Re: [PATCH net 2/2] act_ct: support asymmetric conntrack

[Marcelo Ricardo Leitner]

On Fri, Nov 22, 2019 at 03:39:16PM -0500, Aaron Conole wrote:
> Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx> writes:
> 
> > On Mon, Nov 18, 2019 at 04:21:39PM -0500, Aaron Conole wrote:
> >> Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx> 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@xxxxxxxxxx>
> >> >> ---
> >> >>  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;
> 
> But the maniptype of the first call could be NAT_MANIP_SRC.  In fact,
> that's what I see if the packet is reply direction && !related.

Interesting, ok.

> 
> So, we need the block to invert the manipulation type.  Otherwise, we
> miss the DNAT manipulation.
> 
> So I don't think I can use that block.

Thanks for digging on it.

Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>

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