1 ========= 1 =========
2 dm-verity 2 dm-verity
3 ========= 3 =========
4 4
5 Device-Mapper's "verity" target provides trans 5 Device-Mapper's "verity" target provides transparent integrity checking of
6 block devices using a cryptographic digest pro 6 block devices using a cryptographic digest provided by the kernel crypto API.
7 This target is read-only. 7 This target is read-only.
8 8
9 Construction Parameters 9 Construction Parameters
10 ======================= 10 =======================
11 11
12 :: 12 ::
13 13
14 <version> <dev> <hash_dev> 14 <version> <dev> <hash_dev>
15 <data_block_size> <hash_block_size> 15 <data_block_size> <hash_block_size>
16 <num_data_blocks> <hash_start_block> 16 <num_data_blocks> <hash_start_block>
17 <algorithm> <digest> <salt> 17 <algorithm> <digest> <salt>
18 [<#opt_params> <opt_params>] 18 [<#opt_params> <opt_params>]
19 19
20 <version> 20 <version>
21 This is the type of the on-disk hash forma 21 This is the type of the on-disk hash format.
22 22
23 0 is the original format used in the Chrom 23 0 is the original format used in the Chromium OS.
24 The salt is appended when hashing, diges 24 The salt is appended when hashing, digests are stored continuously and
25 the rest of the block is padded with zer 25 the rest of the block is padded with zeroes.
26 26
27 1 is the current format that should be use 27 1 is the current format that should be used for new devices.
28 The salt is prepended when hashing and e 28 The salt is prepended when hashing and each digest is
29 padded with zeroes to the power of two. 29 padded with zeroes to the power of two.
30 30
31 <dev> 31 <dev>
32 This is the device containing data, the in 32 This is the device containing data, the integrity of which needs to be
33 checked. It may be specified as a path, l 33 checked. It may be specified as a path, like /dev/sdaX, or a device number,
34 <major>:<minor>. 34 <major>:<minor>.
35 35
36 <hash_dev> 36 <hash_dev>
37 This is the device that supplies the hash 37 This is the device that supplies the hash tree data. It may be
38 specified similarly to the device path and 38 specified similarly to the device path and may be the same device. If the
39 same device is used, the hash_start should 39 same device is used, the hash_start should be outside the configured
40 dm-verity device. 40 dm-verity device.
41 41
42 <data_block_size> 42 <data_block_size>
43 The block size on a data device in bytes. 43 The block size on a data device in bytes.
44 Each block corresponds to one digest on th 44 Each block corresponds to one digest on the hash device.
45 45
46 <hash_block_size> 46 <hash_block_size>
47 The size of a hash block in bytes. 47 The size of a hash block in bytes.
48 48
49 <num_data_blocks> 49 <num_data_blocks>
50 The number of data blocks on the data devi 50 The number of data blocks on the data device. Additional blocks are
51 inaccessible. You can place hashes to the 51 inaccessible. You can place hashes to the same partition as data, in this
52 case hashes are placed after <num_data_blo 52 case hashes are placed after <num_data_blocks>.
53 53
54 <hash_start_block> 54 <hash_start_block>
55 This is the offset, in <hash_block_size>-b 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. 56 to the root block of the hash tree.
57 57
58 <algorithm> 58 <algorithm>
59 The cryptographic hash algorithm used for 59 The cryptographic hash algorithm used for this device. This should
60 be the name of the algorithm, like "sha1". 60 be the name of the algorithm, like "sha1".
61 61
62 <digest> 62 <digest>
63 The hexadecimal encoding of the cryptograp 63 The hexadecimal encoding of the cryptographic hash of the root hash block
64 and the salt. This hash should be trusted 64 and the salt. This hash should be trusted as there is no other authenticity
65 beyond this point. 65 beyond this point.
