234 lines
9.9 KiB
Plaintext
234 lines
9.9 KiB
Plaintext
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.
|
|
|
|
There's an alternate mode of operation where dm-integrity uses bitmap
|
|
instead of a journal. If a bit in the bitmap is 1, the corresponding
|
|
region's data and integrity tags are not synchronized - if the machine
|
|
crashes, the unsynchronized regions will be recalculated. The bitmap mode
|
|
is faster than the journal mode, because we don't have to write the data
|
|
twice, but it is also less reliable, because if data corruption happens
|
|
when the machine crashes, it may not be detected.
|
|
|
|
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.
|
|
B - bitmap mode - data and metadata are written without any
|
|
synchronization, the driver maintains a bitmap of dirty
|
|
regions where data and metadata don't match. This mode can
|
|
only be used with internal hash.
|
|
R - recovery mode - in this mode, journal is not replayed,
|
|
checksums are not checked and writes to the device are not
|
|
allowed. This mode is useful for data recovery if the
|
|
device cannot be activated in any of the other standard
|
|
modes.
|
|
|
|
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.
|
|
|
|
meta_device:device
|
|
Don't interleave the data and metadata on on device. Use a
|
|
separate device for metadata.
|
|
|
|
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.
|
|
|
|
recalculate
|
|
Recalculate the integrity tags automatically. It is only valid
|
|
when using internal hash.
|
|
|
|
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.
|
|
|
|
block_size:number
|
|
The size of a data block in bytes. The larger the block size the
|
|
less overhead there is for per-block integrity metadata.
|
|
Supported values are 512, 1024, 2048 and 4096 bytes. If not
|
|
specified the default block size is 512 bytes.
|
|
|
|
sectors_per_bit:number
|
|
In the bitmap mode, this parameter specifies the number of
|
|
512-byte sectors that corresponds to one bitmap bit.
|
|
|
|
bitmap_flush_interval:number
|
|
The bitmap flush interval in milliseconds. The metadata buffers
|
|
are synchronized when this interval expires.
|
|
|
|
|
|
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
|
|
SB_FLAG_HAVE_JOURNAL_MAC - a flag is set if journal_mac is used
|
|
SB_FLAG_RECALCULATING - recalculating is in progress
|
|
SB_FLAG_DIRTY_BITMAP - journal area contains the bitmap of dirty
|
|
blocks
|
|
* log2(sectors per block)
|
|
* a position where recalculating finished
|
|
* 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.
|