io_uring: fix io_try_cancel_userdata race for iowq

WARNING: CPU: 1 PID: 5870 at fs/io_uring.c:5975 io_try_cancel_userdata+0x30f/0x540 fs/io_uring.c:5975
CPU: 0 PID: 5870 Comm: iou-wrk-5860 Not tainted 5.14.0-rc6-next-20210820-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:io_try_cancel_userdata+0x30f/0x540 fs/io_uring.c:5975
Call Trace:
 io_async_cancel fs/io_uring.c:6014 [inline]
 io_issue_sqe+0x22d5/0x65a0 fs/io_uring.c:6407
 io_wq_submit_work+0x1dc/0x300 fs/io_uring.c:6511
 io_worker_handle_work+0xa45/0x1840 fs/io-wq.c:533
 io_wqe_worker+0x2cc/0xbb0 fs/io-wq.c:582
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

io_try_cancel_userdata() can be called from io_async_cancel() executing
in the io-wq context, so the warning fires, which is there to alert
anyone accessing task->io_uring->io_wq in a racy way. However,
io_wq_put_and_exit() always first waits for all threads to complete,
so the only detail left is to zero tctx->io_wq after the context is
removed.

note: one little assumption is that when IO_WQ_WORK_CANCEL, the executor
won't touch ->io_wq, because io_wq_destroy() might cancel left pending
requests in such a way.

Cc: stable@vger.kernel.org
Reported-by: syzbot+b0c9d1588ae92866515f@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/dfdd37a80cfa9ffd3e59538929c99cdd55d8699e.1629721757.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5863,7 +5863,7 @@ static int io_try_cancel_userdata(struct io_kiocb *req, u64 sqe_addr)
 	struct io_ring_ctx *ctx = req->ctx;
 	int ret;
 
-	WARN_ON_ONCE(req->task != current);
+	WARN_ON_ONCE(!io_wq_current_is_worker() && req->task != current);
 
 	ret = io_async_cancel_one(req->task->io_uring, sqe_addr, ctx);
 	if (ret != -ENOENT)
@@ -6369,6 +6369,7 @@ static void io_wq_submit_work(struct io_wq_work *work)
 	if (timeout)
 		io_queue_linked_timeout(timeout);
 
+	/* either cancelled or io-wq is dying, so don't touch tctx->iowq */
 	if (work->flags & IO_WQ_WORK_CANCEL)
 		ret = -ECANCELED;
 
@@ -9184,8 +9185,8 @@ static void io_uring_clean_tctx(struct io_uring_task *tctx)
 		 * Must be after io_uring_del_task_file() (removes nodes under
 		 * uring_lock) to avoid race with io_uring_try_cancel_iowq().
 		 */
-		tctx->io_wq = NULL;
 		io_wq_put_and_exit(wq);
+		tctx->io_wq = NULL;
 	}
 }