Based on kernel version 4.8. Page generated on 2016-10-06 23:10 EST.
1 dm-verity 2 ========== 3 4 Device-Mapper's "verity" target provides transparent integrity checking of 5 block devices using a cryptographic digest provided by the kernel crypto API. 6 This target is read-only. 7 8 Construction 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 74 ignore_corruption 75 Log corrupted blocks, but allow read operations to proceed normally. 76 77 restart_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 82 ignore_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 87 use_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 99 fec_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. 103 104 fec_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>. 107 108 fec_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. 111 112 113 Theory of operation 114 =================== 115 116 dm-verity is meant to be set up as part of a verified boot path. This 117 may be anything ranging from a boot using tboot or trustedgrub to just 118 booting from a known-good device (like a USB drive or CD). 119 120 When a dm-verity device is configured, it is expected that the caller 121 has been authenticated in some way (cryptographic signatures, etc). 122 After instantiation, all hashes will be verified on-demand during 123 disk access. If they cannot be verified up to the root node of the 124 tree, the root hash, then the I/O will fail. This should detect 125 tampering with any data on the device and the hash data. 126 127 Cryptographic hashes are used to assert the integrity of the device on a 128 per-block basis. This allows for a lightweight hash computation on first read 129 into the page cache. Block hashes are stored linearly, aligned to the nearest 130 block size. 131 132 If forward error correction (FEC) support is enabled any recovery of 133 corrupted data will be verified using the cryptographic hash of the 134 corresponding data. This is why combining error correction with 135 integrity checking is essential. 136 137 Hash Tree 138 --------- 139 140 Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 141 of some data block on disk is calculated. If it is an intermediary node, 142 the hash of a number of child nodes is calculated. 143 144 Each entry in the tree is a collection of neighboring nodes that fit in one 145 block. The number is determined based on block_size and the size of the 146 selected cryptographic digest algorithm. The hashes are linearly-ordered in 147 this entry and any unaligned trailing space is ignored but included when 148 calculating the parent node. 149 150 The tree looks something like: 151 152 alg = sha256, num_blocks = 32768, block_size = 4096 153 154 [ root ] 155 / . . . \ 156 [entry_0] [entry_1] 157 / . . . \ . . . \ 158 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 159 / ... \ / . . . \ / \ 160 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 161 162 163 On-disk format 164 ============== 165 166 The verity kernel code does not read the verity metadata on-disk header. 167 It only reads the hash blocks which directly follow the header. 168 It is expected that a user-space tool will verify the integrity of the 169 verity header. 170 171 Alternatively, the header can be omitted and the dmsetup parameters can 172 be passed via the kernel command-line in a rooted chain of trust where 173 the command-line is verified. 174 175 Directly following the header (and with sector number padded to the next hash 176 block boundary) are the hash blocks which are stored a depth at a time 177 (starting from the root), sorted in order of increasing index. 178 179 The full specification of kernel parameters and on-disk metadata format 180 is available at the cryptsetup project's wiki page 181 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity 182 183 Status 184 ====== 185 V (for Valid) is returned if every check performed so far was valid. 186 If any check failed, C (for Corruption) is returned. 187 188 Example 189 ======= 190 Set up a device: 191 # dmsetup create vroot --readonly --table \ 192 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 193 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 194 "1234000000000000000000000000000000000000000000000000000000000000" 195 196 A command line tool veritysetup is available to compute or verify 197 the hash tree or activate the kernel device. This is available from 198 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/ 199 (as a libcryptsetup extension). 200 201 Create hash on the device: 202 # veritysetup format /dev/sda1 /dev/sda2 203 ... 204 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 205 206 Activate the device: 207 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 208 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076