[PATCH v2 1/1] ipv6 : ip6mr.c : Fix can't match the IPv6 multicast packets with input net device in netfilter FORWARD chain.

Current kernel change the skb's net device with output device before netfilter FORWARD chain,
that will lead to can't match the IPv6 multicast packets with input net device in netfilter
FORWARD chain.

the test case for reproduce this issue as follow:

IPv6 multicast udp stream from eth0(input device) to eth1(output device)

Dst Addr: Port
{ff03::4}:6789 --->

                    Router
         +---------------------------+
-------> | [eth0] --FORWARD--> [eth1]| ------->
         +---------------------------+

ip6tables -I FORWARD -p udp -i eth0 -o eth1 --dst ff03::4 -j ACCEPT [first rules ]
ip6tables -A FORWARD -p udp -i eth1 -o eth1 --dst ff03::4 -j ACCEPT [second rules]

Before fix this issue, it will match the second rules not the first rules. you can used
command "ip6tables -L -vnx" to check the match result.

Signed-off-by: Jun Zhao <redacted>
---
 net/ipv6/ip6mr.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/net/ipv6/ip6mr.c b/net/ipv6/ip6mr.c
index 4532973..b099dfb 100644
--- a/net/ipv6/ip6mr.c
+++ b/net/ipv6/ip6mr.c

@@ -1901,6 +1901,7 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
 	struct ipv6hdr *ipv6h;
 	struct mif_device *vif = &mrt->vif6_table[vifi];
 	struct net_device *dev;
+	struct net_device *indev;
 	struct dst_entry *dst;
 	struct flowi6 fl6;
 

@@ -1946,6 +1947,7 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
 	 * result in receiving multiple packets.
 	 */
 	dev = vif->dev;
+	indev = skb->dev;
 	skb->dev = dev;
 	vif->pkt_out++;
 	vif->bytes_out += skb->len;

@@ -1960,7 +1962,7 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
 
 	IP6CB(skb)->flags |= IP6SKB_FORWARDED;
 
-	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD, skb, skb->dev, dev,
+	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD, skb, indev, dev,
 		       ip6mr_forward2_finish);
 
 out_free:

-- 
1.7.2.5

From: Jun Zhao <redacted>
Date: Fri,  3 Aug 2012 14:27:51 +0800

This patch has the same problem I mentioned, it means someone who is
now matching on the VIF device will now fail to do son.

You have to understand that you simply cannot change the behavior
here without breaking something for someone.

On Thu, 2012-08-02 at 23:43 -0700, David Miller wrote:

From: Jun Zhao <redacted>
Date: Fri,  3 Aug 2012 14:27:51 +0800

quoted

@@ -1960,7 +1962,7 @@ static int ip6mr_forward2(struct net *net, struct mr6_table *mrt,
 
 	IP6CB(skb)->flags |= IP6SKB_FORWARDED;
 
-	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD, skb, skb->dev, dev,
+	return NF_HOOK(NFPROTO_IPV6, NF_INET_FORWARD, skb, indev, dev,
 		       ip6mr_forward2_finish);
 

This patch has the same problem I mentioned, it means someone who is
now matching on the VIF device will now fail to do son.

You have to understand that you simply cannot change the behavior
here without breaking something for someone.

Hi, David:

Can you give me some more details what I break in this fix?

As my understanding, in the ip6mr_forward2(), vifi is the VIF index for
output, we can get the output device used vif->dev and record input
device before change skb->dev; then used FORWARD chain match the rules. 

Tks.