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 paramaters 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 <<
87 Panic the device when a corrupted block is <<
88 not compatible with ignore_corruption and <<
89 <<
90 ignore_zero_blocks 86 ignore_zero_blocks
91 Do not verify blocks that are expected to 87 Do not verify blocks that are expected to contain zeroes and always return
92 zeroes instead. This may be useful if the 88 zeroes instead. This may be useful if the partition contains unused blocks
93 that are not guaranteed to contain zeroes. 89 that are not guaranteed to contain zeroes.
94 90
95 use_fec_from_device <fec_dev> 91 use_fec_from_device <fec_dev>
96 Use forward error correction (FEC) to reco 92 Use forward error correction (FEC) to recover from corruption if hash
97 verification fails. Use encoding data from 93 verification fails. Use encoding data from the specified device. This
98 may be the same device where data and hash 94 may be the same device where data and hash blocks reside, in which case
99 fec_start must be outside data and hash ar 95 fec_start must be outside data and hash areas.
100 96
101 If the encoding data covers additional met 97 If the encoding data covers additional metadata, it must be accessible
102 on the hash device after the hash blocks. 98 on the hash device after the hash blocks.
103 99
104 Note: block sizes for data and hash device 100 Note: block sizes for data and hash devices must match. Also, if the
105 verity <dev> is encrypted the <fec_dev> sh 101 verity <dev> is encrypted the <fec_dev> should be too.
106 102
107 fec_roots <num> 103 fec_roots <num>
108 Number of generator roots. This equals to 104 Number of generator roots. This equals to the number of parity bytes in
109 the encoding data. For example, in RS(M, N 105 the encoding data. For example, in RS(M, N) encoding, the number of roots
110 is M-N. 106 is M-N.
111 107
112 fec_blocks <num> 108 fec_blocks <num>
113 The number of encoding data blocks on the 109 The number of encoding data blocks on the FEC device. The block size for
114 the FEC device is <data_block_size>. 110 the FEC device is <data_block_size>.
115 111
116 fec_start <offset> 112 fec_start <offset>
117 This is the offset, in <data_block_size> b 113 This is the offset, in <data_block_size> blocks, from the start of the
118 FEC device to the beginning of the encodin 114 FEC device to the beginning of the encoding data.
119 115
120 check_at_most_once 116 check_at_most_once
121 Verify data blocks only the first time the 117 Verify data blocks only the first time they are read from the data device,
122 rather than every time. This reduces the 118 rather than every time. This reduces the overhead of dm-verity so that it
123 can be used on systems that are memory and 119 can be used on systems that are memory and/or CPU constrained. However, it
124 provides a reduced level of security becau 120 provides a reduced level of security because only offline tampering of the
125 data device's content will be detected, no 121 data device's content will be detected, not online tampering.
126 122
127 Hash blocks are still verified each time t 123 Hash blocks are still verified each time they are read from the hash device,
128 since verification of hash blocks is less 124 since verification of hash blocks is less performance critical than data
129 blocks, and a hash block will not be verif 125 blocks, and a hash block will not be verified any more after all the data
130 blocks it covers have been verified anyway 126 blocks it covers have been verified anyway.
131 127
132 root_hash_sig_key_desc <key_description> 128 root_hash_sig_key_desc <key_description>
133 This is the description of the USER_KEY th 129 This is the description of the USER_KEY that the kernel will lookup to get
134 the pkcs7 signature of the roothash. The p 130 the pkcs7 signature of the roothash. The pkcs7 signature is used to validate
135 the root hash during the creation of the d 131 the root hash during the creation of the device mapper block device.
136 Verification of roothash depends on the co 132 Verification of roothash depends on the config DM_VERITY_VERIFY_ROOTHASH_SIG
137 being set in the kernel. The signatures a !! 133 being set in the kernel.
138 trusted keyring by default, or the seconda <<
139 DM_VERITY_VERIFY_ROOTHASH_SIG_SECONDARY_KE <<
140 trusted keyring includes by default the bu <<
141 also gain new certificates at run time if <<
142 already in the secondary trusted keyring. <<
143 <<
144 try_verify_in_tasklet <<
145 If verity hashes are in cache, verify data <<
146 of workqueue. This option can reduce IO la <<
147 134
148 Theory of operation 135 Theory of operation
149 =================== 136 ===================
150 137
151 dm-verity is meant to be set up as part of a v 138 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 139 may be anything ranging from a boot using tboot or trustedgrub to just
153 booting from a known-good device (like a USB d 140 booting from a known-good device (like a USB drive or CD).
154 141
155 When a dm-verity device is configured, it is e 142 When a dm-verity device is configured, it is expected that the caller
156 has been authenticated in some way (cryptograp 143 has been authenticated in some way (cryptographic signatures, etc).
157 After instantiation, all hashes will be verifi 144 After instantiation, all hashes will be verified on-demand during
158 disk access. If they cannot be verified up to 145 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. 146 tree, the root hash, then the I/O will fail. This should detect
160 tampering with any data on the device and the 147 tampering with any data on the device and the hash data.
