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Documentation / device-mapper / verity.txt




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Based on kernel version 4.1. Page generated on 2015-06-28 12:08 EST.

1	dm-verity
2	==========
3	
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.
7	
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>]
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	
74	ignore_corruption
75	    Log corrupted blocks, but allow read operations to proceed normally.
76	
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.
81	
82	Theory of operation
83	===================
84	
85	dm-verity is meant to be set up as part of a verified boot path.  This
86	may be anything ranging from a boot using tboot or trustedgrub to just
87	booting from a known-good device (like a USB drive or CD).
88	
89	When a dm-verity device is configured, it is expected that the caller
90	has been authenticated in some way (cryptographic signatures, etc).
91	After instantiation, all hashes will be verified on-demand during
92	disk access.  If they cannot be verified up to the root node of the
93	tree, the root hash, then the I/O will fail.  This should detect
94	tampering with any data on the device and the hash data.
95	
96	Cryptographic hashes are used to assert the integrity of the device on a
97	per-block basis. This allows for a lightweight hash computation on first read
98	into the page cache. Block hashes are stored linearly, aligned to the nearest
99	block size.
100	
101	Hash Tree
102	---------
103	
104	Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
105	of some data block on disk is calculated. If it is an intermediary node,
106	the hash of a number of child nodes is calculated.
107	
108	Each entry in the tree is a collection of neighboring nodes that fit in one
109	block.  The number is determined based on block_size and the size of the
110	selected cryptographic digest algorithm.  The hashes are linearly-ordered in
111	this entry and any unaligned trailing space is ignored but included when
112	calculating the parent node.
113	
114	The tree looks something like:
115	
116	alg = 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	
127	On-disk format
128	==============
129	
130	The verity kernel code does not read the verity metadata on-disk header.
131	It only reads the hash blocks which directly follow the header.
132	It is expected that a user-space tool will verify the integrity of the
133	verity header.
134	
135	Alternatively, the header can be omitted and the dmsetup parameters can
136	be passed via the kernel command-line in a rooted chain of trust where
137	the command-line is verified.
138	
139	Directly following the header (and with sector number padded to the next hash
140	block 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	
143	The full specification of kernel parameters and on-disk metadata format
144	is available at the cryptsetup project's wiki page
145	  https://gitlab.com/cryptsetup/cryptsetup/wikis/DMVerity
146	
147	Status
148	======
149	V (for Valid) is returned if every check performed so far was valid.
150	If any check failed, C (for Corruption) is returned.
151	
152	Example
153	=======
154	Set 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	
160	A command line tool veritysetup is available to compute or verify
161	the hash tree or activate the kernel device. This is available from
162	the cryptsetup upstream repository https://gitlab.com/cryptsetup/cryptsetup/
163	(as a libcryptsetup extension).
164	
165	Create hash on the device:
166	  # veritysetup format /dev/sda1 /dev/sda2
167	  ...
168	  Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
169	
170	Activate the device:
171	  # veritysetup create vroot /dev/sda1 /dev/sda2 \
172	    4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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