Subject: Re: 2.6.25-rc: Null dereference in ip_defrag

From: Patrick McHardy <[email-masked]>
Date: Mon, 17 Mar 2008 18:43:32 +0100

Pavel Emelyanov wrote:
> Phil Oester wrote:
>> And the packets causing the problem are all multicast fragments being
>> generated by Quagga's OSPFD (see debug output below).  Tried manually generating
>> some multicast fragments with iperf, but couldn't reproduce it.  
>>
>> Any ideas?
> 
> This is the same as the problem fixed here:
> 
>     commit 4136cd523eb0c0bd53173e16fd7406d31d05824f
>     [IPV4]: route: fix crash ip_route_input
> 
> The sk_buff does not have a valid dev sometimes in ip_defrag() :(
> and you have to setup conntrack rules to make packets go this way.
> But unlike the above problem we cannot even trust the skb->dst to
> be not NULL...

We can on output. Usually we don't even see fragments in conntrack
on output since we've defer fragmentation until all netfilter hooks
have been processed. Quagga is generating fragments using raw sockets
and IP_HDRINCL though.

 > Can you check with this patch, please (untested, but should work)?

This is getting pretty ugly. Shouldn't

int ip_defrag(struct sk_buff *skb, u32 user)
{
...
-         net = skb->dev->nd_net;
+         net = skb->dev ? skb->dev->nd_net : skb->dst->dev->nd_net;

work as well?