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

[Aaron Conole]

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.

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.

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