dm: add integrity target

The dm-integrity target emulates a block device that has additional
per-sector tags that can be used for storing integrity information.

A general problem with storing integrity tags with every sector is that
writing the sector and the integrity tag must be atomic - i.e. in case of
crash, either both sector and integrity tag or none of them is written.

To guarantee write atomicity the dm-integrity target uses a journal. It
writes sector data and integrity tags into a journal, commits the journal
and then copies the data and integrity tags to their respective location.

The dm-integrity target can be used with the dm-crypt target - in this
situation the dm-crypt target creates the integrity data and passes them
to the dm-integrity target via bio_integrity_payload attached to the bio.
In this mode, the dm-crypt and dm-integrity targets provide authenticated
disk encryption - if the attacker modifies the encrypted device, an I/O
error is returned instead of random data.

The dm-integrity target can also be used as a standalone target, in this
mode it calculates and verifies the integrity tag internally. In this
mode, the dm-integrity target can be used to detect silent data
corruption on the disk or in the I/O path.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 189 insertions, 0 deletions

diff --git a/Documentation/device-mapper/dm-integrity.txt b/Documentation/device-mapper/dm-integrity.txt
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+The dm-integrity target emulates a block device that has additional
+per-sector tags that can be used for storing integrity information.
+
+A general problem with storing integrity tags with every sector is that
+writing the sector and the integrity tag must be atomic - i.e. in case of
+crash, either both sector and integrity tag or none of them is written.
+
+To guarantee write atomicity, the dm-integrity target uses journal, it
+writes sector data and integrity tags into a journal, commits the journal
+and then copies the data and integrity tags to their respective location.
+
+The dm-integrity target can be used with the dm-crypt target - in this
+situation the dm-crypt target creates the integrity data and passes them
+to the dm-integrity target via bio_integrity_payload attached to the bio.
+In this mode, the dm-crypt and dm-integrity targets provide authenticated
+disk encryption - if the attacker modifies the encrypted device, an I/O
+error is returned instead of random data.
+
+The dm-integrity target can also be used as a standalone target, in this
+mode it calculates and verifies the integrity tag internally. In this
+mode, the dm-integrity target can be used to detect silent data
+corruption on the disk or in the I/O path.
+
+
+When loading the target for the first time, the kernel driver will format
+the device. But it will only format the device if the superblock contains
+zeroes. If the superblock is neither valid nor zeroed, the dm-integrity
+target can't be loaded.
+
+To use the target for the first time:
+1. overwrite the superblock with zeroes
+2. load the dm-integrity target with one-sector size, the kernel driver
+	will format the device
+3. unload the dm-integrity target
+4. read the "provided_data_sectors" value from the superblock
+5. load the dm-integrity target with the the target size
+	"provided_data_sectors"
+6. if you want to use dm-integrity with dm-crypt, load the dm-crypt target
+	with the size "provided_data_sectors"
+
+
+Target arguments:
+
+1. the underlying block device
+
+2. the number of reserved sector at the beginning of the device - the
+	dm-integrity won't read of write these sectors
+
+3. the size of the integrity tag (if "-" is used, the size is taken from
+	the internal-hash algorithm)
+
+4. mode:
+	D - direct writes (without journal) - in this mode, journaling is
+		not used and data sectors and integrity tags are written
+		separately. In case of crash, it is possible that the data
+		and integrity tag doesn't match.
+	J - journaled writes - data and integrity tags are written to the
+		journal and atomicity is guaranteed. In case of crash,
+		either both data and tag or none of them are written. The
+		journaled mode degrades write throughput twice because the
+		data have to be written twice.
+
+5. the number of additional arguments
+
+Additional arguments:
+
+journal-sectors:number
+	The size of journal, this argument is used only if formatting the
+	device. If the device is already formatted, the value from the
+	superblock is used.
+
+interleave-sectors:number
+	The number of interleaved sectors. This values is rounded down to
+	a power of two. If the device is already formatted, the value from
+	the superblock is used.
+
+buffer-sectors:number
+	The number of sectors in one buffer. The value is rounded down to
+	a power of two.
+
+	The tag area is accessed using buffers, the buffer size is
+	configurable. The large buffer size means that the I/O size will
