netfilter: nf_flowtable: skip device lookup from interface index

Use the output device from the route that we cache in the flowtable
entry.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_ip.c

@@ -235,13 +235,10 @@ nf_flow_offload_ip_hook(void *priv, struct sk_buff *skb,
 	if (tuplehash == NULL)
 		return NF_ACCEPT;
 
-	outdev = dev_get_by_index_rcu(state->net, tuplehash->tuple.oifidx);
-	if (!outdev)
-		return NF_ACCEPT;
-
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 	rt = (struct rtable *)flow->tuplehash[dir].tuple.dst_cache;
+	outdev = rt->dst.dev;
 
 	if (unlikely(nf_flow_exceeds_mtu(skb, flow->tuplehash[dir].tuple.mtu)) &&
 	    (ip_hdr(skb)->frag_off & htons(IP_DF)) != 0)
@@ -452,13 +449,10 @@ nf_flow_offload_ipv6_hook(void *priv, struct sk_buff *skb,
 	if (tuplehash == NULL)
 		return NF_ACCEPT;
 
-	outdev = dev_get_by_index_rcu(state->net, tuplehash->tuple.oifidx);
-	if (!outdev)
-		return NF_ACCEPT;
-
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
 	rt = (struct rt6_info *)flow->tuplehash[dir].tuple.dst_cache;
+	outdev = rt->dst.dev;
 
 	if (unlikely(nf_flow_exceeds_mtu(skb, flow->tuplehash[dir].tuple.mtu)))
 		return NF_ACCEPT;