openvswitch: handle DNAT tuple collision

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

For example, two openvswitch flows:
nw_dst=10.0.0.10,tp_dst=10, actions=ct(commit,table=2,nat(dst=20.0.0.1:20))
nw_dst=10.0.0.20,tp_dst=10, actions=ct(commit,table=2,nat(dst=20.0.0.1:20))

Assuming two TCP clients initiating the following connections:
10.0.0.10:5000->10.0.0.10:10
10.0.0.10:5000->10.0.0.20:10

Both tuples would translate to 10.0.0.10:5000->20.0.0.1:20 causing
nf_conntrack_confirm() to fail because of tuple collision.

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.

Reported-at: https://bugzilla.redhat.com/1877128Suggested-by: Florian Westphal <fw@strlen.de>
Fixes: 05752523 ("openvswitch: Interface with NAT.")
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/openvswitch/conntrack.c

@@ -905,8 +905,8 @@ static int ovs_ct_nat(struct net *net, struct sw_flow_key *key,
 	}
 	err = ovs_ct_nat_execute(skb, ct, ctinfo, &info->range, maniptype);
 
-	if (err == NF_ACCEPT &&
-	    ct->status & IPS_SRC_NAT && ct->status & IPS_DST_NAT) {
+	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
@@ -914,6 +914,10 @@ static int ovs_ct_nat(struct net *net, struct sw_flow_key *key,
 
 			err = ovs_ct_nat_execute(skb, ct, ctinfo, &info->range,
 						 maniptype);
+		} else if (CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
+			err = ovs_ct_nat_execute(skb, ct, ctinfo, NULL,
+						 NF_NAT_MANIP_SRC);
+		}
 	}
 
 	/* Mark NAT done if successful and update the flow key. */