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AFNElJ/M8dIV3CexsaRDcZuOvuhebcgp10rOFbMS/kXnkDpSfjH5aMEbTfe4GLtITcT60hUiNfKC0QUzEQ==@kvack.org X-Gm-Message-State: AOJu0YwyQJBlR6wzJgD/kX7VWpCeDkSKBbmgcLiu3xgSeh08NIS70xka T/VOZ20yWu9qF+me4ES8wiBfVQPi5LRiWaMVL+Sy6R9xwctvX83vGb1ucQ6oRofj4PBJV/kMiih BSEFPr2TpEETg3+F8hU2W X-Received: from pgww20.prod.google.com ([2002:a05:6a02:2c94:b0:c76:5ef4:ec3]) (user=richardycc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a21:6d91:b0:3a8:236a:7df2 with SMTP id adf61e73a8af0-3aa5abc86a4mr12929465637.44.1778230245628; Fri, 08 May 2026 01:50:45 -0700 (PDT) Date: Fri, 8 May 2026 08:49:33 +0000 In-Reply-To: Mime-Version: 1.0 References: X-Mailer: git-send-email 2.54.0.563.g4f69b47b94-goog Message-ID: <20260508084933.3730661-1-richardycc@google.com> Subject: [PATCH v2] zram: fix use-after-free in zram_writeback_endio From: Richard Chang To: Minchan Kim , Sergey Senozhatsky , Jens Axboe , Andrew Morton Cc: bgeffon@google.com, liumartin@google.com, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, linux-mm@kvack.org, Richard Chang Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: k1chsfrmjyyzc3g1gec9qubtdeyi779s X-Rspam-User: X-Rspamd-Queue-Id: 3827F180005 X-Rspamd-Server: rspam07 X-HE-Tag: 1778230247-718465 X-HE-Meta: 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 yJOlOBNQ 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: A crash was observed in zram_writeback_endio due to a NULL pointer dereference in wake_up. The root cause is a race condition between the bio completion handler (zram_writeback_endio) and the writeback task. In zram_writeback_endio, wake_up() is called on &wb_ctl->done_wait after releasing wb_ctl->done_lock. This creates a race window where the writeback task can see num_inflight become 0, return, and free wb_ctl before zram_writeback_endio calls wake_up(). CPU 0 (zram_writeback_endio) CPU 1 (writeback_store) ============================ ============================ zram_writeback_slots zram_submit_wb_request zram_submit_wb_request wait_event(wb_ctl->done_wait) spin_lock(&wb_ctl->done_lock); list_add(&req->entry, &wb_ctl->done_reqs); spin_unlock(&wb_ctl->done_lock); wake_up(&wb_ctl->done_wait); zram_complete_done_reqs spin_lock(&wb_ctl->done_lock); list_add(&req->entry, &wb_ctl->done_reqs); spin_unlock(&wb_ctl->done_lock); while (num_inflight) > 0) spin_lock(&wb_ctl->done_lock); list_del(&req->entry); spin_unlock(&wb_ctl->done_lock); // num_inflight becomes 0 atomic_dec(num_inflight); // Leave zram_writeback_slots // Free wb_ctl release_wb_ctl(wb_ctl); // UAF crash! wake_up(&wb_ctl->done_wait); This patch fixes this race by using RCU. By protecting wb_ctl with rcu_read_lock() in zram_writeback_endio and using kfree_rcu() to free it, we ensure that wb_ctl remains valid during the execution of zram_writeback_endio. Fixes: f405066a1f0d ("zram: introduce writeback bio batching") Suggested-by: Sergey Senozhatsky Suggested-by: Minchan Kim Signed-off-by: Richard Chang --- V1 -> V2: use RCU to manage the wb_ctl lifetime drivers/block/zram/zram_drv.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c index aebc710f0d6a..07111455eecf 100644 --- a/drivers/block/zram/zram_drv.c +++ b/drivers/block/zram/zram_drv.c @@ -33,6 +33,7 @@ #include #include #include +#include #include "zram_drv.h" @@ -504,6 +505,7 @@ struct zram_wb_ctl { wait_queue_head_t done_wait; spinlock_t done_lock; atomic_t num_inflight; + struct rcu_head rcu; }; struct zram_wb_req { @@ -847,7 +849,7 @@ static void release_wb_ctl(struct zram_wb_ctl *wb_ctl) release_wb_req(req); } - kfree(wb_ctl); + kfree_rcu(wb_ctl, rcu); } static struct zram_wb_ctl *init_wb_ctl(struct zram *zram) @@ -964,11 +966,13 @@ static void zram_writeback_endio(struct bio *bio) struct zram_wb_ctl *wb_ctl = bio->bi_private; unsigned long flags; + rcu_read_lock(); spin_lock_irqsave(&wb_ctl->done_lock, flags); list_add(&req->entry, &wb_ctl->done_reqs); spin_unlock_irqrestore(&wb_ctl->done_lock, flags); wake_up(&wb_ctl->done_wait); + rcu_read_unlock(); } static void zram_submit_wb_request(struct zram *zram, -- 2.54.0.563.g4f69b47b94-goog