From: Felix Fietkau <nbd@nbd.name>
Date: Thu, 15 Mar 2018 18:21:43 +0100
Subject: [PATCH] netfilter: nf_flow_table: clean up and fix dst handling

dst handling in the code is inconsistent and possibly wrong. In my test,
skb_dst(skb) holds the dst entry after routing but before NAT, so the
code could possibly return the same dst entry for both directions of a
connection.
Additionally, there was some confusion over the dst entry vs the address
passed as parameter to rt_nexthop/rt6_nexthop.

Do an explicit dst lookup for both ends of the connection and always use
the source address for it. When running the IP hook, use the dst entry
for the opposite direction for determining the route.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
---

--- a/net/netfilter/nf_flow_table_ip.c
+++ b/net/netfilter/nf_flow_table_ip.c
@@ -238,7 +238,7 @@ nf_flow_offload_ip_hook(void *priv, stru
 
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
-	rt = (const struct rtable *)flow->tuplehash[dir].tuple.dst_cache;
+	rt = (const struct rtable *)flow->tuplehash[!dir].tuple.dst_cache;
 
 	if (unlikely(nf_flow_exceeds_mtu(skb, flow->tuplehash[dir].tuple.mtu)) &&
 	    (ip_hdr(skb)->frag_off & htons(IP_DF)) != 0)
@@ -455,7 +455,7 @@ nf_flow_offload_ipv6_hook(void *priv, st
 
 	dir = tuplehash->tuple.dir;
 	flow = container_of(tuplehash, struct flow_offload, tuplehash[dir]);
-	rt = (struct rt6_info *)flow->tuplehash[dir].tuple.dst_cache;
+	rt = (struct rt6_info *)flow->tuplehash[!dir].tuple.dst_cache;
 
 	if (unlikely(nf_flow_exceeds_mtu(skb, flow->tuplehash[dir].tuple.mtu)))
 		return NF_ACCEPT;
--- a/net/netfilter/nft_flow_offload.c
+++ b/net/netfilter/nft_flow_offload.c
@@ -17,27 +17,38 @@ struct nft_flow_offload {
 	struct nft_flowtable	*flowtable;
 };
 
-static int nft_flow_route(const struct nft_pktinfo *pkt,
-			  const struct nf_conn *ct,
-			  struct nf_flow_route *route,
-			  enum ip_conntrack_dir dir)
+static struct dst_entry *
+nft_flow_dst(const struct nf_conn *ct, enum ip_conntrack_dir dir,
+	     const struct nft_pktinfo *pkt)
 {
-	struct dst_entry *this_dst = skb_dst(pkt->skb);
-	struct dst_entry *other_dst = NULL;
+	struct dst_entry *dst;
 	struct flowi fl;
 
 	memset(&fl, 0, sizeof(fl));
 	switch (nft_pf(pkt)) {
 	case NFPROTO_IPV4:
-		fl.u.ip4.daddr = ct->tuplehash[!dir].tuple.dst.u3.ip;
+		fl.u.ip4.daddr = ct->tuplehash[dir].tuple.src.u3.ip;
 		break;
 	case NFPROTO_IPV6:
-		fl.u.ip6.daddr = ct->tuplehash[!dir].tuple.dst.u3.in6;
+		fl.u.ip6.daddr = ct->tuplehash[dir].tuple.src.u3.in6;
 		break;
 	}
 
-	nf_route(nft_net(pkt), &other_dst, &fl, false, nft_pf(pkt));
-	if (!other_dst)
+	nf_route(nft_net(pkt), &dst, &fl, false, nft_pf(pkt));
+
+	return dst;
+}
+
+static int nft_flow_route(const struct nft_pktinfo *pkt,
+			  const struct nf_conn *ct,
+			  struct nf_flow_route *route,
+			  enum ip_conntrack_dir dir)
+{
+	struct dst_entry *this_dst, *other_dst;
+
+	this_dst = nft_flow_dst(ct, dir, pkt);
+	other_dst = nft_flow_dst(ct, !dir, pkt);
+	if (!this_dst || !other_dst)
 		return -ENOENT;
 
 	route->tuple[dir].dst		= this_dst;
