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

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Date:	Wed, 30 Oct 2013 15:13:54 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	Florian Westphal <fw@...len.de>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, pablo@...filter.org,
	netfilter-devel@...r.kernel.org, yoshfuji@...ux-ipv6.org,
	kadlec@...ckhole.kfki.hu, kaber@...sh.net, mleitner@...hat.com
Subject: Re: [patch net-next RFC] netfilter: ip6_tables: use reasm skb for
 matching

Wed, Oct 30, 2013 at 02:41:00PM CET, fw@...len.de wrote:
>Jiri Pirko <jiri@...nulli.us> 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?


Because each fragment need to be pushed through separately.

What different approach would you suggest?

Thanks

Jiri
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