5 Device-Mapper's "verity" target provides transparent integrity checking of
6 block devices using a cryptographic digest provided by the kernel crypto API.
7 This target is read-only.
9 Construction Parameters
10 =======================
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>]
21 This is the type of the on-disk hash format.
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.
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.
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,
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
43 The block size on a data device in bytes.
44 Each block corresponds to one digest on the hash device.
47 The size of a hash block in bytes.
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>.
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.
59 The cryptographic hash algorithm used for this device. This should
60 be the name of the algorithm, like "sha1".
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
68 The hexadecimal encoding of the salt value.
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.
75 Example of optional parameters section:
79 Log corrupted blocks, but allow read operations to proceed normally.
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
87 Panic the device when a corrupted block is discovered. This option is
88 not compatible with ignore_corruption and restart_on_corruption.
91 Do not verify blocks that are expected to contain zeroes and always return
92 zeroes instead. This may be useful if the partition contains unused blocks
93 that are not guaranteed to contain zeroes.
95 use_fec_from_device <fec_dev>
96 Use forward error correction (FEC) to recover from corruption if hash
97 verification fails. Use encoding data from the specified device. This
98 may be the same device where data and hash blocks reside, in which case
99 fec_start must be outside data and hash areas.
101 If the encoding data covers additional metadata, it must be accessible
102 on the hash device after the hash blocks.
104 Note: block sizes for data and hash devices must match. Also, if the
105 verity <dev> is encrypted the <fec_dev> should be too.
108 Number of generator roots. This equals to the number of parity bytes in
109 the encoding data. For example, in RS(M, N) encoding, the number of roots
113 The number of encoding data blocks on the FEC device. The block size for
114 the FEC device is <data_block_size>.
117 This is the offset, in <data_block_size> blocks, from the start of the
118 FEC device to the beginning of the encoding data.
121 Verify data blocks only the first time they are read from the data device,
122 rather than every time. This reduces the overhead of dm-verity so that it
123 can be used on systems that are memory and/or CPU constrained. However, it
124 provides a reduced level of security because only offline tampering of the
125 data device's content will be detected, not online tampering.
127 Hash blocks are still verified each time they are read from the hash device,
128 since verification of hash blocks is less performance critical than data
129 blocks, and a hash block will not be verified any more after all the data
130 blocks it covers have been verified anyway.
132 root_hash_sig_key_desc <key_description>
133 This is the description of the USER_KEY that the kernel will lookup to get
134 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate
135 the root hash during the creation of the device mapper block device.
136 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG
137 being set in the kernel.
142 dm-verity is meant to be set up as part of a verified boot path. This
143 may be anything ranging from a boot using tboot or trustedgrub to just
144 booting from a known-good device (like a USB drive or CD).
146 When a dm-verity device is configured, it is expected that the caller
147 has been authenticated in some way (cryptographic signatures, etc).
148 After instantiation, all hashes will be verified on-demand during
149 disk access. If they cannot be verified up to the root node of the
150 tree, the root hash, then the I/O will fail. This should detect
151 tampering with any data on the device and the hash data.
153 Cryptographic hashes are used to assert the integrity of the device on a
154 per-block basis. This allows for a lightweight hash computation on first read
155 into the page cache. Block hashes are stored linearly, aligned to the nearest
158 If forward error correction (FEC) support is enabled any recovery of
159 corrupted data will be verified using the cryptographic hash of the
160 corresponding data. This is why combining error correction with
161 integrity checking is essential.
166 Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
167 of some data block on disk is calculated. If it is an intermediary node,
168 the hash of a number of child nodes is calculated.
170 Each entry in the tree is a collection of neighboring nodes that fit in one
171 block. The number is determined based on block_size and the size of the
172 selected cryptographic digest algorithm. The hashes are linearly-ordered in
173 this entry and any unaligned trailing space is ignored but included when
174 calculating the parent node.
176 The tree looks something like:
178 alg = sha256, num_blocks = 32768, block_size = 4096
186 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
187 / ... \ / . . . \ / \
188 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
194 The verity kernel code does not read the verity metadata on-disk header.
195 It only reads the hash blocks which directly follow the header.
196 It is expected that a user-space tool will verify the integrity of the
199 Alternatively, the header can be omitted and the dmsetup parameters can
200 be passed via the kernel command-line in a rooted chain of trust where
201 the command-line is verified.
203 Directly following the header (and with sector number padded to the next hash
204 block boundary) are the hash blocks which are stored a depth at a time
205 (starting from the root), sorted in order of increasing index.
207 The full specification of kernel parameters and on-disk metadata format
208 is available at the cryptsetup project's wiki page
210 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
214 V (for Valid) is returned if every check performed so far was valid.
215 If any check failed, C (for Corruption) is returned.
221 # dmsetup create vroot --readonly --table \
222 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
223 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
224 "1234000000000000000000000000000000000000000000000000000000000000"
226 A command line tool veritysetup is available to compute or verify
227 the hash tree or activate the kernel device. This is available from
228 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
229 (as a libcryptsetup extension).
231 Create hash on the device::
233 # veritysetup format /dev/sda1 /dev/sda2
235 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
237 Activate the device::
239 # veritysetup create vroot /dev/sda1 /dev/sda2 \
240 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076