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.
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>]
17 This is the type of the on-disk hash format.
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.
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.
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,
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
39 The block size on a data device in bytes.
40 Each block corresponds to one digest on the hash device.
43 The size of a hash block in bytes.
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>.
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.
55 The cryptographic hash algorithm used for this device. This should
56 be the name of the algorithm, like "sha1".
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
64 The hexadecimal encoding of the salt value.
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.
71 Example of optional parameters section:
75 Log corrupted blocks, but allow read operations to proceed normally.
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
82 use_fec_from_device <fec_dev>
83 Use forward error correction (FEC) to recover from corruption if hash
84 verification fails. Use encoding data from the specified device. This
85 may be the same device where data and hash blocks reside, in which case
86 fec_start must be outside data and hash areas.
88 If the encoding data covers additional metadata, it must be accessible
89 on the hash device after the hash blocks.
91 Note: block sizes for data and hash devices must match. Also, if the
92 verity <dev> is encrypted the <fec_dev> should be too.
95 Number of generator roots. This equals to the number of parity bytes in
96 the encoding data. For example, in RS(M, N) encoding, the number of roots
100 The number of encoding data blocks on the FEC device. The block size for
101 the FEC device is <data_block_size>.
104 This is the offset, in <data_block_size> blocks, from the start of the
105 FEC device to the beginning of the encoding data.
111 dm-verity is meant to be set up as part of a verified boot path. This
112 may be anything ranging from a boot using tboot or trustedgrub to just
113 booting from a known-good device (like a USB drive or CD).
115 When a dm-verity device is configured, it is expected that the caller
116 has been authenticated in some way (cryptographic signatures, etc).
117 After instantiation, all hashes will be verified on-demand during
118 disk access. If they cannot be verified up to the root node of the
119 tree, the root hash, then the I/O will fail. This should detect
120 tampering with any data on the device and the hash data.
122 Cryptographic hashes are used to assert the integrity of the device on a
123 per-block basis. This allows for a lightweight hash computation on first read
124 into the page cache. Block hashes are stored linearly, aligned to the nearest
127 If forward error correction (FEC) support is enabled any recovery of
128 corrupted data will be verified using the cryptographic hash of the
129 corresponding data. This is why combining error correction with
130 integrity checking is essential.
135 Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
136 of some data block on disk is calculated. If it is an intermediary node,
137 the hash of a number of child nodes is calculated.
139 Each entry in the tree is a collection of neighboring nodes that fit in one
140 block. The number is determined based on block_size and the size of the
141 selected cryptographic digest algorithm. The hashes are linearly-ordered in
142 this entry and any unaligned trailing space is ignored but included when
143 calculating the parent node.
145 The tree looks something like:
147 alg = sha256, num_blocks = 32768, block_size = 4096
153 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
154 / ... \ / . . . \ / \
155 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
161 The verity kernel code does not read the verity metadata on-disk header.
162 It only reads the hash blocks which directly follow the header.
163 It is expected that a user-space tool will verify the integrity of the
166 Alternatively, the header can be omitted and the dmsetup parameters can
167 be passed via the kernel command-line in a rooted chain of trust where
168 the command-line is verified.
170 Directly following the header (and with sector number padded to the next hash
171 block boundary) are the hash blocks which are stored a depth at a time
172 (starting from the root), sorted in order of increasing index.
174 The full specification of kernel parameters and on-disk metadata format
175 is available at the cryptsetup project's wiki page
176 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
180 V (for Valid) is returned if every check performed so far was valid.
181 If any check failed, C (for Corruption) is returned.
186 # dmsetup create vroot --readonly --table \
187 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
188 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
189 "1234000000000000000000000000000000000000000000000000000000000000"
191 A command line tool veritysetup is available to compute or verify
192 the hash tree or activate the kernel device. This is available from
193 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
194 (as a libcryptsetup extension).
196 Create hash on the device:
197 # veritysetup format /dev/sda1 /dev/sda2
199 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
202 # veritysetup create vroot /dev/sda1 /dev/sda2 \
203 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076