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 zeros. 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 zeros 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 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. 97 98fec_roots 99 Number of generator roots. This equals to the number of parity bytes in 100 the encoding data. For example, in RS(M, N) encoding, the number of roots 101 is M-N. 102 103fec_blocks 104 The number of encoding data blocks on the FEC device. The block size for 105 the FEC device is <data_block_size>. 106 107fec_start 108 This is the offset, in <data_block_size> blocks, from the start of the 109 FEC device to the beginning of the encoding data. 110 111 112Theory of operation 113=================== 114 115dm-verity is meant to be set up as part of a verified boot path. This 116may be anything ranging from a boot using tboot or trustedgrub to just 117booting from a known-good device (like a USB drive or CD). 118 119When a dm-verity device is configured, it is expected that the caller 120has been authenticated in some way (cryptographic signatures, etc). 121After instantiation, all hashes will be verified on-demand during 122disk access. If they cannot be verified up to the root node of the 123tree, the root hash, then the I/O will fail. This should detect 124tampering with any data on the device and the hash data. 125 126Cryptographic hashes are used to assert the integrity of the device on a 127per-block basis. This allows for a lightweight hash computation on first read 128into the page cache. Block hashes are stored linearly, aligned to the nearest 129block size. 130 131Hash Tree 132--------- 133 134Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 135of some data block on disk is calculated. If it is an intermediary node, 136the hash of a number of child nodes is calculated. 137 138Each entry in the tree is a collection of neighboring nodes that fit in one 139block. The number is determined based on block_size and the size of the 140selected cryptographic digest algorithm. The hashes are linearly-ordered in 141this entry and any unaligned trailing space is ignored but included when 142calculating the parent node. 143 144The tree looks something like: 145 146alg = sha256, num_blocks = 32768, block_size = 4096 147 148 [ root ] 149 / . . . \ 150 [entry_0] [entry_1] 151 / . . . \ . . . \ 152 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 153 / ... \ / . . . \ / \ 154 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 155 156 157On-disk format 158============== 159 160The verity kernel code does not read the verity metadata on-disk header. 161It only reads the hash blocks which directly follow the header. 162It is expected that a user-space tool will verify the integrity of the 163verity header. 164 165Alternatively, the header can be omitted and the dmsetup parameters can 166be passed via the kernel command-line in a rooted chain of trust where 167the command-line is verified. 168 169Directly following the header (and with sector number padded to the next hash 170block boundary) are the hash blocks which are stored a depth at a time 171(starting from the root), sorted in order of increasing index. 172 173The full specification of kernel parameters and on-disk metadata format 174is available at the cryptsetup project's wiki page 175 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 176 177Status 178====== 179V (for Valid) is returned if every check performed so far was valid. 180If any check failed, C (for Corruption) is returned. 181 182Example 183======= 184Set up a device: 185 # dmsetup create vroot --readonly --table \ 186 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 187 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 188 "1234000000000000000000000000000000000000000000000000000000000000" 189 190A command line tool veritysetup is available to compute or verify 191the hash tree or activate the kernel device. This is available from 192the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 193(as a libcryptsetup extension). 194 195Create hash on the device: 196 # veritysetup format /dev/sda1 /dev/sda2 197 ... 198 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 199 200Activate the device: 201 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 202 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 203