Subject: Re: Yet another bridge netfilter crash

From: Patrick McHardy <[email-masked]>
Date: Wed, 04 Aug 2010 18:30:58 +0200

Am 23.07.2010 17:26, schrieb Herbert Xu:
> On Fri, Jul 23, 2010 at 05:17:42PM +0200, Patrick McHardy wrote:
>>
>>> There's also the matter of fragments jumping between bridges.
>>
>> Conntrack zones can be used to avoid that, but that currently needs
>> manual configuration.
> 
> I think this is something that we need to fix.  Because as it
> stands, it can still crash if you get the wrong nf_bridge.
> 
> The reason is that skb->dev does not hold a ref count.  So the
> reassembly code just throws it away and always uses the dev of
> the last fragment.
> 
> This breaks when two bridges combine to reassemble a single
> packet, as the nf_bridge attribute of the reassembled packet
> may come from an skb whose device is now dead.  This is then
> used to fill in the skb->dev (via nf_bridge->physindev).

We could perform a new device lookup on reassembly as we do
when expiring a fragment queue, but we probably shouldn't even
be reassembling fragments from different bridges. One way to
avoid this would be to automatically assign each bridge device
to a different conntrack zone, but conntrack zones are limited
to 2^16 and this might also have other unwanted side-effects.

Until we come up with something better the best fix seems to
be to perform the device lookup based on the iif.