Based on kernel version 3.19. Page generated on 2015-02-13 21:16 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 15 <version> 16 This is the type of the on-disk hash format. 17 18 0 is the original format used in the Chromium OS. 19 The salt is appended when hashing, digests are stored continuously and 20 the rest of the block is padded with zeros. 21 22 1 is the current format that should be used for new devices. 23 The salt is prepended when hashing and each digest is 24 padded with zeros to the power of two. 25 26 <dev> 27 This is the device containing data, the integrity of which needs to be 28 checked. It may be specified as a path, like /dev/sdaX, or a device number, 29 <major>:<minor>. 30 31 <hash_dev> 32 This is the device that supplies the hash tree data. It may be 33 specified similarly to the device path and may be the same device. If the 34 same device is used, the hash_start should be outside the configured 35 dm-verity device. 36 37 <data_block_size> 38 The block size on a data device in bytes. 39 Each block corresponds to one digest on the hash device. 40 41 <hash_block_size> 42 The size of a hash block in bytes. 43 44 <num_data_blocks> 45 The number of data blocks on the data device. Additional blocks are 46 inaccessible. You can place hashes to the same partition as data, in this 47 case hashes are placed after <num_data_blocks>. 48 49 <hash_start_block> 50 This is the offset, in <hash_block_size>-blocks, from the start of hash_dev 51 to the root block of the hash tree. 52 53 <algorithm> 54 The cryptographic hash algorithm used for this device. This should 55 be the name of the algorithm, like "sha1". 56 57 <digest> 58 The hexadecimal encoding of the cryptographic hash of the root hash block 59 and the salt. This hash should be trusted as there is no other authenticity 60 beyond this point. 61 62 <salt> 63 The hexadecimal encoding of the salt value. 64 65 Theory of operation 66 =================== 67 68 dm-verity is meant to be set up as part of a verified boot path. This 69 may be anything ranging from a boot using tboot or trustedgrub to just 70 booting from a known-good device (like a USB drive or CD). 71 72 When a dm-verity device is configured, it is expected that the caller 73 has been authenticated in some way (cryptographic signatures, etc). 74 After instantiation, all hashes will be verified on-demand during 75 disk access. If they cannot be verified up to the root node of the 76 tree, the root hash, then the I/O will fail. This should detect 77 tampering with any data on the device and the hash data. 78 79 Cryptographic hashes are used to assert the integrity of the device on a 80 per-block basis. This allows for a lightweight hash computation on first read 81 into the page cache. Block hashes are stored linearly, aligned to the nearest 82 block size. 83 84 Hash Tree 85 --------- 86 87 Each node in the tree is a cryptographic hash. If it is a leaf node, the hash 88 of some data block on disk is calculated. If it is an intermediary node, 89 the hash of a number of child nodes is calculated. 90 91 Each entry in the tree is a collection of neighboring nodes that fit in one 92 block. The number is determined based on block_size and the size of the 93 selected cryptographic digest algorithm. The hashes are linearly-ordered in 94 this entry and any unaligned trailing space is ignored but included when 95 calculating the parent node. 96 97 The tree looks something like: 98 99 alg = sha256, num_blocks = 32768, block_size = 4096 100 101 [ root ] 102 / . . . \ 103 [entry_0] [entry_1] 104 / . . . \ . . . \ 105 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127] 106 / ... \ / . . . \ / \ 107 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767 108 109 110 On-disk format 111 ============== 112 113 The verity kernel code does not read the verity metadata on-disk header. 114 It only reads the hash blocks which directly follow the header. 115 It is expected that a user-space tool will verify the integrity of the 116 verity header. 117 118 Alternatively, the header can be omitted and the dmsetup parameters can 119 be passed via the kernel command-line in a rooted chain of trust where 120 the command-line is verified. 121 122 Directly following the header (and with sector number padded to the next hash 123 block boundary) are the hash blocks which are stored a depth at a time 124 (starting from the root), sorted in order of increasing index. 125 126 The full specification of kernel parameters and on-disk metadata format 127 is available at the cryptsetup project's wiki page 128 http://code.google.com/p/cryptsetup/wiki/DMVerity 129 130 Status 131 ====== 132 V (for Valid) is returned if every check performed so far was valid. 133 If any check failed, C (for Corruption) is returned. 134 135 Example 136 ======= 137 Set up a device: 138 # dmsetup create vroot --readonly --table \ 139 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\ 140 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\ 141 "1234000000000000000000000000000000000000000000000000000000000000" 142 143 A command line tool veritysetup is available to compute or verify 144 the hash tree or activate the kernel device. This is available from 145 the cryptsetup upstream repository http://code.google.com/p/cryptsetup/ 146 (as a libcryptsetup extension). 147 148 Create hash on the device: 149 # veritysetup format /dev/sda1 /dev/sda2 150 ... 151 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 152 153 Activate the device: 154 # veritysetup create vroot /dev/sda1 /dev/sda2 \ 155 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076