io_uring: check if iowq is killed before queuing

task work can be executed after the task has gone through io_uring
termination, whether it's the final task_work run or the fallback path.
In this case, task work will find ->io_wq being already killed and
null'ed, which is a problem if it then tries to forward the request to
io_queue_iowq(). Make io_queue_iowq() fail requests in this case.

Note that it also checks PF_KTHREAD, because the user can first close
a DEFER_TASKRUN ring and shortly after kill the task, in which case
->iowq check would race.

Cc: stable@vger.kernel.org
Fixes: 50c5225 ("block: implement async io_uring discard cmd")
Fixes: 773af69 ("io_uring: always reissue from task_work context")
Reported-by: Will <willsroot@protonmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/63312b4a2c2bb67ad67b857d17a300e1d3b078e8.1734637909.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -514,7 +514,11 @@ static void io_queue_iowq(struct io_kiocb *req)
 	struct io_uring_task *tctx = req->tctx;
 
 	BUG_ON(!tctx);
-	BUG_ON(!tctx->io_wq);
+
+	if ((current->flags & PF_KTHREAD) || !tctx->io_wq) {
+		io_req_task_queue_fail(req, -ECANCELED);
+		return;
+	}
 
 	/* init ->work of the whole link before punting */
 	io_prep_async_link(req);