Subject: Re: [BUG] skb corruption and kernel panic at forwarding with fragmentation

From: Florian Westphal <[email-masked]>
Date: Thu, 7 Jan 2016 00:49:43 +0100

Florian Westphal <[email-masked]> wrote:
> Thadeu Lima de Souza Cascardo <[email-masked]> wrote:
> > On Wed, Jan 06, 2016 at 11:11:41PM +0300, Konstantin Khlebnikov wrote:

[ skb_gso_segment uses skb->cb[], causes oops if ip_fragment is invoked
  on segmented skbs ]

> > I have hit this as well, this fixes it for me on an older kernel. Can you try it
> > on latest kernel?
> 
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index d8a1745..f44bc91 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -216,6 +216,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
> >  	netdev_features_t features;
> >  	struct sk_buff *segs;
> >  	int ret = 0;
> > +	struct inet_skb_parm ipcb;
> >  
> >  	if (skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
> >  		return ip_finish_output2(skb);
> > @@ -227,6 +228,10 @@ static int ip_finish_output_gso(struct sk_buff *skb)
> >  	 * 2) skb arrived via virtio-net, we thus get TSO/GSO skbs directly
> >  	 * from host network stack.
> >  	 */
> > +	/* We need to save IPCB here because skb_gso_segment will use
> > +	 * SKB_GSO_CB.
> > +	 */
> > +	ipcb = *IPCB(skb);
> >  	features = netif_skb_features(skb);
> >  	segs = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
> >  	if (IS_ERR_OR_NULL(segs)) {
> > @@ -241,6 +246,7 @@ static int ip_finish_output_gso(struct sk_buff *skb)
> >  		int err;
> >  
> >  		segs->next = NULL;
> > +		*IPCB(segs) = ipcb;
> >  		err = ip_fragment(segs, ip_finish_output2);
> >  
> >  		if (err && ret == 0)
> 
> I'm worried that this doesn't solve all cases. f.e. xfrm may also
> call skb_gso_segment(), and it will call into ipv4/ipv6 netfilter
> postrouting + ipv4 output functions...
> 
> nfqnl_enqueue_packet() is also affected.

... but it seems that those three are the only affected callers
of skb_gso_segment (tbf is ok since skb isn't owned by anyone,
ovs does save/restore already).

I think this patch is the right way, we just need similar
save/restore in nfqnl_enqueue_packet and xfrm_output_gso().

The latter two can be used by either ipv4 or ipv6 so it might
be preferable to just save/restore sizeof(struct skb_gso_cb);
or a union of inet_skb_parm+inet6_skb_parm.

Cascardo, can you cook a patch?

Thanks!