dm-snap: reject the transient exception store for snapshot-merge

snapshot_merge_next_chunks() calls s->store->type->prepare_merge() without
checking it. The transient store ("N") implements neither prepare_merge nor
commit_merge, and snapshot_ctr() validates those callbacks only on the handover
path (__validate_exception_handover()), so a directly constructed snapshot-merge
calls a NULL prepare_merge on resume:

  snapshot_merge_resume -> snapshot_merge_next_chunks -> prepare_merge(...)

commit_merge() is NULL for the same reason. Userspace only issues documented
device-mapper ioctls (CAP_SYS_ADMIN); this is a misconfiguration the kernel must
reject rather than oops on.

Reject the combination in snapshot_ctr() at table-load time (-EINVAL), the same
check the handover path already performs; this also covers the commit_merge()
call site, which a guard local to the prepare_merge call would miss.

Tested on Linux v6.6.157 KASAN: unpatched, the dmsetup reproducer oopses in
snapshot_merge_next_chunks(); with this patch, the table load fails with -EINVAL
and no oops. Also present on v6.6.145; still present on the linux-dm tree
(555f3d7be91a) and on torvalds/linux master (62f4c998b297) as of 2026-09-22.

Note that this is not a security bug because only root can trigger it.

Signed-off-by: Ma Xinmeng <1564938642@qq.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

0 files changed, 0 insertions, 0 deletions