[dm-devel] [PATCH v2 0/3] Fix dm-crypt zoned block device support

[Damien Le Moal]

Mike,

Zone append BIOs (REQ_OP_ZONE_APPEND) always specify the start sector
of the zone to be written instead of the actual location sector to
write. The write location is determined by the device and returned to
the host upon completion of the operation.

This interface, while simple and efficient for writing into sequential
zones of a zoned block device, is incompatible with the use of sector
values to calculate a cypher block IV. All data written in a zone is
encrypted using an IV calculated from the first sectors of the zone,
but read operation will specify any sector within the zone, resulting
in an IV mismatch between encryption and decryption. Reads fail in that
case.

Using a single sector value (e.g. the zone start sector) for all read
and writes into a zone can solve this problem, but at the cost of
weakening the cypher chosen by the user. Emulating zone append using
regular writes would be another potential solution, but it is complex
and would add a lot of overhead.

Instead, to solve this problem, explicitly disable support for zone
append operations in dm-crypt if the target was setup using a cypher IV
mode using sector values. The null and random IV modes can still be used
with zone append operations. This lack of support for zone append is
exposed to the user by setting the dm-crypt target queue limit
max_zone_append_sectors to 0. This change is done in patches 1 and 2.

Patch 3 fixes zonefs to fall back to using regular write when
max_zone_append_sectors is 0 (Note: I can take this patch through the
zonefs tree. But since I have nothing else for an eventual rc8 and next
cycle, you can take it too. No chance of conflict).

Overall, these changes do not break user space:
1) There is no interface allowing a user to use zone append write
without a file system. So applications using directly a raw dm-crypt
device will continue working using regular write operations.
2) btrfs zoned support was added in 5.12. Anybody trying btrfs-zoned on
top of dm-crypt would have faced the read failures already. So there
are no existing deployments to preserve. Same for zonefs.

For file systems, using zone append with encryption will need to be
supported within the file system (e.g. fscrypt). In this case, cypher IV
calculation can rely for instance on file block offsets as these are
known before a zone append operation write these blocks to disk at
unknown locations.

Reviews and comments are very much welcome.

Changes from v1:
* Addressed Johannes comments by renaming the CRYPT_IV_NO_SECTORS flag
  to CRYPT_IV_ZONE_APPEND to avoid a double negation with !test_bit().
  This also clarifies that the flag is used solely to check zone append
  support.
* Removed btrfs patch (former patch 3) as David is taking that patch
  through the btrfs tree misc-next branch.
* Added reviewed-by, Fixes and Cc tags.

Damien Le Moal (3):
  dm: Introduce zone append support control
  dm crypt: Fix zoned block device support
  zonefs: fix synchronous write to sequential zone files

 drivers/md/dm-crypt.c         | 49 ++++++++++++++++++++++++++++-------
 drivers/md/dm-table.c         | 41 +++++++++++++++++++++++++++++
 fs/zonefs/super.c             | 16 +++++++++---
 fs/zonefs/zonefs.h            |  2 ++
 include/linux/device-mapper.h |  6 +++++
 5 files changed, 101 insertions(+), 13 deletions(-)

-- 
2.30.2