mm: zswap: fix objcg use-after-free in entry destruction

In the per-memcg LRU universe, LRU removal uses entry->objcg to determine
which list count needs to be decreased.  Drop the objcg reference after
updating the LRU, to fix a possible use-after-free.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: a65b0e7 ("zswap: make shrinking memcg-aware")
Signed-off-by: Johannes Weiner <[email protected]>
Acked-by: Yosry Ahmed <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/zswap.c

@@ -536,10 +536,6 @@ static struct zpool *zswap_find_zpool(struct zswap_entry *entry)
  */
 static void zswap_free_entry(struct zswap_entry *entry)
 {
-	if (entry->objcg) {
-		obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
-		obj_cgroup_put(entry->objcg);
-	}
 	if (!entry->length)
 		atomic_dec(&zswap_same_filled_pages);
 	else {
@@ -548,6 +544,10 @@ static void zswap_free_entry(struct zswap_entry *entry)
 		atomic_dec(&entry->pool->nr_stored);
 		zswap_pool_put(entry->pool);
 	}
+	if (entry->objcg) {
+		obj_cgroup_uncharge_zswap(entry->objcg, entry->length);
+		obj_cgroup_put(entry->objcg);
+	}
 	zswap_entry_cache_free(entry);
 	atomic_dec(&zswap_stored_pages);
 	zswap_update_total_size();