ublk_drv: do not add a re-issued request aborted previously to ioucmd's task_work

In ublk_queue_rq(), Assume current request is a re-issued request aborted
previously in monitor_work because the ubq_daemon(ioucmd's task) is
PF_EXITING. For this request, we cannot call
io_uring_cmd_complete_in_task() anymore because at that moment io_uring
context may be freed in case that no inflight ioucmd exists. Otherwise,
we may cause null-deref in ctx->fallback_work.

Add a check on UBLK_IO_FLAG_ABORTED to prevent the above situation. This
check is safe and makes sense.

Note: monitor_work sets UBLK_IO_FLAG_ABORTED and ends this request
(releasing the tag). Then the request is restarted(allocating the tag)
and we are here. Since releasing/allocating a tag implies smp_mb(),
finding UBLK_IO_FLAG_ABORTED guarantees that here is a re-issued request
aborted previously.
Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220815023633.259825-4-ZiyangZhang@linux.alibaba.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -756,9 +756,25 @@ static blk_status_t ublk_queue_rq(struct blk_mq_hw_ctx *hctx,
 		if (task_work_add(ubq->ubq_daemon, &data->work, notify_mode))
 			goto fail;
 	} else {
-		struct io_uring_cmd *cmd = ubq->ios[rq->tag].cmd;
+		struct ublk_io *io = &ubq->ios[rq->tag];
+		struct io_uring_cmd *cmd = io->cmd;
 		struct ublk_uring_cmd_pdu *pdu = ublk_get_uring_cmd_pdu(cmd);
 
+		/*
+		 * If the check pass, we know that this is a re-issued request aborted
+		 * previously in monitor_work because the ubq_daemon(cmd's task) is
+		 * PF_EXITING. We cannot call io_uring_cmd_complete_in_task() anymore
+		 * because this ioucmd's io_uring context may be freed now if no inflight
+		 * ioucmd exists. Otherwise we may cause null-deref in ctx->fallback_work.
+		 *
+		 * Note: monitor_work sets UBLK_IO_FLAG_ABORTED and ends this request(releasing
+		 * the tag). Then the request is re-started(allocating the tag) and we are here.
+		 * Since releasing/allocating a tag implies smp_mb(), finding UBLK_IO_FLAG_ABORTED
+		 * guarantees that here is a re-issued request aborted previously.
+		 */
+		if ((io->flags & UBLK_IO_FLAG_ABORTED))
+			goto fail;
+
 		pdu->req = rq;
 		io_uring_cmd_complete_in_task(cmd, ublk_rq_task_work_cb);
 	}