66 66
67 <salt> 67 <salt>
68 The hexadecimal encoding of the salt value 68 The hexadecimal encoding of the salt value.
69 69
70 <#opt_params> 70 <#opt_params>
71 Number of optional parameters. If there ar 71 Number of optional parameters. If there are no optional parameters,
72 the optional parameters section can be ski 72 the optional parameters section can be skipped or #opt_params can be zero.
73 Otherwise #opt_params is the number of fol 73 Otherwise #opt_params is the number of following arguments.
74 74
75 Example of optional parameters section: 75 Example of optional parameters section:
76 1 ignore_corruption 76 1 ignore_corruption
77 77
78 ignore_corruption 78 ignore_corruption
79 Log corrupted blocks, but allow read opera 79 Log corrupted blocks, but allow read operations to proceed normally.
80 80
81 restart_on_corruption 81 restart_on_corruption
82 Restart the system when a corrupted block 82 Restart the system when a corrupted block is discovered. This option is
83 not compatible with ignore_corruption and 83 not compatible with ignore_corruption and requires user space support to
84 avoid restart loops. 84 avoid restart loops.
85 85
86 panic_on_corruption 86 panic_on_corruption
87 Panic the device when a corrupted block is 87 Panic the device when a corrupted block is discovered. This option is
88 not compatible with ignore_corruption and 88 not compatible with ignore_corruption and restart_on_corruption.
89 89
90 ignore_zero_blocks 90 ignore_zero_blocks
91 Do not verify blocks that are expected to 91 Do not verify blocks that are expected to contain zeroes and always return
92 zeroes instead. This may be useful if the 92 zeroes instead. This may be useful if the partition contains unused blocks
93 that are not guaranteed to contain zeroes. 93 that are not guaranteed to contain zeroes.
94 94
95 use_fec_from_device <fec_dev> 95 use_fec_from_device <fec_dev>
96 Use forward error correction (FEC) to reco 96 Use forward error correction (FEC) to recover from corruption if hash
97 verification fails. Use encoding data from 97 verification fails. Use encoding data from the specified device. This
98 may be the same device where data and hash 98 may be the same device where data and hash blocks reside, in which case
99 fec_start must be outside data and hash ar 99 fec_start must be outside data and hash areas.
100 100
101 If the encoding data covers additional met 101 If the encoding data covers additional metadata, it must be accessible
102 on the hash device after the hash blocks. 102 on the hash device after the hash blocks.
103 103
104 Note: block sizes for data and hash device 104 Note: block sizes for data and hash devices must match. Also, if the
105 verity <dev> is encrypted the <fec_dev> sh 105 verity <dev> is encrypted the <fec_dev> should be too.
106 106
107 fec_roots <num> 107 fec_roots <num>
108 Number of generator roots. This equals to 108 Number of generator roots. This equals to the number of parity bytes in
109 the encoding data. For example, in RS(M, N 109 the encoding data. For example, in RS(M, N) encoding, the number of roots
110 is M-N. 110 is M-N.
111 111
112 fec_blocks <num> 112 fec_blocks <num>
113 The number of encoding data blocks on the 113 The number of encoding data blocks on the FEC device. The block size for
114 the FEC device is <data_block_size>. 114 the FEC device is <data_block_size>.
115 115
116 fec_start <offset> 116 fec_start <offset>
117 This is the offset, in <data_block_size> b 117 This is the offset, in <data_block_size> blocks, from the start of the
118 FEC device to the beginning of the encodin 118 FEC device to the beginning of the encoding data.
119 119
120 check_at_most_once 120 check_at_most_once
121 Verify data blocks only the first time the 121 Verify data blocks only the first time they are read from the data device,
122 rather than every time. This reduces the 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 123 can be used on systems that are memory and/or CPU constrained. However, it
124 provides a reduced level of security becau 124 provides a reduced level of security because only offline tampering of the
125 data device's content will be detected, no 125 data device's content will be detected, not online tampering.
