datapath: handle DNAT tuple collision.

Upstream commit:
    commit 8aa7b526dc0b5dbf40c1b834d76a667ad672a410
    Author: Dumitru Ceara <[email protected]>
    Date:   Wed Oct 7 17:48:03 2020 +0200

    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/1877128
    Suggested-by: Florian Westphal <[email protected]>
    Fixes: 05752523e565 ("openvswitch: Interface with NAT.")
    Signed-off-by: Dumitru Ceara <[email protected]>
    Signed-off-by: Jakub Kicinski <[email protected]>

Fixes: f8f97cd ("datapath: Interface with NAT.")
Signed-off-by: Dumitru Ceara <[email protected]>
Acked-by: Paolo Valerio <[email protected]>
Signed-off-by: Ilya Maximets <[email protected]>

datapath/conntrack.c

@@ -978,15 +978,19 @@ 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 (maniptype == NF_NAT_MANIP_SRC)
-			maniptype = NF_NAT_MANIP_DST;
-		else
-			maniptype = NF_NAT_MANIP_SRC;
-
-		err = ovs_ct_nat_execute(skb, ct, ctinfo, &info->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 = 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. */