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Documentation / device-mapper / verity.txt

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Based on kernel version 4.8. Page generated on 2016-10-06 23:10 EST.

1	dm-verity
2	==========
4	Device-Mapper's "verity" target provides transparent integrity checking of
5	block devices using a cryptographic digest provided by the kernel crypto API.
6	This target is read-only.
8	Construction 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>]
16	<version>
17	    This is the type of the on-disk hash format.
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.
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.
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>.
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.
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.
42	<hash_block_size>
43	    The size of a hash block in bytes.
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>.
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.
54	<algorithm>
55	    The cryptographic hash algorithm used for this device.  This should
56	    be the name of the algorithm, like "sha1".
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.
63	<salt>
64	    The hexadecimal encoding of the salt value.
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.
71	    Example of optional parameters section:
72	        1 ignore_corruption
74	ignore_corruption
75	    Log corrupted blocks, but allow read operations to proceed normally.
77	restart_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.
82	ignore_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.
87	use_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.
93	    If the encoding data covers additional metadata, it must be accessible
94	    on the hash device after the hash blocks.
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.
99	fec_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.
104	fec_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>.
108	fec_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.
113	Theory of operation
114	===================
116	dm-verity is meant to be set up as part of a verified boot path.  This
117	may be anything ranging from a boot using tboot or trustedgrub to just
118	booting from a known-good device (like a USB drive or CD).
120	When a dm-verity device is configured, it is expected that the caller
121	has been authenticated in some way (cryptographic signatures, etc).
122	After instantiation, all hashes will be verified on-demand during
123	disk access.  If they cannot be verified up to the root node of the
124	tree, the root hash, then the I/O will fail.  This should detect
125	tampering with any data on the device and the hash data.
127	Cryptographic hashes are used to assert the integrity of the device on a
128	per-block basis. This allows for a lightweight hash computation on first read
129	into the page cache. Block hashes are stored linearly, aligned to the nearest
130	block size.
132	If forward error correction (FEC) support is enabled any recovery of
133	corrupted data will be verified using the cryptographic hash of the
134	corresponding data. This is why combining error correction with
135	integrity checking is essential.
137	Hash Tree
138	---------
140	Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
141	of some data block on disk is calculated. If it is an intermediary node,
142	the hash of a number of child nodes is calculated.
144	Each entry in the tree is a collection of neighboring nodes that fit in one
145	block.  The number is determined based on block_size and the size of the
146	selected cryptographic digest algorithm.  The hashes are linearly-ordered in
147	this entry and any unaligned trailing space is ignored but included when
148	calculating the parent node.
150	The tree looks something like:
152	alg = sha256, num_blocks = 32768, block_size = 4096
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
163	On-disk format
164	==============
166	The verity kernel code does not read the verity metadata on-disk header.
167	It only reads the hash blocks which directly follow the header.
168	It is expected that a user-space tool will verify the integrity of the
169	verity header.
171	Alternatively, the header can be omitted and the dmsetup parameters can
172	be passed via the kernel command-line in a rooted chain of trust where
173	the command-line is verified.
175	Directly following the header (and with sector number padded to the next hash
176	block boundary) are the hash blocks which are stored a depth at a time
177	(starting from the root), sorted in order of increasing index.
179	The full specification of kernel parameters and on-disk metadata format
180	is available at the cryptsetup project's wiki page
181	  https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
183	Status
184	======
185	V (for Valid) is returned if every check performed so far was valid.
186	If any check failed, C (for Corruption) is returned.
188	Example
189	=======
190	Set 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"
196	A command line tool veritysetup is available to compute or verify
197	the hash tree or activate the kernel device. This is available from
198	the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
199	(as a libcryptsetup extension).
201	Create hash on the device:
202	  # veritysetup format /dev/sda1 /dev/sda2
203	  ...
204	  Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
206	Activate the device:
207	  # veritysetup create vroot /dev/sda1 /dev/sda2 \
208	    4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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