== 126 127dm-verity is meant to be set up as part of a verified boot path. This 128may be anything ranging from a boot using tboot or trustedgrub to just 129booting from a known-good device (like a USB drive or CD). 130 131When a dm-verity device is configured, it is expected that the caller 132has been authenticated in some way (cryptographic signatures, etc). 133After instantiation, all hashes will be verified on-demand during 134disk access. If they cannot be verified up to the root node of the 135tree, the root hash, then the I/O will fail. This should detect 136tampering with any data on the device and the hash data. 137 138Cryptographic hashes are used to assert the integrity of the device on a 139per-block basis. This allows for a lightweight hash computation on first read 140into the page cache. Block hashes are stored linearly, aligned to the nearest 141block size. 142 143If forward error correction (FEC) support is enabled any recovery of 144corrupted data will be verified using the cryptographic hash of the 145corresponding data. This is why combining error correction with 146integrity checking is essential. 147 148Hash Tree 149--------- 150 151Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 152of some data block on disk is calculated. If it is an intermediary node, 153the hash of a number of child nodes is calculated. 154 155Each entry in the tree is a collection of neighboring nodes that fit in one 156block. The number is determined based on block_size and the size of the 157selected cryptographic digest algorithm. The hashes are linearly-ordered in 158this entry and any unaligned trailing space is ignored but included when 159calculating the parent node. 160 161The tree looks something like: 162 163alg = 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 174On-disk format 175============== 176 177The verity kernel code does not read the verity metadata on-disk header. 178It only reads the hash blocks which directly follow the header. 179It is expected that a user-space tool will verify the integrity of the 180verity header. 181 182Alternatively, the header can be omitted and the dmsetup parameters can 183be passed via the kernel command-line in a rooted chain of trust where 184the command-line is verified. 185 186Directly following the header (and with sector number padded to the next hash 187block 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 190The full specification of kernel parameters and on-disk metadata format 191is available at the cryptsetup project's wiki page 192 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 193 194Status 195====== 196V (for Valid) is returned if every check performed so far was valid. 197If any check failed, C (for Corruption) is returned. 198 199Example 200======= 201Set 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 207A command line tool veritysetup is available to compute or verify 208the hash tree or activate the kernel device. This is available from 209the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 210(as a libcryptsetup extension). 211 212Create hash on the device: 213 # veritysetup format /dev/sda1 /dev/sda2 214 ... 215 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 216 217Activate the device: 218 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 219 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 220
