io_uring: limit fixed table size by RLIMIT_NOFILE

Limit the number of files in io_uring fixed tables by RLIMIT_NOFILE,
that's the first and the simpliest restriction that we should impose.

Cc: stable@vger.kernel.org
Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b2756c340aed7d6c0b302c26dab50c6c5907f4ce.1629451684.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7733,6 +7733,8 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
 		return -EINVAL;
 	if (nr_args > IORING_MAX_FIXED_FILES)
 		return -EMFILE;
+	if (nr_args > rlimit(RLIMIT_NOFILE))
+		return -EMFILE;
 	ret = io_rsrc_node_switch_start(ctx);
 	if (ret)
 		return ret;