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 128root_hash_sig_key_desc <key_description> 129 This is the description of the USER_KEY that the kernel will lookup to get 130 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate 131 the root hash during the creation of the device mapper block device. 132 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG 133 being set in the kernel. 134 135Theory of operation 136=================== 137 138dm-verity is meant to be set up as part of a verified boot path. This 139may be anything ranging from a boot using tboot or trustedgrub to just 140booting from a known-good device (like a USB drive or CD). 141 142When a dm-verity device is configured, it is expected that the caller 143has been authenticated in some way (cryptographic signatures, etc). 144After instantiation, all hashes will be verified on-demand during 145disk access. If they cannot be verified up to the root node of the 146tree, the root hash, then the I/O will fail. This should detect 147tampering with any data on the device and the hash data. 148 149Cryptographic hashes are used to assert the integrity of the device on a 150per-block basis. This allows for a lightweight hash computation on first read 151into the page cache. Block hashes are stored linearly, aligned to the nearest 152block size. 153 154If forward error correction (FEC) support is enabled any recovery of 155corrupted data will be verified using the cryptographic hash of the 156corresponding data. This is why combining error correction with 157integrity checking is essential. 158 159Hash Tree 160--------- 161 162Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 163of some data block on disk is calculated. If it is an intermediary node, 164the hash of a number of child nodes is calculated. 165 166Each entry in the tree is a collection of neighboring nodes that fit in one 167block. The number is determined based on block_size and the size of the 168selected cryptographic digest algorithm. The hashes are linearly-ordered in 169this entry and any unaligned trailing space is ignored but included when 170calculating the parent node. 171 172The tree looks something like: 173 174 alg = sha256, num_blocks = 32768, block_size = 4096 175 176:: 177 178 [ root ] 179 / . . . \ 180 [entry_0] [entry_1] 181 / . . . \ . . . \ 182 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 183 / ... \ / . . . \ / \ 184 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 185 186 187On-disk format 188============== 189 190The verity kernel code does not read the verity metadata on-disk header. 191It only reads the hash blocks which directly follow the header. 192It is expected that a user-space tool will verify the integrity of the 193verity header. 194 195Alternatively, the header can be omitted and the dmsetup parameters can 196be passed via the kernel command-line in a rooted chain of trust where 197the command-line is verified. 198 199Directly following the header (and with sector number padded to the next hash 200block boundary) are the hash blocks which are stored a depth at a time 201(starting from the root), sorted in order of increasing index. 202 203The full specification of kernel parameters and on-disk metadata format 204is available at the cryptsetup project's wiki page 205 206 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 207 208Status 209====== 210V (for Valid) is returned if every check performed so far was valid. 211If any check failed, C (for Corruption) is returned. 212 213Example 214======= 215Set up a device:: 216 217 # dmsetup create vroot --readonly --table \ 218 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 219 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 220 "1234000000000000000000000000000000000000000000000000000000000000" 221 222A command line tool veritysetup is available to compute or verify 223the hash tree or activate the kernel device. This is available from 224the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 225(as a libcryptsetup extension). 226 227Create hash on the device:: 228 229 # veritysetup format /dev/sda1 /dev/sda2 230 ... 231 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 232 233Activate the device:: 234 235 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 236 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 237