[NETFILTER]: Fix xfrm lookup after SNAT

To find out if a packet needs to be handled by IPsec after SNAT, packets
are currently rerouted in POST_ROUTING and a new xfrm lookup is done. This
breaks SNAT of non-unicast packets to non-local addresses because the
packet is routed as incoming packet and no neighbour entry is bound to the
dst_entry. In general, it seems to be a bad idea to replace the dst_entry
after the packet was already sent to the output routine because its state
might not match what's expected.

This patch changes the xfrm lookup in POST_ROUTING to re-use the original
dst_entry without routing the packet again. This means no policy routing
can be used for transport mode transforms (which keep the original route)
when packets are SNATed to match the policy, but it looks like the best
we can do for now.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 44 insertions, 3 deletions

diff --git a/net/ipv4/netfilter.c b/net/ipv4/netfilter.c
index 52a3d7c..ed42cdc 100644
--- a/net/ipv4/netfilter.c
+++ b/net/ipv4/netfilter.c
@@ -78,6 +78,47 @@ int ip_route_me_harder(struct sk_buff **pskb)
 }
 EXPORT_SYMBOL(ip_route_me_harder);
 
+#ifdef CONFIG_XFRM
+int ip_xfrm_me_harder(struct sk_buff **pskb)
+{
+	struct flowi fl;
+	unsigned int hh_len;
+	struct dst_entry *dst;
+
+	if (IPCB(*pskb)->flags & IPSKB_XFRM_TRANSFORMED)
+		return 0;
+	if (xfrm_decode_session(*pskb, &fl, AF_INET) < 0)
+		return -1;
+
+	dst = (*pskb)->dst;
+	if (dst->xfrm)
+		dst = ((struct xfrm_dst *)dst)->route;
+	dst_hold(dst);
+
+	if (xfrm_lookup(&dst, &fl, (*pskb)->sk, 0) < 0)
+		return -1;
+
+	dst_release((*pskb)->dst);
+	(*pskb)->dst = dst;
+
+	/* Change in oif may mean change in hh_len. */
+	hh_len = (*pskb)->dst->dev->hard_header_len;
+	if (skb_headroom(*pskb) < hh_len) {
+		struct sk_buff *nskb;
+
+		nskb = skb_realloc_headroom(*pskb, hh_len);
+		if (!nskb)
+			return -1;
+		if ((*pskb)->sk)
+			skb_set_owner_w(nskb, (*pskb)->sk);
+		kfree_skb(*pskb);
+		*pskb = nskb;
+	}
+	return 0;
+}
+EXPORT_SYMBOL(ip_xfrm_me_harder);
+#endif
+
 void (*ip_nat_decode_session)(struct sk_buff *, struct flowi *);
 EXPORT_SYMBOL(ip_nat_decode_session);
 
diff --git a/net/ipv4/netfilter/ip_nat_standalone.c b/net/ipv4/netfilter/ip_nat_standalone.c
index 92c5499..7c3f7d3 100644
--- a/net/ipv4/netfilter/ip_nat_standalone.c
+++ b/net/ipv4/netfilter/ip_nat_standalone.c
@@ -235,19 +235,19 @@ ip_nat_out(unsigned int hooknum,
 		return NF_ACCEPT;
 
 	ret = ip_nat_fn(hooknum, pskb, in, out, okfn);
+#ifdef CONFIG_XFRM
 	if (ret != NF_DROP && ret != NF_STOLEN
 	    && (ct = ip_conntrack_get(*pskb, &ctinfo)) != NULL) {
 		enum ip_conntrack_dir dir = CTINFO2DIR(ctinfo);
 
 		if (ct->tuplehash[dir].tuple.src.ip !=
 		    ct->tuplehash[!dir].tuple.dst.ip
-#ifdef CONFIG_XFRM
 		    || ct->tuplehash[dir].tuple.src.u.all !=
 		       ct->tuplehash[!dir].tuple.dst.u.all
-#endif
 		    )
-			return ip_route_me_harder(pskb) == 0 ? ret : NF_DROP;
+			return ip_xfrm_me_harder(pskb) == 0 ? ret : NF_DROP;
 	}
+#endif
 	return ret;
 }