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Documentation / device-mapper / dm-integrity.txt




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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.

1	The dm-integrity target emulates a block device that has additional
2	per-sector tags that can be used for storing integrity information.
3	
4	A general problem with storing integrity tags with every sector is that
5	writing the sector and the integrity tag must be atomic - i.e. in case of
6	crash, either both sector and integrity tag or none of them is written.
7	
8	To guarantee write atomicity, the dm-integrity target uses journal, it
9	writes sector data and integrity tags into a journal, commits the journal
10	and then copies the data and integrity tags to their respective location.
11	
12	The dm-integrity target can be used with the dm-crypt target - in this
13	situation the dm-crypt target creates the integrity data and passes them
14	to the dm-integrity target via bio_integrity_payload attached to the bio.
15	In this mode, the dm-crypt and dm-integrity targets provide authenticated
16	disk encryption - if the attacker modifies the encrypted device, an I/O
17	error is returned instead of random data.
18	
19	The dm-integrity target can also be used as a standalone target, in this
20	mode it calculates and verifies the integrity tag internally. In this
21	mode, the dm-integrity target can be used to detect silent data
22	corruption on the disk or in the I/O path.
23	
24	
25	When loading the target for the first time, the kernel driver will format
26	the device. But it will only format the device if the superblock contains
27	zeroes. If the superblock is neither valid nor zeroed, the dm-integrity
28	target can't be loaded.
29	
30	To use the target for the first time:
31	1. overwrite the superblock with zeroes
32	2. load the dm-integrity target with one-sector size, the kernel driver
33		will format the device
34	3. unload the dm-integrity target
35	4. read the "provided_data_sectors" value from the superblock
36	5. load the dm-integrity target with the the target size
37		"provided_data_sectors"
38	6. if you want to use dm-integrity with dm-crypt, load the dm-crypt target
39		with the size "provided_data_sectors"
40	
41	
42	Target arguments:
43	
44	1. the underlying block device
45	
46	2. the number of reserved sector at the beginning of the device - the
47		dm-integrity won't read of write these sectors
48	
49	3. the size of the integrity tag (if "-" is used, the size is taken from
50		the internal-hash algorithm)
51	
52	4. mode:
53		D - direct writes (without journal) - in this mode, journaling is
54			not used and data sectors and integrity tags are written
55			separately. In case of crash, it is possible that the data
56			and integrity tag doesn't match.
57		J - journaled writes - data and integrity tags are written to the
58			journal and atomicity is guaranteed. In case of crash,
59			either both data and tag or none of them are written. The
60			journaled mode degrades write throughput twice because the
61			data have to be written twice.
62		R - recovery mode - in this mode, journal is not replayed,
63			checksums are not checked and writes to the device are not
64			allowed. This mode is useful for data recovery if the
65			device cannot be activated in any of the other standard
66			modes.
67	
68	5. the number of additional arguments
69	
70	Additional arguments:
71	
72	journal_sectors:number
73		The size of journal, this argument is used only if formatting the
74		device. If the device is already formatted, the value from the
75		superblock is used.
76	
77	interleave_sectors:number
78		The number of interleaved sectors. This values is rounded down to
79		a power of two. If the device is already formatted, the value from
80		the superblock is used.
81	
82	buffer_sectors:number
83		The number of sectors in one buffer. The value is rounded down to
84		a power of two.
85	
86		The tag area is accessed using buffers, the buffer size is
87		configurable. The large buffer size means that the I/O size will
88		be larger, but there could be less I/Os issued.
89	
90	journal_watermark:number
91		The journal watermark in percents. When the size of the journal
92		exceeds this watermark, the thread that flushes the journal will
93		be started.
94	
95	commit_time:number
96		Commit time in milliseconds. When this time passes, the journal is
97		written. The journal is also written immediatelly if the FLUSH
98		request is received.
99	
100	internal_hash:algorithm(:key)	(the key is optional)
101		Use internal hash or crc.
102		When this argument is used, the dm-integrity target won't accept
103		integrity tags from the upper target, but it will automatically
104		generate and verify the integrity tags.
