Antonio Quartulli
2013-Sep-26 20:19 UTC
[RFC net] bridge: clean the nf_bridge status when forwarding the skb
From: Antonio Quartulli <antonio@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@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);
}
--
1.8.1.5