Subject: Re: [RFC net] bridge: clean the nf_bridge status when forwarding the skb

From: Antonio Quartulli <[email-masked]>
Date: Thu, 26 Sep 2013 23:16:48 +0200

On Thu, Sep 26, 2013 at 02:10:21PM -0700, Stephen Hemminger wrote:
> On Thu, 26 Sep 2013 22:19:50 +0200
> Antonio Quartulli <[email-masked]> wrote:
> 
> > From: Antonio Quartulli <[email-masked]>
> > 
> > Even if enslaving a bridge interface into another bridge is
> > forbidden, it is still possible to create a chain of
> > virtual interfaces including two distinct bridges.
> > 
> > In this case, the skb entering the second bridge could have
> > the nf_bridge field already set due to a previous operation
> > and consequently lead to a wrong processing of the packet
> > itself.
> > 
> > To prevent this behaviour release and set to NULL the
> > nf_bridge field of the skb when exiting the bridge interface.
> > 
> > Signed-off-by: Antonio Quartulli <[email-masked]>
> > ---
> > 
> > I am not sure if this is a wanted behaviour or a real BUG. I found this
> > "misbehaviour" while testing batman-adv with the following configuration:
> > 
> > - br0 (bridge interface) having bat0 and eth0 as slaves
> > - bat0 (which is a virtual interface provided by the batman-adv module and that
> >   works similarly to a bridge - to some extends) having br1 as slave
> > - br1 (second bridge interface) having eth1 as slave
> > 
> > Then follow these events:
> > - a broadcast packet arrives on eth0
> > - the skb enters br0 and skb->nf_bridge gets initialised and used
> > - the skb enters bat0 and the packet *gets encapsulated in the batman-adv packet
> >   which adds a batman-adv header and another Ethernet header*
> > - the skb enters br1 and gets ruined because nf_bridge_maybe_copy_header() (in
> >   br_dev_queue_push_xmit()) will try to restore an header that does not make
> >   sense anymore.
> > 
> > With this patch the nf_bridge gets de-initialised before exiting br0 and
> > therefore it is processed properly inside br1: nf_bridge_maybe_copy_header()
> > does not take place at all because nf_bridge is never initialised (the packet is
> > non-IP since it is a batman-adv packet)
> > 
> > To the developers of the bridge module I would like to ask:
> > 1) is skb->nf_bridge allowed to be non NULL when entering br_dev_xmit() ? If so,
> >    when is this supposed to happen?
> > 
> > 2) do you think this patch is logically correct but the nf_bridge release should
> >    be done in batman-adv since it is the one re-encapsulating the packet?
> > 
> > 
> > I hope I have made the problem clear.
> > 
> > Best regards,
> > 
> > 
> >  net/bridge/br_forward.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/net/bridge/br_forward.c b/net/bridge/br_forward.c
> > index 4b81b14..65864bc 100644
> > --- a/net/bridge/br_forward.c
> > +++ b/net/bridge/br_forward.c
> > @@ -49,6 +49,11 @@ int br_dev_queue_push_xmit(struct sk_buff *skb)
> >  	} else {
> >  		skb_push(skb, ETH_HLEN);
> >  		br_drop_fake_rtable(skb);
> > +
> > +		/* clean the NF bridge data */
> > +		nf_bridge_put(skb->nf_bridge);
> > +		skb->nf_bridge = NULL;
> > +
> >  		dev_queue_xmit(skb);
> >  	}
> >  

Regarding CONFIG_BRIDGE_NETFILTER you are right, thanks.

> 
> I think the header will also be garbage if bridge on bridge with netfilter is used.
> See nf_bridge_save_header.

What header are you referring to? nf_bridge_save_header() saves the header in
skb->nf_bridge->data, which is freed during nf_bridge_put() (assuming
->use reached 0).

-- 
Antonio Quartulli