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Documentation / device-mapper / verity.txt




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Based on kernel version 3.9. Page generated on 2013-05-02 22:56 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	
15	<version>
16	    This is the type of the on-disk hash format.
17	
18	    0 is the original format used in the Chromium OS.
19	      The salt is appended when hashing, digests are stored continuously and
20	      the rest of the block is padded with zeros.
21	
22	    1 is the current format that should be used for new devices.
23	      The salt is prepended when hashing and each digest is
24	      padded with zeros to the power of two.
25	
26	<dev>
27	    This is the device containing data, the integrity of which needs to be
28	    checked.  It may be specified as a path, like /dev/sdaX, or a device number,
29	    <major>:<minor>.
30	
31	<hash_dev>
32	    This is the device that supplies the hash tree data.  It may be
33	    specified similarly to the device path and may be the same device.  If the
34	    same device is used, the hash_start should be outside the configured
35	    dm-verity device.
36	
37	<data_block_size>
38	    The block size on a data device in bytes.
39	    Each block corresponds to one digest on the hash device.
40	
41	<hash_block_size>
42	    The size of a hash block in bytes.
43	
44	<num_data_blocks>
45	    The number of data blocks on the data device.  Additional blocks are
46	    inaccessible.  You can place hashes to the same partition as data, in this
47	    case hashes are placed after <num_data_blocks>.
48	
49	<hash_start_block>
50	    This is the offset, in <hash_block_size>-blocks, from the start of hash_dev
51	    to the root block of the hash tree.
52	
53	<algorithm>
54	    The cryptographic hash algorithm used for this device.  This should
55	    be the name of the algorithm, like "sha1".
56	
57	<digest>
58	    The hexadecimal encoding of the cryptographic hash of the root hash block
59	    and the salt.  This hash should be trusted as there is no other authenticity
60	    beyond this point.
61	
62	<salt>
63	    The hexadecimal encoding of the salt value.
64	
65	Theory of operation
66	===================
67	
68	dm-verity is meant to be set up as part of a verified boot path.  This
69	may be anything ranging from a boot using tboot or trustedgrub to just
70	booting from a known-good device (like a USB drive or CD).
71	
72	When a dm-verity device is configured, it is expected that the caller
73	has been authenticated in some way (cryptographic signatures, etc).
74	After instantiation, all hashes will be verified on-demand during
75	disk access.  If they cannot be verified up to the root node of the
76	tree, the root hash, then the I/O will fail.  This should detect
77	tampering with any data on the device and the hash data.
78	
79	Cryptographic hashes are used to assert the integrity of the device on a
80	per-block basis. This allows for a lightweight hash computation on first read
81	into the page cache. Block hashes are stored linearly, aligned to the nearest
82	block size.
83	
84	Hash Tree
85	---------
86	
87	Each node in the tree is a cryptographic hash.  If it is a leaf node, the hash
88	of some data block on disk is calculated. If it is an intermediary node,
89	the hash of a number of child nodes is calculated.
90	
91	Each entry in the tree is a collection of neighboring nodes that fit in one
92	block.  The number is determined based on block_size and the size of the
93	selected cryptographic digest algorithm.  The hashes are linearly-ordered in
94	this entry and any unaligned trailing space is ignored but included when
95	calculating the parent node.
96	
97	The tree looks something like:
98	
99	alg = sha256, num_blocks = 32768, block_size = 4096
100	
101	                                 [   root    ]
102	                                /    . . .    \
103	                     [entry_0]                 [entry_1]
104	                    /  . . .  \                 . . .   \
105	         [entry_0_0]   . . .  [entry_0_127]    . . . .  [entry_1_127]
106	           / ... \             /   . . .  \             /           \
107	     blk_0 ... blk_127  blk_16256   blk_16383      blk_32640 . . . blk_32767
108	
109	
110	On-disk format
111	==============
112	
113	The verity kernel code does not read the verity metadata on-disk header.
114	It only reads the hash blocks which directly follow the header.
115	It is expected that a user-space tool will verify the integrity of the
116	verity header.
117	
118	Alternatively, the header can be omitted and the dmsetup parameters can
119	be passed via the kernel command-line in a rooted chain of trust where
120	the command-line is verified.
121	
122	Directly following the header (and with sector number padded to the next hash
123	block boundary) are the hash blocks which are stored a depth at a time
124	(starting from the root), sorted in order of increasing index.
125	
126	The full specification of kernel parameters and on-disk metadata format
127	is available at the cryptsetup project's wiki page
128	  http://code.google.com/p/cryptsetup/wiki/DMVerity
129	
130	Status
131	======
132	V (for Valid) is returned if every check performed so far was valid.
133	If any check failed, C (for Corruption) is returned.
134	
135	Example
136	=======
137	Set up a device:
138	  # dmsetup create vroot --readonly --table \
139	    "0 2097152 verity 1 /dev/sda1 /dev/sda2 4096 4096 262144 1 sha256 "\
140	    "4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076 "\
141	    "1234000000000000000000000000000000000000000000000000000000000000"
142	
143	A command line tool veritysetup is available to compute or verify
144	the hash tree or activate the kernel device. This is available from
145	the cryptsetup upstream repository http://code.google.com/p/cryptsetup/
146	(as a libcryptsetup extension).
147	
148	Create hash on the device:
149	  # veritysetup format /dev/sda1 /dev/sda2
150	  ...
151	  Root hash: 4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
152	
153	Activate the device:
154	  # veritysetup create vroot /dev/sda1 /dev/sda2 \
155	    4392712ba01368efdf14b05c76f9e4df0d53664630b5d48632ed17a137f39076
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