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Wed, 20 May 2026 01:54:57 -0700 (PDT) Date: Wed, 20 May 2026 16:54:51 +0800 From: Ming Lei To: Tetsuo Handa Cc: Bart Van Assche , Jens Axboe , Christoph Hellwig , Damien Le Moal , linux-block , LKML , Andrew Morton Subject: Re: [PATCH v2] loop: Fix NULL pointer dereference by synchronizing lo_release and loop_queue_rq Message-ID: References: <9b2032d6-3f36-4d2b-8128-985c08a4fa37@I-love.SAKURA.ne.jp> <20260518174013.4b72dd50a5bcb89daaed1f62@linux-foundation.org> <94076bc9-2c09-4bb6-8468-b6b8af419cb9@I-love.SAKURA.ne.jp> <1ab8c579-eb76-4227-8a72-6ec819135219@I-love.SAKURA.ne.jp> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1ab8c579-eb76-4227-8a72-6ec819135219@I-love.SAKURA.ne.jp> On Wed, May 20, 2026 at 05:20:01PM +0900, Tetsuo Handa wrote: > On 2026/05/20 16:49, Ming Lei wrote: > > On Wed, May 20, 2026 at 03:36:12PM +0900, Tetsuo Handa wrote: > >> On 2026/05/20 12:06, Ming Lei wrote: > >>> The IO after close(loop) should be from writeback. rcu/sruc isn't necessary, > >> > >> Gemini's comment is that drain_workqueue() is not sufficient for waiting for > >> do_req_filebacked(REQ_OP_WRITE) requests with cmd->use_aio == true case to complete. > > > > Anything cleared in __loop_clr_fd() is not used by lo_rw_aio_complete() & lo_complete_rq(). > > "struct inode *inode = file_inode(iocb->ki_filp);" in kiocb_end_write() from > lo_rw_aio_do_completion() can dereference "struct file *" with refcount == 0 (UAF) > because fput() in __loop_clr_fd() can be the last reference to that file. OK, you are right. It can be handled by adding sync_blockdev(lo->lo_device) in __loop_clr_fd() because IO can be from writeback only when loop disk is closed. Please feel free to test the following change: diff --git a/drivers/block/loop.c b/drivers/block/loop.c index 0000913f7efc..bbd15974a082 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; + sync_blockdev(lo->lo_device); + spin_lock_irq(&lo->lo_lock); filp = lo->lo_backing_file; lo->lo_backing_file = NULL; > > > > So why isn't drain_workqueue() enough for cmd->use_aio? > > In addition to possible UAF above, the assumption at > https://elixir.bootlin.com/linux/v7.1-rc4/source/drivers/block/loop.c#L1134 > is currently broken due to this race problem. That shouldn't be one issue otherwise bio based driver can't work with updating limits. Thanks, Ming