tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1dm-verity
2==========
3
4Device-Mapper's "verity" target provides transparent integrity checking of
5block devices using a cryptographic digest provided by the kernel crypto API.
6This target is read-only.
7
8Construction 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>]
15
16<version>
17 This is the type of the on-disk hash format.
18
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.
22
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.
26
27<dev>
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,
30 <major>:<minor>.
31
32<hash_dev>
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
36 dm-verity device.
37
38<data_block_size>
39 The block size on a data device in bytes.
40 Each block corresponds to one digest on the hash device.
41
42<hash_block_size>
43 The size of a hash block in bytes.
44
45<num_data_blocks>
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>.
49
50<hash_start_block>
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.
53
54<algorithm>
55 The cryptographic hash algorithm used for this device. This should
56 be the name of the algorithm, like "sha1".
57
58<digest>
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
61 beyond this point.
62
63<salt>
64 The hexadecimal encoding of the salt value.
65
66<#opt_params>
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.
70
71 Example of optional parameters section:
72 1 ignore_corruption
73
74ignore_corruption
75 Log corrupted blocks, but allow read operations to proceed normally.
76
77restart_on_corruption
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
80 avoid restart loops.
81
82ignore_zero_blocks
83 Do not verify blocks that are expected to contain zeroes and always return
84 zeroes instead. This may be useful if the partition contains unused blocks
85 that are not guaranteed to contain zeroes.
86
87use_fec_from_device <fec_dev>
88 Use forward error correction (FEC) to recover from corruption if hash
89 verification fails. Use encoding data from the specified device. This
90 may be the same device where data and hash blocks reside, in which case
91 fec_start must be outside data and hash areas.
92
93 If the encoding data covers additional metadata, it must be accessible
94 on the hash device after the hash blocks.
95
96 Note: block sizes for data and hash devices must match. Also, if the
97 verity <dev> is encrypted the <fec_dev> should be too.
98
99fec_roots <num>
100 Number of generator roots. This equals to the number of parity bytes in
101 the encoding data. For example, in RS(M, N) encoding, the number of roots
102 is M-N.
103
104fec_blocks <num>
105 The number of encoding data blocks on the FEC device. The block size for
106 the FEC device is <data_block_size>.
107
108fec_start <offset>
109 This is the offset, in <data_block_size> blocks, from the start of the
110 FEC device to the beginning of the encoding data.
111
112
113Theory of operation
114===================
115
116dm-verity is meant to be set up as part of a verified boot path. This
117may be anything ranging from a boot using tboot or trustedgrub to just
118booting from a known-good device (like a USB drive or CD).
119
120When a dm-verity device is configured, it is expected that the caller
121has been authenticated in some way (cryptographic signatures, etc).
122After instantiation, all hashes will be verified on-demand during
123disk access. If they cannot be verified up to the root node of the
124tree, the root hash, then the I/O will fail. This should detect
125tampering with any data on the device and the hash data.
126
127Cryptographic hashes are used to assert the integrity of the device on a
128per-block basis. This allows for a lightweight hash computation on first read
129into the page cache. Block hashes are stored linearly, aligned to the nearest
130block size.
131
132If forward error correction (FEC) support is enabled any recovery of
133corrupted data will be verified using the cryptographic hash of the
134corresponding data. This is why combining error correction with
135integrity checking is essential.
136
137Hash Tree
138---------
139
140Each node in the tree is a cryptographic hash. If it is a leaf node, the hash
141of some data block on disk is calculated. If it is an intermediary node,
142the hash of a number of child nodes is calculated.
143
144Each entry in the tree is a collection of neighboring nodes that fit in one
145block. The number is determined based on block_size and the size of the
146selected cryptographic digest algorithm. The hashes are linearly-ordered in
147this entry and any unaligned trailing space is ignored but included when
148calculating the parent node.
149
150The tree looks something like:
151
152alg = sha256, num_blocks = 32768, block_size = 4096
153
154 [ root ]
155 / . . . \
156 [entry_0] [entry_1]
157 / . . . \ . . . \
158 [entry_0_0] . . . [entry_0_127] . . . . [entry_1_127]
159 / ... \ / . . . \ / \
160 blk_0 ... blk_127 blk_16256 blk_16383 blk_32640 . . . blk_32767
161
162
163On-disk format
164==============
165
166The verity kernel code does not read the verity metadata on-disk header.
167It only reads the hash blocks which directly follow the header.
168It is expected that a user-space tool will verify the integrity of the
169verity header.
170
171Alternatively, the header can be omitted and the dmsetup parameters can
172be passed via the kernel command-line in a rooted chain of trust where
173the command-line is verified.
174
175Directly following the header (and with sector number padded to the next hash
176block boundary) are the hash blocks which are stored a depth at a time
177(starting from the root), sorted in order of increasing index.
178
179The full specification of kernel parameters and on-disk metadata format
180is available at the cryptsetup project's wiki page
181 https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
182
183Status
184======
185V (for Valid) is returned if every check performed so far was valid.
186If any check failed, C (for Corruption) is returned.
187
188Example
189=======
190Set up a device:
191 # dmsetup create vroot --readonly --table \
192 "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
193 "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
194 "1234000000000000000000000000000000000000000000000000000000000000"
195
196A command line tool veritysetup is available to compute or verify
197the hash tree or activate the kernel device. This is available from
198the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
199(as a libcryptsetup extension).
200
201Create hash on the device:
202 # veritysetup format /dev/sda1 /dev/sda2
203 ...
204 Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
205
206Activate the device:
207 # veritysetup create vroot /dev/sda1 /dev/sda2 \
208 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076