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

[Stephen Hemminger]

On Thu, 26 Sep 2013 22:19:50 +0200
Antonio Quartulli <antonio at meshcoding.com> wrote:

> From: Antonio Quartulli <antonio at open-mesh.com>
> 
> 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 <antonio at open-mesh.com>
> ---
> 
> 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);
>  	}
>  

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