Subject: Re: Occasional oops with IPSec and IPv6.

From: Nick Bowler <[email-masked]>
Date: Fri, 18 Nov 2011 16:21:29 -0500

On 2011-11-18 22:06 +0200, Timo Teräs wrote:
> It's still headroom underrun.
> 
> I'm not too familiar with the relevant IPv6 code, but it seems to be
> mostly modelled after the IPv4 side. Looking at the back trace offset
> inside ipv6_fragment, I'd say it was taking the "fast path" for
> constructing the fragments. So first guess is that the headroom check
> for allowing fast path to happen is not right.
> 
> Since the code seems to be treating separately hlen and struct frag_hdr,
> I'm wondering if the following patch would be in place?
> 
> diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> index 1c9bf8b..c35d9fc 100644
> --- a/net/ipv6/ip6_output.c
> +++ b/net/ipv6/ip6_output.c
> @@ -675,7 +675,7 @@ int ip6_fragment(struct sk_buff *skb, int
> (*output)(struct sk_buff *))
>  			/* Correct geometry. */
>  			if (frag->len > mtu ||
>  			    ((frag->len & 7) && frag->next) ||
> -			    skb_headroom(frag) < hlen)
> +			    skb_headroom(frag) < hlen + sizeof(struct frag_hdr))
>  				goto slow_path_clean;
> 
>  			/* Partially cloned skb? */
> 
> 
> Alternatively, we could just run the "slow path" unconditionally with
> the test load to see if it fixes the issue. At least that'd be pretty
> good test if it's a problem in the ipv6 fragmentation code or something
> else.

Good call.  I replaced the "correct geometry" check with an
unconditional "goto slow_path_clean;", and I can no longer reproduce the
crash.  So at the very least, I have a workaround now.  (I still have
Herbert Xu's six patches applied on top of Linus' master).

I then tried the smaller change above, but this does not correct the
issue.

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)