io_uring: don't touch scm_fp_list after queueing skb

It's safer to not touch scm_fp_list after we queued an skb to which it
was assigned, there might be races lurking if we screw subtle sync
guarantees on the io_uring side.

Fixes: 6b06314c ("io_uring: add file set registration")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8631,8 +8631,12 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
 		refcount_add(skb->truesize, &sk->sk_wmem_alloc);
 		skb_queue_head(&sk->sk_receive_queue, skb);
 
-		for (i = 0; i < nr_files; i++)
-			fput(fpl->fp[i]);
+		for (i = 0; i < nr; i++) {
+			struct file *file = io_file_from_index(ctx, i + offset);
+
+			if (file)
+				fput(file);
+		}
 	} else {
 		kfree_skb(skb);
 		free_uid(fpl->user);