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Subject: Re: unable to handle kernel NULL pointer dereference in skb_dequeue
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Andrej Ota <andrej@ota.si>
Cc: linux-kernel@vger.kernel.org, gvs@zemos.net,
        Rami Rosen <ramirose@gmail.com>, netdev <netdev@vger.kernel.org>
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	 <1291381791.2897.250.camel@edumazet-laptop>
	 <aa43945b38cee751a8231084866b074d@192.168.253.23>
Content-Type: text/plain; charset="UTF-8"
Date: Fri, 03 Dec 2010 15:46:35 +0100
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Le vendredi 03 décembre 2010 à 15:37 +0100, Andrej Ota a écrit :
> >> Patch that works for me is below. Now I only hope I haven't
> >> (re)introduced a memory leak...
> 
> > Problem comes from commit 55c95e738da85 (fix return value of
> > __pppoe_xmit() method)
> > 
> > I am not sure patch is OK
> 
> 
> Me neither. That's why I wrote "works for me". All I dare say is that it
> works better than current code and is probably no worse than it was before
> above mentioned commit. Apart from that, there is no point in having return
> value for __pppoe_xmit if return value isn't needed.
> 
> Easiest way of triggering this BUG is by terminating PPPoE on the server
> side, which then hits "if (!dev) { goto abort; }". This in turn calls
> "kfree_skb(skb); return 0;" which returns to pppoe_rcv_core which then
> goto-s to "abort_put" which again calls "kfree_skb(skb)". Voila the bug.
> 
> I don't know how to trigger "if (skb_cow_head(skb, ..." to see if I have
> just caused another BUG. However, if I read file comments at the top, I see
> a comment from 19/07/01 stating that I have to delete original skb if code
> succeeds and never delete it on failure. About the skb copy mentioned in
> the same comment, I don't know. 2001 was many commits ago.

Well, all I wanted to say was that _I_ was not sure, but probably other
network guys have a better diagnostic.

Rami, could you re-explain the rationale of your patch ?

Thanks


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