126 126
127 Hash blocks are still verified each time t 127 Hash blocks are still verified each time they are read from the hash device,
128 since verification of hash blocks is less 128 since verification of hash blocks is less performance critical than data
129 blocks, and a hash block will not be verif 129 blocks, and a hash block will not be verified any more after all the data
130 blocks it covers have been verified anyway 130 blocks it covers have been verified anyway.
131 131
132 root_hash_sig_key_desc <key_description> 132 root_hash_sig_key_desc <key_description>
133 This is the description of the USER_KEY th 133 This is the description of the USER_KEY that the kernel will lookup to get
134 the pkcs7 signature of the roothash. The p 134 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate
135 the root hash during the creation of the d 135 the root hash during the creation of the device mapper block device.
136 Verification of roothash depends on the co 136 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG
137 being set in the kernel. The signatures a 137 being set in the kernel. The signatures are checked against the builtin
138 trusted keyring by default, or the seconda 138 trusted keyring by default, or the secondary trusted keyring if
139 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KE 139 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KEYRING is set. The secondary
140 trusted keyring includes by default the bu 140 trusted keyring includes by default the builtin trusted keyring, and it can
141 also gain new certificates at run time if 141 also gain new certificates at run time if they are signed by a certificate
142 already in the secondary trusted keyring. 142 already in the secondary trusted keyring.
143 143
144 try_verify_in_tasklet 144 try_verify_in_tasklet
145 If verity hashes are in cache, verify data 145 If verity hashes are in cache, verify data blocks in kernel tasklet instead
146 of workqueue. This option can reduce IO la 146 of workqueue. This option can reduce IO latency.
147 147
148 Theory of operation 148 Theory of operation
149 =================== 149 ===================
150 150
151 dm-verity is meant to be set up as part of a v 151 dm-verity is meant to be set up as part of a verified boot path. This
152 may be anything ranging from a boot using tboo 152 may be anything ranging from a boot using tboot or trustedgrub to just
153 booting from a known-good device (like a USB d 153 booting from a known-good device (like a USB drive or CD).
154 154
155 When a dm-verity device is configured, it is e 155 When a dm-verity device is configured, it is expected that the caller
156 has been authenticated in some way (cryptograp 156 has been authenticated in some way (cryptographic signatures, etc).
157 After instantiation, all hashes will be verifi 157 After instantiation, all hashes will be verified on-demand during
158 disk access. If they cannot be verified up to 158 disk access. If they cannot be verified up to the root node of the
159 tree, the root hash, then the I/O will fail. 159 tree, the root hash, then the I/O will fail. This should detect
160 tampering with any data on the device and the 160 tampering with any data on the device and the hash data.
161 161
162 Cryptographic hashes are used to assert the in 162 Cryptographic hashes are used to assert the integrity of the device on a
163 per-block basis. This allows for a lightweight 163 per-block basis. This allows for a lightweight hash computation on first read
164 into the page cache. Block hashes are stored l 164 into the page cache. Block hashes are stored linearly, aligned to the nearest
165 block size. 165 block size.
166 166
167 If forward error correction (FEC) support is e 167 If forward error correction (FEC) support is enabled any recovery of
168 corrupted data will be verified using the cryp 168 corrupted data will be verified using the cryptographic hash of the
169 corresponding data. This is why combining erro 169 corresponding data. This is why combining error correction with
170 integrity checking is essential. 170 integrity checking is essential.
171 171
172 Hash Tree 172 Hash Tree
173 --------- 173 ---------
174 174
175 Each node in the tree is a cryptographic hash. 175 Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
176 of some data block on disk is calculated. If i 176 of some data block on disk is calculated. If it is an intermediary node,
177 the hash of a number of child nodes is calcula 177 the hash of a number of child nodes is calculated.
178 178
179 Each entry in the tree is a collection of neig 179 Each entry in the tree is a collection of neighboring nodes that fit in one
180 block. The number is determined based on bloc 180 block. The number is determined based on block_size and the size of the
181 selected cryptographic digest algorithm. The 181 selected cryptographic digest algorithm. The hashes are linearly-ordered in
182 this entry and any unaligned trailing space is 182 this entry and any unaligned trailing space is ignored but included when
183 calculating the parent node. 183 calculating the parent node.
