struct rtable *ip_route_output_flow(struct net *net, struct flowi4 *flp4,
				    const struct sock *sk)
{
	struct rtable *rt = __ip_route_output_key(net, flp4);

	if (IS_ERR(rt))
		return rt;

	if (flp4->flowi4_proto)
		rt = (struct rtable *)xfrm_lookup_route(net, &rt->dst,
							flowi4_to_flowi(flp4),
							sk, 0);

	return rt;
}

The product dereferences a pointer that it expects to be valid but is NULL.