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 zeros.
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 zeros 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
82Theory of operation
83===================
84
85dm-verity is meant to be set up as part of a verified boot path.  This
86may be anything ranging from a boot using tboot or trustedgrub to just
87booting from a known-good device (like a USB drive or CD).
88
89When a dm-verity device is configured, it is expected that the caller
90has been authenticated in some way (cryptographic signatures, etc).
91After instantiation, all hashes will be verified on-demand during
92disk access.  If they cannot be verified up to the root node of the
93tree, the root hash, then the I/O will fail.  This should detect
94tampering with any data on the device and the hash data.
95
96Cryptographic hashes are used to assert the integrity of the device on a
97per-block basis. This allows for a lightweight hash computation on first read
98into the page cache. Block hashes are stored linearly, aligned to the nearest
99block size.
100
101Hash Tree
102---------
103
104Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
105of some data block on disk is calculated. If it is an intermediary node,
106the hash of a number of child nodes is calculated.
107
108Each entry in the tree is a collection of neighboring nodes that fit in one
109block.  The number is determined based on block_size and the size of the
110selected cryptographic digest algorithm.  The hashes are linearly-ordered in
111this entry and any unaligned trailing space is ignored but included when
112calculating the parent node.
113
114The tree looks something like:
115
116alg = sha256, num_blocks = 32768, block_size = 4096
117
118                                 [   root    ]
119                                /    . . .    \
120                     [entry_0]                 [entry_1]
121                    /  . . .  \                 . . .   \
122         [entry_0_0]   . . .  [entry_0_127]    . . . .  [entry_1_127]
123           / ... \             /   . . .  \             /           \
124     blk_0 ... blk_127  blk_16256   blk_16383      blk_32640 . . . blk_32767
125
126
127On-disk format
128==============
129
130The verity kernel code does not read the verity metadata on-disk header.
131It only reads the hash blocks which directly follow the header.
132It is expected that a user-space tool will verify the integrity of the
133verity header.
134
135Alternatively, the header can be omitted and the dmsetup parameters can
136be passed via the kernel command-line in a rooted chain of trust where
137the command-line is verified.
138
139Directly following the header (and with sector number padded to the next hash
140block boundary) are the hash blocks which are stored a depth at a time
141(starting from the root), sorted in order of increasing index.
142
143The full specification of kernel parameters and on-disk metadata format
144is available at the cryptsetup project's wiki page
145  https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
146
147Status
148======
149V (for Valid) is returned if every check performed so far was valid.
150If any check failed, C (for Corruption) is returned.
151
152Example
153=======
154Set up a device:
155  # dmsetup create vroot --readonly --table \
156    "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
157    "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
158    "1234000000000000000000000000000000000000000000000000000000000000"
159
160A command line tool veritysetup is available to compute or verify
161the hash tree or activate the kernel device. This is available from
162the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
163(as a libcryptsetup extension).
164
165Create hash on the device:
166  # veritysetup format /dev/sda1 /dev/sda2
167  ...
168  Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
169
170Activate the device:
171  # veritysetup create vroot /dev/sda1 /dev/sda2 \
172    4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
173