io_uring: fix races with file table unregister

Fixed file table quiesce might unlock ->uring_lock, potentially letting
new requests to be submitted, don't allow those requests to use the
table as they will race with unregistration.
Reported-and-tested-by: van fantasy <g1042620637@gmail.com>
Fixes: 05f3fb3c ("io_uring: avoid ring quiesce for fixed file set unregister and update")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>

fs/io_uring.c

@@ -9768,11 +9768,19 @@ static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
 
 static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
 {
+	unsigned nr = ctx->nr_user_files;
 	int ret;
 
 	if (!ctx->file_data)
 		return -ENXIO;
+
+	/*
+	 * Quiesce may unlock ->uring_lock, and while it's not held
+	 * prevent new requests using the table.
+	 */
+	ctx->nr_user_files = 0;
 	ret = io_rsrc_ref_quiesce(ctx->file_data, ctx);
+	ctx->nr_user_files = nr;
 	if (!ret)
 		__io_sqe_files_unregister(ctx);
 	return ret;