netdev - Re: Problematic commits in the ipsec tree

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Message-ID: <20130823110323.GB808@order.stressinduktion.org>
Date:	Fri, 23 Aug 2013 13:03:23 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Steffen Klassert <steffen.klassert@...unet.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: Problematic commits in the ipsec tree

Hello!

On Fri, Aug 23, 2013 at 10:58:07AM +0200, Steffen Klassert wrote:
> On Thu, Aug 22, 2013 at 03:53:42PM +0200, Hannes Frederic Sowa wrote:
> > On Thu, Aug 22, 2013 at 12:47:24PM +0200, Steffen Klassert wrote:
> > > Hannes,
> > > 
> > > I have two problematic commits from you in the ipsec tree. The first one is:
> > > 
> > > commit 0ea9d5e3e (xfrm: introduce helper for safe determination of mtu)
> > > 
> > > This breakes pmtu discovery for IPv4 because now we use the device mtu
> > > instead of the reduced IPsec mtu in xfrm4_tunnel_check_size() if a IPv4
> > > socket is at the skb.
> > 
> > I am currently testing this following patch. It should restore old behavior
> > for ipv4 sockets.
> > 
> > diff --git a/include/net/xfrm.h b/include/net/xfrm.h
> > index ac5b025..65d3529 100644
> > --- a/include/net/xfrm.h
> > +++ b/include/net/xfrm.h
> > @@ -1730,8 +1730,6 @@ static inline int xfrm_skb_dst_mtu(struct sk_buff *skb)
> >  
> >  	if (sk && skb->protocol == htons(ETH_P_IPV6))
> >  		return ip6_skb_dst_mtu(skb);
> > -	else if (sk && skb->protocol == htons(ETH_P_IP))
> > -		return ip_skb_dst_mtu(skb);
> >  	return dst_mtu(skb_dst(skb));
> >  }
> 
> This looks still fragile. xfrm_skb_dst_mtu() is called from
> __xfrm6_output() and from xfrm4_tunnel_check_size().
> We will have the same bug again as soon as somebody thinks that
> it is save to call it from xfrm6_tunnel_check_size() too. So I
> think it is better not to call it from xfrm4_tunnel_check_size().

Hm, I don't think I can follow you completly here. I searched for allocations
of ipv6 skbs (where they originated from a socket) and checked these
allocations also initialize the skb->protocol field (the second patch).

I wonder if ip6_skb_dst_mtu was correct all along and if we should just
switch to dst_mtu(skb_dst(skb)) in all cases?

> Also, why do we need xfrm_skb_dst_mtu() at all? When it is called
> from __xfrm6_output(), we know that this is IPv6. So we can call
> ip6_skb_dst_mtu() directly as it was before your change.

The problem was that ip6_skb_dst_mtu dereferences the ipv6_pinfo socket to
check for IPV6_PMTUDISC_PROBE. If IPv4 is encapsulated in IPv6 and the packet
originated from the same host (sk != NULL) we call down to ip6_skb_dst_mtu.
This function checks that ipv6_pinfo is not NULL, so it worked by accident
currently.

My plan would be to check that all IPv4 skb allocations use the correct
skb->protocol, too, or if we can use the skb->inner_protocol field to mark
packets in the tunnel drivers from which protocol they originated.

Greetings,

  Hannes

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