+	be larger, but there could be less I/Os issued.
+
+journal-watermark:number
+	The journal watermark in percents. When the size of the journal
+	exceeds this watermark, the thread that flushes the journal will
+	be started.
+
+commit-time:number
+	Commit time in milliseconds. When this time passes, the journal is
+	written. The journal is also written immediatelly if the FLUSH
+	request is received.
+
+internal-hash:algorithm(:key)	(the key is optional)
+	Use internal hash or crc.
+	When this argument is used, the dm-integrity target won't accept
+	integrity tags from the upper target, but it will automatically
+	generate and verify the integrity tags.
+
+	You can use a crc algorithm (such as crc32), then integrity target
+	will protect the data against accidental corruption.
+	You can also use a hmac algorithm (for example
+	"hmac(sha256):0123456789abcdef"), in this mode it will provide
+	cryptographic authentication of the data without encryption.
+
+	When this argument is not used, the integrity tags are accepted
+	from an upper layer target, such as dm-crypt. The upper layer
+	target should check the validity of the integrity tags.
+
+journal-crypt:algorithm(:key)	(the key is optional)
+	Encrypt the journal using given algorithm to make sure that the
+	attacker can't read the journal. You can use a block cipher here
+	(such as "cbc(aes)") or a stream cipher (for example "chacha20",
+	"salsa20", "ctr(aes)" or "ecb(arc4)").
+
+	The journal contains history of last writes to the block device,
+	an attacker reading the journal could see the last sector nubmers
+	that were written. From the sector numbers, the attacker can infer
+	the size of files that were written. To protect against this
+	situation, you can encrypt the journal.
+
+journal-mac:algorithm(:key)	(the key is optional)
+	Protect sector numbers in the journal from accidental or malicious
+	modification. To protect against accidental modification, use a
+	crc algorithm, to protect against malicious modification, use a
+	hmac algorithm with a key.
+
+	This option is not needed when using internal-hash because in this
+	mode, the integrity of journal entries is checked when replaying
+	the journal. Thus, modified sector number would be detected at
+	this stage.
+
+
+The journal mode (D/J), buffer-sectors, journal-watermark, commit-time can
+be changed when reloading the target (load an inactive table and swap the
+tables with suspend and resume). The other arguments should not be changed
+when reloading the target because the layout of disk data depend on them
+and the reloaded target would be non-functional.
+
+
+The layout of the formatted block device:
+* reserved sectors (they are not used by this target, they can be used for
+  storing LUKS metadata or for other purpose), the size of the reserved
+  area is specified in the target arguments
+* superblock (4kiB)
+	* magic string - identifies that the device was formatted
+	* version
+	* log2(interleave sectors)
+	* integrity tag size
+	* the number of journal sections
+	* provided data sectors - the number of sectors that this target
+	  provides (i.e. the size of the device minus the size of all
+	  metadata and padding). The user of this target should not send
+	  bios that access data beyond the "provided data sectors" limit.
+	* flags - a flag is set if journal-mac is used
+* journal
+	The journal is divided into sections, each section contains:
+	* metadata area (4kiB), it contains journal entries
+	  every journal entry contains:
+		* logical sector (specifies where the data and tag should
+		  be written)
+		* last 8 bytes of data
+		* integrity tag (the size is specified in the superblock)
+	    every metadata sector ends with
+		* mac (8-bytes), all the macs in 8 metadata sectors form a
+		  64-byte value. It is used to store hmac of sector
+		  numbers in the journal section, to protect against a
+		  possibility that the attacker tampers with sector
+		  numbers in the journal.
+		* commit id
+	* data area (the size is variable; it depends on how many journal
+	  entries fit into the metadata area)
+	    every sector in the data area contains:
+		* data (504 bytes of data, the last 8 bytes are stored in
+		  the journal entry)
+		* commit id
+	To test if the whole journal section was written correctly, every
+	512-byte sector of the journal ends with 8-byte commit id. If the
+	commit id matches on all sectors in a journal section, then it is
+	assumed that the section was written correctly. If the commit id
+	doesn't match, the section was written partially and it should not
+	be replayed.
+* one or more runs of interleaved tags and data. Each run contains:
+	* tag area - it contains integrity tags. There is one tag for each
+	  sector in the data area
+	* data area - it contains data sectors. The number of data sectors
+	  in one run must be a power of two. log2 of this value is stored
+	  in the superblock.