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 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.

	  If unsure, say N.

config DM_INTEGRITY

	tristate "Integrity target"

	depends on BLK_DEV_DM

	select BLK_DEV_INTEGRITY

	select DM_BUFIO

	select CRYPTO

	select ASYNC_XOR

	---help---

	   This is the integrity target.

endif # MD

obj-$(CONFIG_DM_CACHE_SMQ)	+= dm-cache-smq.o

obj-$(CONFIG_DM_ERA)		+= dm-era.o

obj-$(CONFIG_DM_LOG_WRITES)	+= dm-log-writes.o

obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o

ifeq ($(CONFIG_DM_UEVENT),y)

dm-mod-objs			+= dm-uevent.o