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Tue, 19 May 2026 04:29:02 -0700 (PDT) Date: Tue, 19 May 2026 19:28:53 +0800 From: Ming Lei To: Hongling Zeng Cc: Hongling Zeng , axboe@kernel.dk, ming.lei@canonical.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Tetsuo Handa Subject: Re: [PATCH] block/loop: Fix NULL pointer dereference in lo_rw_aio() Message-ID: References: <20260519040103.22776-1-zenghongling@kylinos.cn> <6A0BFDE2.5000808@126.com> <6A0C29F6.5080704@126.com> Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <6A0C29F6.5080704@126.com> On Tue, May 19, 2026 at 05:14:30PM +0800, Hongling Zeng wrote: > Hi Ming, > > Tetsuo’s syzkaller trace confirms the root cause: > > Timeline: > T6142: lo_rw_aio(loop3) starting read with raw_refcnt=0x0, refcnt=1 > T44: lo_rw_aio(loop3) starting read with raw_refcnt=0x0, refcnt=1 > T6148: __loop_clr_fd(loop3) clearing lo_backing_file with raw_refcnt=0x0, > refcnt=1 > T180: lo_rw_aio(loop3) starting write with NULL file (already cleared?) > T180: CRASH - null-ptr-deref IO could be from writeback or early close, so the following patch should help: diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0000913f7efc..24654a03db71 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -1118,6 +1118,8 @@ static void __loop_clr_fd(struct loop_device *lo) struct file *filp; gfp_t gfp = lo->old_gfp_mask; + drain_workqueue(lo->workqueue); + spin_lock_irq(&lo->lo_lock); filp = lo->lo_backing_file; lo->lo_backing_file = NULL; @@ -1857,9 +1859,6 @@ static blk_status_t loop_queue_rq(struct blk_mq_hw_ctx *hctx, blk_mq_start_request(rq); - if (data_race(READ_ONCE(lo->lo_state)) != Lo_bound) - return BLK_STS_IOERR; - switch (req_op(rq)) { case REQ_OP_FLUSH: case REQ_OP_DISCARD: @@ -1901,6 +1900,11 @@ static void loop_handle_cmd(struct loop_cmd *cmd) int ret = 0; struct mem_cgroup *old_memcg = NULL; + if (data_race(READ_ONCE(lo->lo_state)) != Lo_bound) { + ret = -EIO; + goto failed; + } + if (write && (lo->lo_flags & LO_FLAGS_READ_ONLY)) { ret = -EIO; goto failed; > > Key anomaly: |raw_refcnt=0|while |refcnt=1|. > This means |__loop_clr_fd()|runs while I/O is still active. > Regression introduced by: > 6050fa4c84cc ("loop: don't hold lo_mutex during __loop_clr_fd()") Why do you conclude it is caused by above commit? Thanks, Ming