184 184
185 The tree looks something like: 185 The tree looks something like:
186 186
187 alg = sha256, num_blocks = 32768, bloc 187 alg = sha256, num_blocks = 32768, block_size = 4096
188 188
189 :: 189 ::
190 190
191 [ root ] 191 [ root ]
192 / . . . 192 / . . . \
193 [entry_0] 193 [entry_0] [entry_1]
194 / . . . \ 194 / . . . \ . . . \
195 [entry_0_0] . . . [entry_0_127] 195 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
196 / ... \ / . . . \ 196 / ... \ / . . . \ / \
197 blk_0 ... blk_127 blk_16256 blk_16383 197 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
198 198
199 199
200 On-disk format 200 On-disk format
201 ============== 201 ==============
202 202
203 The verity kernel code does not read the verit 203 The verity kernel code does not read the verity metadata on-disk header.
204 It only reads the hash blocks which directly f 204 It only reads the hash blocks which directly follow the header.
205 It is expected that a user-space tool will ver 205 It is expected that a user-space tool will verify the integrity of the
206 verity header. 206 verity header.
207 207
208 Alternatively, the header can be omitted and t 208 Alternatively, the header can be omitted and the dmsetup parameters can
209 be passed via the kernel command-line in a roo 209 be passed via the kernel command-line in a rooted chain of trust where
210 the command-line is verified. 210 the command-line is verified.
211 211
212 Directly following the header (and with sector 212 Directly following the header (and with sector number padded to the next hash
213 block boundary) are the hash blocks which are 213 block boundary) are the hash blocks which are stored a depth at a time
214 (starting from the root), sorted in order of i 214 (starting from the root), sorted in order of increasing index.
215 215
216 The full specification of kernel parameters an 216 The full specification of kernel parameters and on-disk metadata format
217 is available at the cryptsetup project's wiki 217 is available at the cryptsetup project's wiki page
218 218
219 https://gitlab.com/cryptsetup/cryptsetup/wik 219 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
220 220
221 Status 221 Status
222 ====== 222 ======
223 V (for Valid) is returned if every check perfo 223 V (for Valid) is returned if every check performed so far was valid.
224 If any check failed, C (for Corruption) is ret 224 If any check failed, C (for Corruption) is returned.
225 225
226 Example 226 Example
227 ======= 227 =======
228 Set up a device:: 228 Set up a device::
229 229
230 # dmsetup create vroot --readonly --table \ 230 # dmsetup create vroot --readonly --table \
231 "0 2097152 verity 1 /dev/sda1 /dev/sda2 40 231 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
232 "4392712ba01368efdf14b05c76f9e4df0d5366463 232 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
233 "12340000000000000000000000000000000000000 233 "1234000000000000000000000000000000000000000000000000000000000000"
234 234
235 A command line tool veritysetup is available t 235 A command line tool veritysetup is available to compute or verify
236 the hash tree or activate the kernel device. T 236 the hash tree or activate the kernel device. This is available from
237 the cryptsetup upstream repository https://git 237 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
238 (as a libcryptsetup extension). 238 (as a libcryptsetup extension).
239 239
240 Create hash on the device:: 240 Create hash on the device::
241 241
242 # veritysetup format /dev/sda1 /dev/sda2 242 # veritysetup format /dev/sda1 /dev/sda2
243 ... 243 ...
244 Root hash: 4392712ba01368efdf14b05c76f9e4df0 244 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
245 245
246 Activate the device:: 246 Activate the device::
247 247
248 # veritysetup create vroot /dev/sda1 /dev/sd 248 # veritysetup create vroot /dev/sda1 /dev/sda2 \
249 4392712ba01368efdf14b05c76f9e4df0d53664630 249 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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