[PATCH] dm-pcache: fix use-after-free during cache replay

[Shuangpeng Bai <shuangpeng.kernel@gmail.com>]

kset_replay() drops the last reference to a stale cache key before
using key->cache_pos to acquire a reference to its cache segment. This
can dereference the freed key.

Segment references account for key records that have not yet been
consumed by key-log garbage collection. A reference is acquired for
every successfully recorded key, and the GC path drops one for every
record, including records whose segment generation has become stale.
Replay therefore has to restore the segment reference before deciding
whether the decoded key should be inserted into the request-key tree.

Move cache_seg_get() before the generation check. This preserves the
get/put accounting for stale records while ensuring that the key is not
accessed after cache_key_put().

Fixes: 1d57628ff95b ("dm-pcache: add persistent cache target in device-mapper")

Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
---
 drivers/md/dm-pcache/cache_key.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/drivers/md/dm-pcache/cache_key.c b/drivers/md/dm-pcache/cache_key.c
index e068e878231b..b37d899f2d3c 100644
--- a/drivers/md/dm-pcache/cache_key.c
+++ b/drivers/md/dm-pcache/cache_key.c
@@ -729,6 +729,7 @@ static int kset_replay(struct pcache_cache *cache, struct pcache_cache_kset_onme
 		}
 
 		__set_bit(key->cache_pos.cache_seg->cache_seg_id, cache->seg_map);
+		cache_seg_get(key->cache_pos.cache_seg);
 
 		/* Check if the segment generation is valid for insertion. */
 		if (key->seg_gen < key->cache_pos.cache_seg->gen) {
@@ -739,8 +740,6 @@ static int kset_replay(struct pcache_cache *cache, struct pcache_cache_kset_onme
 			cache_key_insert(&cache->req_key_tree, key, true);
 			spin_unlock(&cache_subtree->tree_lock);
 		}
-
-		cache_seg_get(key->cache_pos.cache_seg);
 	}
 
 	return 0;
-- 
2.43.0