Re: net: GPF in eth_header

[Eric Dumazet]

On Mon, 2016-11-28 at 13:05 -0800, Eric Dumazet wrote:
> 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;

Or more exactly , returning NF_DROP so that skb is really freed ;)


diff --git a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
index f7aab5ab93a5..508739a3ca2a 100644
--- a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
+++ b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
@@ -65,9 +65,9 @@ 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)
-		return NF_STOLEN;
+	/* queued or mangled ... */
+	if (err)
+		return (err == -EINPROGRESS) ? NF_STOLEN : NF_DROP;
 
 	return NF_ACCEPT;
 }