[IPV6]: Do no rely on skb->dst before it is assigned.

Because skb->dst is assigned in ip6_route_input(), it is really
bad to use it in hop-by-hop option handler(s).

Closes: Bug #8450 (Eric Sesterhenn <snakebyte@gmx.de>)
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/exthdrs.c

@@ -660,6 +660,14 @@ EXPORT_SYMBOL_GPL(ipv6_invert_rthdr);
   Hop-by-hop options.
  **********************************/
 
+/*
+ * Note: we cannot rely on skb->dst before we assign it in ip6_route_input().
+ */
+static inline struct inet6_dev *ipv6_skb_idev(struct sk_buff *skb)
+{
+	return skb->dst ? ip6_dst_idev(skb->dst) : __in6_dev_get(skb->dev);
+}
+
 /* Router Alert as of RFC 2711 */
 
 static int ipv6_hop_ra(struct sk_buff **skbp, int optoff)
@@ -688,25 +696,25 @@ static int ipv6_hop_jumbo(struct sk_buff **skbp, int optoff)
 	if (nh[optoff + 1] != 4 || (optoff & 3) != 2) {
 		LIMIT_NETDEBUG(KERN_DEBUG "ipv6_hop_jumbo: wrong jumbo opt length/alignment %d\n",
 			       nh[optoff+1]);
-		IP6_INC_STATS_BH(ip6_dst_idev(skb->dst),
+		IP6_INC_STATS_BH(ipv6_skb_idev(skb),
 				 IPSTATS_MIB_INHDRERRORS);
 		goto drop;
 	}
 
 	pkt_len = ntohl(*(__be32 *)(nh + optoff + 2));
 	if (pkt_len <= IPV6_MAXPLEN) {
-		IP6_INC_STATS_BH(ip6_dst_idev(skb->dst), IPSTATS_MIB_INHDRERRORS);
+		IP6_INC_STATS_BH(ipv6_skb_idev(skb), IPSTATS_MIB_INHDRERRORS);
 		icmpv6_param_prob(skb, ICMPV6_HDR_FIELD, optoff+2);
 		return 0;
 	}
 	if (ipv6_hdr(skb)->payload_len) {
-		IP6_INC_STATS_BH(ip6_dst_idev(skb->dst), IPSTATS_MIB_INHDRERRORS);
+		IP6_INC_STATS_BH(ipv6_skb_idev(skb), IPSTATS_MIB_INHDRERRORS);
 		icmpv6_param_prob(skb, ICMPV6_HDR_FIELD, optoff);
 		return 0;
 	}
 
 	if (pkt_len > skb->len - sizeof(struct ipv6hdr)) {
-		IP6_INC_STATS_BH(ip6_dst_idev(skb->dst), IPSTATS_MIB_INTRUNCATEDPKTS);
+		IP6_INC_STATS_BH(ipv6_skb_idev(skb), IPSTATS_MIB_INTRUNCATEDPKTS);
 		goto drop;
 	}