105	
106		You can use a crc algorithm (such as crc32), then integrity target
107		will protect the data against accidental corruption.
108		You can also use a hmac algorithm (for example
109		"hmac(sha256):0123456789abcdef"), in this mode it will provide
110		cryptographic authentication of the data without encryption.
111	
112		When this argument is not used, the integrity tags are accepted
113		from an upper layer target, such as dm-crypt. The upper layer
114		target should check the validity of the integrity tags.
115	
116	journal_crypt:algorithm(:key)	(the key is optional)
117		Encrypt the journal using given algorithm to make sure that the
118		attacker can't read the journal. You can use a block cipher here
119		(such as "cbc(aes)") or a stream cipher (for example "chacha20",
120		"salsa20", "ctr(aes)" or "ecb(arc4)").
121	
122		The journal contains history of last writes to the block device,
123		an attacker reading the journal could see the last sector nubmers
124		that were written. From the sector numbers, the attacker can infer
125		the size of files that were written. To protect against this
126		situation, you can encrypt the journal.
127	
128	journal_mac:algorithm(:key)	(the key is optional)
129		Protect sector numbers in the journal from accidental or malicious
130		modification. To protect against accidental modification, use a
131		crc algorithm, to protect against malicious modification, use a
132		hmac algorithm with a key.
133	
134		This option is not needed when using internal-hash because in this
135		mode, the integrity of journal entries is checked when replaying
136		the journal. Thus, modified sector number would be detected at
137		this stage.
138	
139	block_size:number
140		The size of a data block in bytes.  The larger the block size the
141		less overhead there is for per-block integrity metadata.
142		Supported values are 512, 1024, 2048 and 4096 bytes.  If not
143		specified the default block size is 512 bytes.
144	
145	The journal mode (D/J), buffer_sectors, journal_watermark, commit_time can
146	be changed when reloading the target (load an inactive table and swap the
147	tables with suspend and resume). The other arguments should not be changed
148	when reloading the target because the layout of disk data depend on them
149	and the reloaded target would be non-functional.
150	
151	
152	The layout of the formatted block device:
153	* reserved sectors (they are not used by this target, they can be used for
154	  storing LUKS metadata or for other purpose), the size of the reserved
155	  area is specified in the target arguments
156	* superblock (4kiB)
157		* magic string - identifies that the device was formatted
158		* version
159		* log2(interleave sectors)
160		* integrity tag size
161		* the number of journal sections
162		* provided data sectors - the number of sectors that this target
163		  provides (i.e. the size of the device minus the size of all
164		  metadata and padding). The user of this target should not send
165		  bios that access data beyond the "provided data sectors" limit.
166		* flags - a flag is set if journal_mac is used
167	* journal
168		The journal is divided into sections, each section contains:
169		* metadata area (4kiB), it contains journal entries
170		  every journal entry contains:
171			* logical sector (specifies where the data and tag should
172			  be written)
173			* last 8 bytes of data
174			* integrity tag (the size is specified in the superblock)
175		    every metadata sector ends with
176			* mac (8-bytes), all the macs in 8 metadata sectors form a
177			  64-byte value. It is used to store hmac of sector
178			  numbers in the journal section, to protect against a
179			  possibility that the attacker tampers with sector
180			  numbers in the journal.
181			* commit id
182		* data area (the size is variable; it depends on how many journal
183		  entries fit into the metadata area)
184		    every sector in the data area contains:
185			* data (504 bytes of data, the last 8 bytes are stored in
186			  the journal entry)
187			* commit id
188		To test if the whole journal section was written correctly, every
189		512-byte sector of the journal ends with 8-byte commit id. If the
190		commit id matches on all sectors in a journal section, then it is
191		assumed that the section was written correctly. If the commit id
192		doesn't match, the section was written partially and it should not
193		be replayed.
194	* one or more runs of interleaved tags and data. Each run contains:
195		* tag area - it contains integrity tags. There is one tag for each
196		  sector in the data area
197		* data area - it contains data sectors. The number of data sectors
198		  in one run must be a power of two. log2 of this value is stored
199		  in the superblock.
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