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Thu, 21 May 2026 19:55:06 -0700 (PDT) Received: from fedora.tail348456.ts.net ([172.245.82.59]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8cc80ded51esm6110616d6.17.2026.05.21.19.55.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 21 May 2026 19:55:05 -0700 (PDT) From: Ming Lei To: Jens Axboe , linux-block@vger.kernel.org Cc: Ming Lei , syzbot+cd8a9a308e879a4e2c28@syzkaller.appspotmail.com, syzbot+bc273027d5643e48e5b3@syzkaller.appspotmail.com, Tetsuo Handa , Hongling Zeng , Bart Van Assche , Andrew Morton Subject: [PATCH] loop: add sync_blockdev() in __loop_clr_fd() to prevent UAF Date: Fri, 22 May 2026 10:54:51 +0800 Message-ID: <20260522025451.845068-1-tom.leiming@gmail.com> X-Mailer: git-send-email 2.54.0 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-Transfer-Encoding: 8bit The syzbot report describes a NULL pointer dereference in lo_rw_aio() caused by a race between lo_release() and loop_queue_rq(). __loop_clr_fd() clears lo->lo_backing_file while an already-scheduled asynchronous I/O work (lo_rw_aio) is about to execute on a kworker. Since the kworker enters lo_rw_aio() after lo->lo_backing_file has been set to NULL, it dereferences the NULL pointer when initializing the kiocb, leading to a general protection fault. Fix this by adding sync_blockdev() in __loop_clr_fd() to flush all pending writeback I/O before clearing lo->lo_backing_file. Since the loop disk is already closed at this point, no new I/O can be submitted — only writeback remains. Reported-by: syzbot+cd8a9a308e879a4e2c28@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=cd8a9a308e879a4e2c28 Reported-by: syzbot+bc273027d5643e48e5b3@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=bc273027d5643e48e5b3 Analyzed-by: Tetsuo Handa Cc: Hongling Zeng Cc: Tetsuo Handa Cc: Bart Van Assche Cc: Andrew Morton Link: https://lore.kernel.org/linux-block/9b2032d6-3f36-4d2b-8128-985c08a4fa37@I-love.SAKURA.ne.jp/ Signed-off-by: Ming Lei --- drivers/block/loop.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0000913f7efc..023e20a88a8a 100644 --- a/drivers/block/loop.c +++ b/drivers/block/loop.c @@ -1118,6 +1118,9 @@ static void __loop_clr_fd(struct loop_device *lo) struct file *filp; gfp_t gfp = lo->old_gfp_mask; + /* flush outstanding writeback I/O before clearing lo_backing_file */ + sync_blockdev(lo->lo_device); + spin_lock_irq(&lo->lo_lock); filp = lo->lo_backing_file; lo->lo_backing_file = NULL; -- 2.54.0