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. Signed-off-by: Ma Xinmeng <[email protected]> --- drivers/md/dm-snap.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c index dcf34c6b0..f42935361 100644 --- a/drivers/md/dm-snap.c +++ b/drivers/md/dm-snap.c @@ -1320,6 +1320,20 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv) goto bad_store; } + /* + * snapshot-merge requires the optional prepare_merge/commit_merge + * callbacks; the transient exception store does not implement them. + * The handover path checks this in __validate_exception_handover(), + * but a directly constructed snapshot-merge never goes through it. + */ + if (dm_target_is_snapshot_merge(ti) && + (!s->store->type->prepare_merge || + !s->store->type->commit_merge)) { + ti->error = "Snapshot exception store does not support snapshot-merge."; + r = -EINVAL; + goto bad_hash_tables; + } + argv += args_used; argc -= args_used; -- 2.49.0.windows.1
