Re: net: GPF in eth_header

[Eric Dumazet]

On Mon, 2016-11-28 at 11:47 -0800, Eric Dumazet wrote:
> On Mon, 2016-11-28 at 20:34 +0100, Dmitry Vyukov wrote:
> > On Mon, Nov 28, 2016 at 8:04 PM, 'Andrey Konovalov' via syzkaller
> 
> > > Hi Eric,
> > >
> > > As far as I can see, skb_network_offset() becomes negative after
> > > pskb_pull(skb, (u8 *) (fhdr + 1) - skb->data) in nf_ct_frag6_queue().
> > > At least I'm able to detect that with a BUG_ON().
> > >
> > > Also it seems that the issue is only reproducible (at least with the
> > > poc I provided) for a short time after boot.
> > 
> > 
> > Eric,
> > 
> > Is it enough to debug? Or maybe Andrey can trace some values for you.
> 
> Well, now we are talking, if you tell me how many modules you load, it
> might help ;)
> 
> nf_ct_frag6_queue is nowhere to be seen in my kernels, that might
> explain why I could not reproduce the bug.
> 
> Let me try ;)
> 

Might be a bug added in commit daaa7d647f81f3
("netfilter: ipv6: avoid nf_iterate recursion")

Florian, what do you think of dropping a packet that presumably was
mangled badly by nf_ct_frag6_queue() ?

(Like about 48 byte pulled :(, and/or skb->csum changed )

diff --git a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
index f7aab5ab93a5..31aa947332d8 100644
--- a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
+++ b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
@@ -65,8 +65,8 @@ static unsigned int ipv6_defrag(void *priv,
 
 	err = nf_ct_frag6_gather(state->net, skb,
 				 nf_ct6_defrag_user(state->hook, skb));
-	/* queued */
-	if (err == -EINPROGRESS)
+	/* queued or mangled ... */
+	if (err)
 		return NF_STOLEN;
 
 	return NF_ACCEPT;