linux-kernel - Re: [BUG] A panic caused by null pointer dereference aftering updating to

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Message-Id: <20140414.152225.1983723140136958968.davem@davemloft.net>
Date:	Mon, 14 Apr 2014 15:22:25 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	nasa4836@...il.com, jchapman@...alix.com, edumazet@...gle.com,
	joe@...ches.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [BUG] A panic caused by null pointer dereference aftering
 updating to

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 14 Apr 2014 12:06:36 -0700

> On Mon, 2014-04-14 at 14:48 -0400, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Mon, 14 Apr 2014 11:19:26 -0700
>> 
>> > ip_local_out() doesn't use skb->sk
>> 
>> It does Eric.
>> 
> 
> Hmmm, right...
> 
>> We had just such a report with this in the backtrace, when AF_PACKET
>> sends over vxlan devices.
>> 
>> The problem is ip_mc_output().
> 
> So this means that : User socket wanted sk_mc_loop(sk), but because
> vxlan changed skb->sk to internal socket, we were doing something else
> anyway.

Actually the exact opposite is happening.  vxlan does not override skb->sk,
and leaves it as AF_PACKET's socket.  Then we crash in ip_mc_output() because
it only expects IP sockets, not AF_PACKET ones, attached to skb->sk.

> There are a lot of undocumented features here... like
> 
>  	skb->priority = sk->sk_priority;
>  	skb->mark = sk->sk_mark;
> 
> which socket do we want here (in ip_queue_xmit()) ?

Probably that of tunnel.

> Its interesting to see ip6_xmit() already has a 'struct sock *sk'
> parameter...
> 
> This was the preliminary patch I tested :

You still need to make similar transformation to ip_local_out() as mentioned
above.
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