161 148
162 Cryptographic hashes are used to assert the in 149 Cryptographic hashes are used to assert the integrity of the device on a
163 per-block basis. This allows for a lightweight 150 per-block basis. This allows for a lightweight hash computation on first read
164 into the page cache. Block hashes are stored l 151 into the page cache. Block hashes are stored linearly, aligned to the nearest
165 block size. 152 block size.
166 153
167 If forward error correction (FEC) support is e 154 If forward error correction (FEC) support is enabled any recovery of
168 corrupted data will be verified using the cryp 155 corrupted data will be verified using the cryptographic hash of the
169 corresponding data. This is why combining erro 156 corresponding data. This is why combining error correction with
170 integrity checking is essential. 157 integrity checking is essential.
171 158
172 Hash Tree 159 Hash Tree
173 --------- 160 ---------
174 161
175 Each node in the tree is a cryptographic hash. 162 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 163 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 164 the hash of a number of child nodes is calculated.
178 165
179 Each entry in the tree is a collection of neig 166 Each entry in the tree is a collection of neighboring nodes that fit in one
180 block. The number is determined based on bloc 167 block. The number is determined based on block_size and the size of the
181 selected cryptographic digest algorithm. The 168 selected cryptographic digest algorithm. The hashes are linearly-ordered in
182 this entry and any unaligned trailing space is 169 this entry and any unaligned trailing space is ignored but included when
183 calculating the parent node. 170 calculating the parent node.
184 171
185 The tree looks something like: 172 The tree looks something like:
186 173
187 alg = sha256, num_blocks = 32768, bloc 174 alg = sha256, num_blocks = 32768, block_size = 4096
188 175
189 :: 176 ::
190 177
191 [ root ] 178 [ root ]
192 / . . . 179 / . . . \
193 [entry_0] 180 [entry_0] [entry_1]
194 / . . . \ 181 / . . . \ . . . \
195 [entry_0_0] . . . [entry_0_127] 182 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
196 / ... \ / . . . \ 183 / ... \ / . . . \ / \
197 blk_0 ... blk_127 blk_16256 blk_16383 184 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
198 185
199 186
200 On-disk format 187 On-disk format
201 ============== 188 ==============
202 189
203 The verity kernel code does not read the verit 190 The verity kernel code does not read the verity metadata on-disk header.
204 It only reads the hash blocks which directly f 191 It only reads the hash blocks which directly follow the header.
205 It is expected that a user-space tool will ver 192 It is expected that a user-space tool will verify the integrity of the
206 verity header. 193 verity header.
207 194
208 Alternatively, the header can be omitted and t 195 Alternatively, the header can be omitted and the dmsetup parameters can
209 be passed via the kernel command-line in a roo 196 be passed via the kernel command-line in a rooted chain of trust where
210 the command-line is verified. 197 the command-line is verified.
211 198
212 Directly following the header (and with sector 199 Directly following the header (and with sector number padded to the next hash
213 block boundary) are the hash blocks which are 200 block boundary) are the hash blocks which are stored a depth at a time
214 (starting from the root), sorted in order of i 201 (starting from the root), sorted in order of increasing index.
215 202
216 The full specification of kernel parameters an 203 The full specification of kernel parameters and on-disk metadata format
217 is available at the cryptsetup project's wiki 204 is available at the cryptsetup project's wiki page
218 205
219 https://gitlab.com/cryptsetup/cryptsetup/wik 206 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
220 207
221 Status 208 Status
222 ====== 209 ======
223 V (for Valid) is returned if every check perfo 210 V (for Valid) is returned if every check performed so far was valid.
224 If any check failed, C (for Corruption) is ret 211 If any check failed, C (for Corruption) is returned.
225 212
226 Example 213 Example
227 ======= 214 =======
228 Set up a device:: 215 Set up a device::
229 216
230 # dmsetup create vroot --readonly --table \ 217 # dmsetup create vroot --readonly --table \
231 "0 2097152 verity 1 /dev/sda1 /dev/sda2 40 218 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
232 "4392712ba01368efdf14b05c76f9e4df0d5366463 219 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
233 "12340000000000000000000000000000000000000 220 "1234000000000000000000000000000000000000000000000000000000000000"
234 221
235 A command line tool veritysetup is available t 222 A command line tool veritysetup is available to compute or verify
236 the hash tree or activate the kernel device. T 223 the hash tree or activate the kernel device. This is available from
237 the cryptsetup upstream repository https://git 224 the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
238 (as a libcryptsetup extension). 225 (as a libcryptsetup extension).
239 226
240 Create hash on the device:: 227 Create hash on the device::
241 228
242 # veritysetup format /dev/sda1 /dev/sda2 229 # veritysetup format /dev/sda1 /dev/sda2
243 ... 230 ...
244 Root hash: 4392712ba01368efdf14b05c76f9e4df0 231 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
245 232
246 Activate the device:: 233 Activate the device::
247 234
248 # veritysetup create vroot /dev/sda1 /dev/sd 235 # veritysetup create vroot /dev/sda1 /dev/sda2 \
249 4392712ba01368efdf14b05c76f9e4df0d53664630 236 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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