netdev - Re: Revert 462fb2af9788a82a534f8184abfde31574e1cfa0 (bridge : Sanitize skb before it enters the IP stack)

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Date:	Mon, 19 May 2014 22:49:32 +0200
From:	Bart De Schuymer <bdschuym@...dora.be>
To:	Florian Westphal <fw@...len.de>,
	David Newall <davidn@...idnewall.com>
CC:	Stephen Hemminger <stephen@...workplumber.org>,
	Netdev <netdev@...r.kernel.org>, netfilter-devel@...r.kernel.org,
	bridge@...ts.linux-foundation.org
Subject: Re: Revert 462fb2af9788a82a534f8184abfde31574e1cfa0 (bridge : Sanitize
 skb before it enters the IP stack)

Florian Westphal schreef op 19/05/2014 19:09:
> David Newall <davidn@...idnewall.com> wrote:
>
> [ remove lkml and cc nf-devel ]
>
>> I tried to persevere with the commit: I recalculated checksum, which
>> left routes and times improperly updated in options.  Then I tried
>> calling ip_forward_options, which looks like it would correctly
>> update RR and TS (not to mention checksum)m but that bombed because
>> skb_rtable returned NULL.
>
> Yes.  bridge<->netfilter wiring is pure duct tape.
> The glue code will set up a fake rtable for the skb after the
> prerouting hook. [ see br_nf_pre_routing_finish() ].
>
>> I see three ways to progress:
>>
>> 1. Possibly call ip_forward_option, but that requires somebody who
>> understands this code to help;
>> 2. Just recalculate the checksum, leaving crap in the options; or
>> 3. Revert the commit.
>
> I think none of these are an option.
>
> I fail to understand why a bridge should honor/modifiy IP options.
>
> For the 'local delivery' case the ip stack will take care of
> option parsing, for forwarding it should be sufficient to do
> sanity tests (for netfilters sake).
>
>>>From a quick glance, it should be sufficient to edit
> br_parse_ip_options() and remove everything after
>
> memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
>
> A 2nd step would be to move a copy of ip_options_compile()
> into br_netfilter.c and trim it down to only validate the
> ipv4 header without modifying it.

Perhaps it's possible to call ip_options_compile with a skb == NULL, 
like ip_options.c::ip_options_get_finish does. That way we don't need to 
duplicate code.
An alternative would be to make sure that the data pointed to by IPCB 
and BR_INPUT_SKB_CB don't overlap. If this were the case, we could 
indeed just revert the commit that was referred to.


cheers,
Bart

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