linux-kernel - Re: Possible NULL dereference caused by -stable commit ef81bb40bf15f350fe865f31fa42f1082772a576

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Message-ID: <1317211749.2941.6.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Wed, 28 Sep 2011 14:09:09 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...hat.com>,
	linux-kernel@...r.kernel.org, Greg KH <gregkh@...e.de>,
	stable@...nel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	Amos Kong <akong@...hat.com>
Subject: Re: Possible NULL dereference caused by -stable commit
 ef81bb40bf15f350fe865f31fa42f1082772a576

Le mercredi 28 septembre 2011 à 18:51 +0800, Jason Wang a écrit :
> Hi all:
> 
> A possible NULL dereference were noticed by the stable commit 
> ef81bb40bf15f350fe865f31fa42f1082772a576 (which is a backport of 
> 87c48fa3b4630905f98268dde838ee43626a060c). The case happens when bridge 
> froward a packet from guest to a physical nic, at this time no route is 
> attached to the skb which may lead a NULL dereference in 
> ipv6_select_ident(). -Next version have this check so it is fine. The 
> following patch may be used to avoid this but may also lead the ip 
> identification predicable, and this defect is also exist -next version 
> when no route because we still depends on a global variable to generate 
> the identification.  Any thought on this?

1) Discussion on current kernel :

All we need here is not the route but inet_peer, so that inet_getid()
can be called on it.

If no route is given to ipv6_select_ident(), at least we can try to get
inet_peer, and release it before exiting from ipv6_select_ident()

2) On stable kernel, we already use an array
(ipv6_fragmentation_id[FID_HASH_SZ];) to make less predictables
fragments ids.

So we could get dst addr from the packet to be forwarded instead of the
(possibly NULL) route.



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