[PATCH v2 00/11] dm-pcache: validate on-media metadata against a forged cache image

[Bryam Vargas via B4 Relay]

dm-pcache reads its geometry, segment ids, kset headers, key records and
the persisted tail positions from the cache device. Each of those fields
is protected only by a crc32c with a fixed public seed, so whoever
supplies the cache device on a table load (CAP_SYS_ADMIN) controls them
all. Several are used as an array index, a copy length, a backing offset
or a loop bound with no validation, so a forged image turns a table load
into an out-of-bounds access, a BUG_ON or an unterminated loop.

This series validates each field before use and fails the table load with
-EIO instead.

   1  validate seg_id fields
   2  validate geometry fields from on-disk cache_info
   3  validate kset key_num and intra-segment bounds
   4  bound the persisted tail-position offset
   5  reject a kset that overruns its segment
   6  detect a cycle in the last-kset chain during replay
   7  bound the logical key offset from persistent memory
   8  clamp the tail kset read to the segment data region
   9  validate on-media seg_num against the cache device size
  10  validate the persisted dirty_tail chain at load
  11  only hand out initialized cache segments

Patch 9 closes the most serious case. seg_num is the geometry root every
other on-media segment id is bounded against, but it is itself never
checked against the device. Because cache_dev->mapping is the direct map
of the pmem, a forged seg_num larger than the device makes a new-cache
init memset() 12 KiB past the mapping, over ordinary kernel memory --
KASAN, at table load:

  BUG: KASAN: slab-use-after-free in cache_dev_zero_range+0x1d/0x80 [dm_pcache]
  Write of size 12288 ... by task dmsetup
   cache_dev_zero_range <- cache_seg_init <- pcache_cache_start
   <- dm_pcache_ctr <- dm_table_add_target <- table_load

Patch 10 closes a writeback-side livelock. The persisted dirty_tail is
decoded independently of the key_tail chain that cache_replay() walks and
bounds, and the writeback worker follows it on its own; a forged dirty_tail
whose last-kset chain does not terminate makes cache_writeback_fn() re-arm
with no delay forever. Patch 10 validates that chain at load with the same
hop cap cache_replay() uses and rejects the image with -EIO.

Patch 11 is defensive. get_cache_segment() scans the physical segment count
rather than the initialized cache_info->n_segs, so a forged n_segs smaller
than the device could hand cache_kset_close() a zeroed segment whose data
pointer is NULL. Bounding the allocator to initialized segments removes that
possibility; it does not reject a conforming image.

Tested on a KASAN build, one forged image per field. For patches 1-10 the
unpatched vector faults, corrupts, over-reads or loops at the table load
(patch 10: a forged dirty_tail cycle re-arms the writeback worker until the
hop cap fires), the patched build rejects the image with -EIO, and a valid
image still loads.

This is a large series and it touches most of the on-media parse path, so I'd
rather it be reviewed as a set than piecemeal; I'll hold the next respin until
there is feedback on the whole thing.

One caveat on the testing above: each patch was exercised at its boundary -- a
crafted image with the field forged just past its limit, on a KASAN build. That
is the crafted-image case these patches are about, not a normal-workload run.
Exercising ordinary cache operation over time would take more than I put in
here; a conforming image loads and the other vectors behave as before, but I
have not stress-tested normal use.

Two notes for review. Zheng Gu reviewed v1; v2 reshaped those patches -- patch
1 now bounds against cache_info->n_segs, not the device count, and the series
grew from three to eleven -- so I did not carry the v1 Reviewed-by tags and
would ask for a fresh look. There is also an in-flight cleanup series on this
driver (jianyungao89, "dm-pcache: minor cleanups") touching cache_seg_remain()
and cache_data_alloc(); it does not overlap these fixes, but you may want to
sequence the two.

v2:
 - Grew from 3 patches to 11. Mikulas Patocka's review of v1 showed the
   writeback worker re-armed on a rejected kset forever instead of
   stopping; auditing the same forged-image threat model surfaced the
   remaining fields, including the unbounded seg_num geometry root (patch 9,
   the out-of-bounds write above), the persisted dirty_tail chain the
   writeback worker follows independently of replay (patch 10) and the
   uninitialized-segment allocation reaching cache_kset_close() (patch 11).

v1: https://lore.kernel.org/all/20260619-b4-disp-997773a1-v1-0-14df8aedb323@xxxxxxxxx/
---
Bryam Vargas (11):
      dm-pcache: validate seg_id fields from persistent memory
      dm-pcache: validate geometry fields from on-disk cache_info
      dm-pcache: validate kset key_num and intra-segment bounds
      dm-pcache: bound the persisted tail-position offset
      dm-pcache: reject a kset that overruns its segment
      dm-pcache: detect a cycle in the last-kset chain during replay
      dm-pcache: bound the logical key offset from persistent memory
      dm-pcache: clamp the tail kset read to the segment data region
      dm-pcache: validate on-media seg_num against the cache device size
      dm-pcache: validate the persisted dirty_tail chain at load
      dm-pcache: only hand out initialized cache segments

 drivers/md/dm-pcache/cache.c           |  29 +++++++++
 drivers/md/dm-pcache/cache.h           |  38 +++++++++++
 drivers/md/dm-pcache/cache_dev.c       |  22 ++++++-
 drivers/md/dm-pcache/cache_gc.c        |  31 +++++++--
 drivers/md/dm-pcache/cache_key.c       | 113 ++++++++++++++++++++++++++++++++-
 drivers/md/dm-pcache/cache_segment.c   |  12 +++-
 drivers/md/dm-pcache/cache_writeback.c |  38 ++++++++---
 7 files changed, 261 insertions(+), 22 deletions(-)
---
base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
change-id: 20260717-b4-disp-4c0a0039-0cc25500aee1

Best regards,
-- 
Bryam Vargas <hexlabsecurity@xxxxxxxxx>