static int __mkroute_input(struct sk_buff *skb,
			   const struct fib_result *res,
			   struct in_device *in_dev,
			   __be32 daddr, __be32 saddr, u32 tos)
{
	struct fib_nh_exception *fnhe;
	struct rtable *rth;
	int err;
	struct in_device *out_dev;
	bool do_cache;
	u32 itag = 0;

	/* get a working reference to the output device */
	out_dev = __in_dev_get_rcu(FIB_RES_DEV(*res));
	if (!out_dev) {
		net_crit_ratelimited("Bug in ip_route_input_slow(). Please report.\n");
		return -EINVAL;
	}

	err = fib_validate_source(skb, saddr, daddr, tos, FIB_RES_OIF(*res),
				  in_dev->dev, in_dev, &itag);
	if (err < 0) {
		ip_handle_martian_source(in_dev->dev, in_dev, skb, daddr,
					 saddr);

		goto cleanup;
	}

	do_cache = res->fi && !itag;
	if (out_dev == in_dev && err && IN_DEV_TX_REDIRECTS(out_dev) &&
	    skb->protocol == htons(ETH_P_IP) &&
	    (IN_DEV_SHARED_MEDIA(out_dev) ||
	     inet_addr_onlink(out_dev, saddr, FIB_RES_GW(*res))))
		IPCB(skb)->flags |= IPSKB_DOREDIRECT;

	if (skb->protocol != htons(ETH_P_IP)) {
		/* Not IP (i.e. ARP). Do not create route, if it is
		 * invalid for proxy arp. DNAT routes are always valid.
		 *
		 * Proxy arp feature have been extended to allow, ARP
		 * replies back to the same interface, to support
		 * Private VLAN switch technologies. See arp.c.
		 */
		if (out_dev == in_dev &&
		    IN_DEV_PROXY_ARP_PVLAN(in_dev) == 0) {
			err = -EINVAL;
			goto cleanup;
		}
	}

	fnhe = find_exception(&FIB_RES_NH(*res), daddr);
	if (do_cache) {
		if (fnhe) {
			rth = rcu_dereference(fnhe->fnhe_rth_input);
			if (rth && rth->dst.expires &&
			    time_after(jiffies, rth->dst.expires)) {
				ip_del_fnhe(&FIB_RES_NH(*res), daddr);
				fnhe = NULL;
			} else {
				goto rt_cache;
			}
		}

		rth = rcu_dereference(FIB_RES_NH(*res).nh_rth_input);

rt_cache:
		if (rt_cache_valid(rth)) {
			skb_dst_set_noref(skb, &rth->dst);
			goto out;
		}
	}

	rth = rt_dst_alloc(out_dev->dev, 0, res->type,
			   IN_DEV_CONF_GET(in_dev, NOPOLICY),
			   IN_DEV_CONF_GET(out_dev, NOXFRM), do_cache);
	if (!rth) {
		err = -ENOBUFS;
		goto cleanup;
	}

	rth->rt_is_input = 1;
	if (res->table)
		rth->rt_table_id = res->table->tb_id;
	RT_CACHE_STAT_INC(in_slow_tot);

	rth->dst.input = ip_forward;

	rt_set_nexthop(rth, daddr, res, fnhe, res->fi, res->type, itag,
		       do_cache);
	set_lwt_redirect(rth);
	skb_dst_set(skb, &rth->dst);
out:
	err = 0;
 cleanup:
	return err;
}

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