Subject: Re: [patch net-next RFC] netfilter: ip6_tables: use reasm skb for matching

From: Florian Westphal <[email-masked]>
Date: Wed, 30 Oct 2013 14:41:00 +0100

Jiri Pirko <[email-masked]> wrote:
> Currently, when ipv6 fragment goes through the netfilter, match
> functions are called on them directly. This might cause match function
> to fail. So benefit from the fact that nf_defrag_ipv6 constructs
> reassembled skb for us and use this reassembled skb for matching.
> 
> This patch fixes for example following situation:
> On HOSTA do:
> ip6tables -I INPUT -p icmpv6 -j DROP
> ip6tables -I INPUT -p icmpv6 -m icmp6 --icmpv6-type 128 -j ACCEPT
> 
> and on HOSTB you do:
> ping6 HOSTA -s2000    (MTU is 1500)
> 
> Incoming echo requests will be filtered out on HOSTA. This issue does
> not occur with smaller packets than MTU (where fragmentation does not happen).

[..]
> diff --git a/net/ipv6/netfilter/ip6_tables.c b/net/ipv6/netfilter/ip6_tables.c
> index 44400c2..5421beb0 100644
> --- a/net/ipv6/netfilter/ip6_tables.c
> +++ b/net/ipv6/netfilter/ip6_tables.c
> @@ -328,6 +328,7 @@ ip6t_do_table(struct sk_buff *skb,
>  	const struct xt_table_info *private;
>  	struct xt_action_param acpar;
>  	unsigned int addend;
> +	struct sk_buff *reasm = skb->nfct_reasm ? skb->nfct_reasm : skb;
>  
>  	/* Initialization */
>  	indev = in ? in->name : nulldevname;
> @@ -363,7 +364,7 @@ ip6t_do_table(struct sk_buff *skb,
>  
>  		IP_NF_ASSERT(e);
>  		acpar.thoff = 0;
> -		if (!ip6_packet_match(skb, indev, outdev, &e->ipv6,
> +		if (!ip6_packet_match(reasm, indev, outdev, &e->ipv6,
>  		    &acpar.thoff, &acpar.fragoff, &acpar.hotdrop)) {

[..]

This is a bit backwards, I think.
- We gather frags
- Then we invoke ip6t_do_table for each individual fragment

So basically your patch is equivalent to
for_each_frag( )
  ip6t_do_table(reassembled_skb)

Which makes no sense to me - why traverse the ruleset n times with the same
packet?