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From: Johannes Weiner <hannes@cmpxchg.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Nhat Pham <nphamcs@gmail.com>,
	Yosry Ahmed <yosryahmed@google.com>,
	Chengming Zhou <zhouchengming@bytedance.com>,
	linux-mm@kvack.org,
	linux-kernel@vger.kernel.org
Subject: [PATCH] mm: zswap: fix objcg use-after-free in entry destruction
Date: Mon, 29 Jan 2024 20:34:38 -0500
Message-ID: <20240130013438.565167-1-hannes@cmpxchg.org>
Precedence: bulk
MIME-Version: 1.0
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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.

Fixes: a65b0e7607cc ("zswap: make shrinking memcg-aware")
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 mm/zswap.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index de68a5928527..7f88b3a77e4a 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -522,10 +522,6 @@ 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 {
@@ -534,6 +530,10 @@ 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();
-- 
2.43.0