net/sched: act_ct: handle DNAT tuple collision

This this the counterpart of 8aa7b52 ("openvswitch: handle DNAT
tuple collision") for act_ct. From that commit changelog:

"""
With multiple DNAT rules it's possible that after destination
translation the resulting tuples collide.

...

Netfilter handles this case by allocating a null binding for SNAT at
egress by default.  Perform the same operation in openvswitch for DNAT
if no explicit SNAT is requested by the user and allocate a null binding
for SNAT for packets in the "original" direction.
"""

Fixes: 95219af ("act_ct: support asymmetric conntrack")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/act_ct.c

@@ -904,14 +904,19 @@ static int tcf_ct_act_nat(struct sk_buff *skb,
 	}
 
 	err = ct_nat_execute(skb, ct, ctinfo, range, maniptype);
-	if (err == NF_ACCEPT &&
-	    ct->status & IPS_SRC_NAT && ct->status & IPS_DST_NAT) {
-		if (maniptype == NF_NAT_MANIP_SRC)
-			maniptype = NF_NAT_MANIP_DST;
-		else
-			maniptype = NF_NAT_MANIP_SRC;
-
-		err = ct_nat_execute(skb, ct, ctinfo, range, maniptype);
+	if (err == NF_ACCEPT && ct->status & IPS_DST_NAT) {
+		if (ct->status & IPS_SRC_NAT) {
+			if (maniptype == NF_NAT_MANIP_SRC)
+				maniptype = NF_NAT_MANIP_DST;
+			else
+				maniptype = NF_NAT_MANIP_SRC;
+
+			err = ct_nat_execute(skb, ct, ctinfo, range,
+					     maniptype);
+		} else if (CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
+			err = ct_nat_execute(skb, ct, ctinfo, NULL,
+					     NF_NAT_MANIP_SRC);
+		}
 	}
 	return err;
 #else