xref: /linux/Documentation/admin-guide/device-mapper/dm-integrity.rst (revision 26fbb4c8c7c3ee9a4c3b4de555a8587b5a19154e)
1============
2dm-integrity
3============
4
5The dm-integrity target emulates a block device that has additional
6per-sector tags that can be used for storing integrity information.
7
8A general problem with storing integrity tags with every sector is that
9writing the sector and the integrity tag must be atomic - i.e. in case of
10crash, either both sector and integrity tag or none of them is written.
11
12To guarantee write atomicity, the dm-integrity target uses journal, it
13writes sector data and integrity tags into a journal, commits the journal
14and then copies the data and integrity tags to their respective location.
15
16The dm-integrity target can be used with the dm-crypt target - in this
17situation the dm-crypt target creates the integrity data and passes them
18to the dm-integrity target via bio_integrity_payload attached to the bio.
19In this mode, the dm-crypt and dm-integrity targets provide authenticated
20disk encryption - if the attacker modifies the encrypted device, an I/O
21error is returned instead of random data.
22
23The dm-integrity target can also be used as a standalone target, in this
24mode it calculates and verifies the integrity tag internally. In this
25mode, the dm-integrity target can be used to detect silent data
26corruption on the disk or in the I/O path.
27
28There's an alternate mode of operation where dm-integrity uses bitmap
29instead of a journal. If a bit in the bitmap is 1, the corresponding
30region's data and integrity tags are not synchronized - if the machine
31crashes, the unsynchronized regions will be recalculated. The bitmap mode
32is faster than the journal mode, because we don't have to write the data
33twice, but it is also less reliable, because if data corruption happens
34when the machine crashes, it may not be detected.
35
36When loading the target for the first time, the kernel driver will format
37the device. But it will only format the device if the superblock contains
38zeroes. If the superblock is neither valid nor zeroed, the dm-integrity
39target can't be loaded.
40
41To use the target for the first time:
42
431. overwrite the superblock with zeroes
442. load the dm-integrity target with one-sector size, the kernel driver
45   will format the device
463. unload the dm-integrity target
474. read the "provided_data_sectors" value from the superblock
485. load the dm-integrity target with the target size
49   "provided_data_sectors"
506. if you want to use dm-integrity with dm-crypt, load the dm-crypt target
51   with the size "provided_data_sectors"
52
53
54Target arguments:
55
561. the underlying block device
57
582. the number of reserved sector at the beginning of the device - the
59   dm-integrity won't read of write these sectors
60
613. the size of the integrity tag (if "-" is used, the size is taken from
62   the internal-hash algorithm)
63
644. mode:
65
66	D - direct writes (without journal)
67		in this mode, journaling is
68		not used and data sectors and integrity tags are written
69		separately. In case of crash, it is possible that the data
70		and integrity tag doesn't match.
71	J - journaled writes
72		data and integrity tags are written to the
73		journal and atomicity is guaranteed. In case of crash,
74		either both data and tag or none of them are written. The
75		journaled mode degrades write throughput twice because the
76		data have to be written twice.
77	B - bitmap mode - data and metadata are written without any
78		synchronization, the driver maintains a bitmap of dirty
79		regions where data and metadata don't match. This mode can
80		only be used with internal hash.
81	R - recovery mode - in this mode, journal is not replayed,
82		checksums are not checked and writes to the device are not
83		allowed. This mode is useful for data recovery if the
84		device cannot be activated in any of the other standard
85		modes.
86
875. the number of additional arguments
88
89Additional arguments:
90
91journal_sectors:number
92	The size of journal, this argument is used only if formatting the
93	device. If the device is already formatted, the value from the
94	superblock is used.
95
96interleave_sectors:number
97	The number of interleaved sectors. This values is rounded down to
98	a power of two. If the device is already formatted, the value from
99	the superblock is used.
100
101meta_device:device
102	Don't interleave the data and metadata on the device. Use a
103	separate device for metadata.
104
105buffer_sectors:number
106	The number of sectors in one buffer. The value is rounded down to
107	a power of two.
108
109	The tag area is accessed using buffers, the buffer size is
110	configurable. The large buffer size means that the I/O size will
111	be larger, but there could be less I/Os issued.
112
113journal_watermark:number
114	The journal watermark in percents. When the size of the journal
115	exceeds this watermark, the thread that flushes the journal will
116	be started.
117
118commit_time:number
119	Commit time in milliseconds. When this time passes, the journal is
120	written. The journal is also written immediately if the FLUSH
121	request is received.
122
123internal_hash:algorithm(:key)	(the key is optional)
124	Use internal hash or crc.
125	When this argument is used, the dm-integrity target won't accept
126	integrity tags from the upper target, but it will automatically
127	generate and verify the integrity tags.
128
129	You can use a crc algorithm (such as crc32), then integrity target
130	will protect the data against accidental corruption.
131	You can also use a hmac algorithm (for example
132	"hmac(sha256):0123456789abcdef"), in this mode it will provide
133	cryptographic authentication of the data without encryption.
134
135	When this argument is not used, the integrity tags are accepted
136	from an upper layer target, such as dm-crypt. The upper layer
137	target should check the validity of the integrity tags.
