io_uring: Fix race for sqes with userspace

commit 935d1e45908afb8853c497f2c2bbbb685dec51dc upstream.

io_ring_submit() finalises with
1. io_commit_sqring(), which releases sqes to the userspace
2. Then calls to io_queue_link_head(), accessing released head's sqe

Reorder them.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NXiaoguang Wang <xiaoguang.wang@linux.alibaba.com>

fs/io_uring.c

@@ -2794,13 +2794,14 @@ static int io_ring_submit(struct io_ring_ctx *ctx, unsigned int to_submit)
 		submit++;
 		io_submit_sqe(ctx, &s, statep, &link);
 	}
-	io_commit_sqring(ctx);
 
 	if (link)
 		io_queue_link_head(ctx, link, &link->submit, shadow_req);
 	if (statep)
 		io_submit_state_end(statep);
 
+	io_commit_sqring(ctx);
+
 	return submit;
 }