io_uring: protect fixed file indexing with array_index_nospec()

We index the file tables with a user given value. After we check
it's within our limits, use array_index_nospec() to prevent any
spectre attacks here.
Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2321,6 +2321,7 @@ static int io_req_set_file(struct io_ring_ctx *ctx, const struct sqe_submit *s,
 		if (unlikely(!ctx->user_files ||
 		    (unsigned) fd >= ctx->nr_user_files))
 			return -EBADF;
+		fd = array_index_nospec(fd, ctx->nr_user_files);
 		if (!ctx->user_files[fd])
 			return -EBADF;
 		req->file = ctx->user_files[fd];