[ovs-dev] [PATCH net 2/2] act_ct: support asymmetric conntrack

[Marcelo Ricardo Leitner]

On Mon, Nov 18, 2019 at 04:21:39PM -0500, Aaron Conole wrote:
> Marcelo Ricardo Leitner <marcelo.leitner at gmail.com> writes:
> 
> > On Fri, Nov 08, 2019 at 04:07:14PM -0500, Aaron Conole wrote:
> >> The act_ct TC module shares a common conntrack and NAT infrastructure
> >> exposed via netfilter.  It's possible that a packet needs both SNAT and
> >> DNAT manipulation, due to e.g. tuple collision.  Netfilter can support
> >> this because it runs through the NAT table twice - once on ingress and
> >> again after egress.  The act_ct action doesn't have such capability.
> >> 
> >> Like netfilter hook infrastructure, we should run through NAT twice to
> >> keep the symmetry.
> >> 
> >> Fixes: b57dc7c13ea9 ("net/sched: Introduce action ct")
> >> 
> >> Signed-off-by: Aaron Conole <aconole at redhat.com>
> >> ---
> >>  net/sched/act_ct.c | 13 ++++++++++++-
> >>  1 file changed, 12 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/net/sched/act_ct.c b/net/sched/act_ct.c
> >> index fcc46025e790..f3232a00970f 100644
> >> --- a/net/sched/act_ct.c
> >> +++ b/net/sched/act_ct.c
> >> @@ -329,6 +329,7 @@ static int tcf_ct_act_nat(struct sk_buff *skb,
> >>  			  bool commit)
> >>  {
> >>  #if IS_ENABLED(CONFIG_NF_NAT)
> >> +	int err;
> >>  	enum nf_nat_manip_type maniptype;
> >>  
> >>  	if (!(ct_action & TCA_CT_ACT_NAT))
> >> @@ -359,7 +360,17 @@ static int tcf_ct_act_nat(struct sk_buff *skb,
> >>  		return NF_ACCEPT;
> >>  	}
> >>  
> >> -	return ct_nat_execute(skb, ct, ctinfo, range, maniptype);
> >> +	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);
> >> +	}
> >
> > I keep thinking about this and I'm not entirely convinced that this
> > shouldn't be simpler. More like:
> >
> > if (DNAT)
> > 	DNAT
> > if (SNAT)
> > 	SNAT
> >
> > So it always does DNAT before SNAT, similarly to what iptables would
> > do on PRE/POSTROUTING chains.
> 
> I can rewrite the whole function, but I wanted to start with the smaller
> fix that worked.  I also think it needs more testing then (since it's
> something of a rewrite of the function).
> 
> I guess it's not too important - do you think it gives any readability
> to do it this way?  If so, I can respin the patch changing it like you
> describe.

I didn't mean a rewrite, but just to never handle SNAT before DNAT. So
the fix here would be like:

-	return ct_nat_execute(skb, ct, ctinfo, range, maniptype);
+	err = ct_nat_execute(skb, ct, ctinfo, range, maniptype);
+	if (err == NF_ACCEPT && maniptype == NF_NAT_MANIP_DST &&
+	    ct->status & IPS_SRC_NAT && ct->status & IPS_DST_NAT) {
+		maniptype = NF_NAT_MANIP_SRC;
+		err = ct_nat_execute(skb, ct, ctinfo, range, maniptype);
+	}
+	return err;

> >> +	return err;
> >>  #else
> >>  	return NF_ACCEPT;
> >>  #endif
> >> -- 
> >> 2.21.0
> >> 
>