138
139recalculate
140	Recalculate the integrity tags automatically. It is only valid
141	when using internal hash.
142
143journal_crypt:algorithm(:key)	(the key is optional)
144	Encrypt the journal using given algorithm to make sure that the
145	attacker can't read the journal. You can use a block cipher here
146	(such as "cbc(aes)") or a stream cipher (for example "chacha20",
147	"salsa20" or "ctr(aes)").
148
149	The journal contains history of last writes to the block device,
150	an attacker reading the journal could see the last sector numbers
151	that were written. From the sector numbers, the attacker can infer
152	the size of files that were written. To protect against this
153	situation, you can encrypt the journal.
154
155journal_mac:algorithm(:key)	(the key is optional)
156	Protect sector numbers in the journal from accidental or malicious
157	modification. To protect against accidental modification, use a
158	crc algorithm, to protect against malicious modification, use a
159	hmac algorithm with a key.
160
161	This option is not needed when using internal-hash because in this
162	mode, the integrity of journal entries is checked when replaying
163	the journal. Thus, modified sector number would be detected at
164	this stage.
165
166block_size:number
167	The size of a data block in bytes.  The larger the block size the
168	less overhead there is for per-block integrity metadata.
169	Supported values are 512, 1024, 2048 and 4096 bytes.  If not
170	specified the default block size is 512 bytes.
171
172sectors_per_bit:number
173	In the bitmap mode, this parameter specifies the number of
174	512-byte sectors that corresponds to one bitmap bit.
175
176bitmap_flush_interval:number
177	The bitmap flush interval in milliseconds. The metadata buffers
178	are synchronized when this interval expires.
179
180allow_discards
181	Allow block discard requests (a.k.a. TRIM) for the integrity device.
182	Discards are only allowed to devices using internal hash.
183
184fix_padding
185	Use a smaller padding of the tag area that is more
186	space-efficient. If this option is not present, large padding is
187	used - that is for compatibility with older kernels.
188
189legacy_recalculate
190	Allow recalculating of volumes with HMAC keys. This is disabled by
191	default for security reasons - an attacker could modify the volume,
192	set recalc_sector to zero, and the kernel would not detect the
193	modification.
194
195The journal mode (D/J), buffer_sectors, journal_watermark, commit_time and
196allow_discards can be changed when reloading the target (load an inactive
197table and swap the tables with suspend and resume). The other arguments
198should not be changed when reloading the target because the layout of disk
199data depend on them and the reloaded target would be non-functional.
200
201
202Status line:
203
2041. the number of integrity mismatches
2052. provided data sectors - that is the number of sectors that the user
206   could use
2073. the current recalculating position (or '-' if we didn't recalculate)
208
209
210The layout of the formatted block device:
211
212* reserved sectors
213    (they are not used by this target, they can be used for
214    storing LUKS metadata or for other purpose), the size of the reserved
215    area is specified in the target arguments
216
217* superblock (4kiB)
218	* magic string - identifies that the device was formatted
219	* version
220	* log2(interleave sectors)
221	* integrity tag size
222	* the number of journal sections
223	* provided data sectors - the number of sectors that this target
224	  provides (i.e. the size of the device minus the size of all
225	  metadata and padding). The user of this target should not send
226	  bios that access data beyond the "provided data sectors" limit.
227	* flags
228	    SB_FLAG_HAVE_JOURNAL_MAC
229		- a flag is set if journal_mac is used
230	    SB_FLAG_RECALCULATING
231		- recalculating is in progress
232	    SB_FLAG_DIRTY_BITMAP
233		- journal area contains the bitmap of dirty
234		  blocks
235	* log2(sectors per block)
236	* a position where recalculating finished
237* journal
238	The journal is divided into sections, each section contains:
239
240	* metadata area (4kiB), it contains journal entries
241
242	  - every journal entry contains:
243
244		* logical sector (specifies where the data and tag should
245		  be written)
246		* last 8 bytes of data
247		* integrity tag (the size is specified in the superblock)
248
249	  - every metadata sector ends with
250
251		* mac (8-bytes), all the macs in 8 metadata sectors form a
252		  64-byte value. It is used to store hmac of sector
253		  numbers in the journal section, to protect against a
254		  possibility that the attacker tampers with sector
255		  numbers in the journal.
256		* commit id
257
258	* data area (the size is variable; it depends on how many journal
259	  entries fit into the metadata area)
260
261	    - every sector in the data area contains:
262
263		* data (504 bytes of data, the last 8 bytes are stored in
264		  the journal entry)
265		* commit id
266
267	To test if the whole journal section was written correctly, every
268	512-byte sector of the journal ends with 8-byte commit id. If the
269	commit id matches on all sectors in a journal section, then it is
270	assumed that the section was written correctly. If the commit id
271	doesn't match, the section was written partially and it should not
272	be replayed.
273
274* one or more runs of interleaved tags and data.
275    Each run contains:
276
277	* tag area - it contains integrity tags. There is one tag for each
278	  sector in the data area
279	* data area - it contains data sectors. The number of data sectors
280	  in one run must be a power of two. log2 of this value is stored
281	  in the superblock.
282