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

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

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@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.

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