[Bridge] [RFC PATCH] net act_vlan: use correct len in skb_pull

[Toshiaki Makita]

On 2019/02/14 4:51, Zahari Doychev wrote:
> The bridge and VLAN code expects that skb->data points to the start of the
> VLAN header instead of the next (network) header. Currently after
> tcf_vlan_act() on ingress filter skb->data points to the next network
> header. In this case the Linux bridge does not forward correctly double
> tagged VLAN packets added using tc vlan action as the outer vlan tag from
> the skb is inserted at the wrong offset after the vlan tag in the payload.
> Making skb->data to point to the VLAN header in tcf_vlan_act() by using
> ETH_HLEN in skb_pull_rcsum() fixes the problem.
> 
> The following commands were used for testing:
> 
> ip link add name br0 type bridge vlan_filtering 1
> ip link set dev br0 up
> 
> ip link set dev net0 up
> ip link set dev net0 master br0
> 
> ip link set dev net1 up
> ip link set dev net1 master br0
> 
> bridge vlan add dev net0 vid 100 master
> bridge vlan add dev br0 vid 100 self
> bridge vlan add dev net1 vid 100 master
> 
> tc qdisc add dev net0 handle ffff: clsact
> tc qdisc add dev net1 handle ffff: clsact
> 
> tc filter add dev net0 ingress pref 1 protocol all flower \
>                   action vlan push id 10 pipe action vlan push id 100
> 
> tc filter add dev net0 egress pref 1 protocol 802.1q flower \
>                   vlan_id 100 vlan_ethtype 802.1q cvlan_id 10 \
>                   action vlan pop pipe action vlan pop
> 
> Signed-off-by: Zahari Doychev <zahari.doychev at linux.com>
> ---
>  net/sched/act_vlan.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/act_vlan.c b/net/sched/act_vlan.c
> index 93fdaf707313..308d7d89f925 100644
> --- a/net/sched/act_vlan.c
> +++ b/net/sched/act_vlan.c
> @@ -86,7 +86,7 @@ static int tcf_vlan_act(struct sk_buff *skb, const struct tc_action *a,
>  
>  out:
>  	if (skb_at_tc_ingress(skb))
> -		skb_pull_rcsum(skb, skb->mac_len);
> +		skb_pull_rcsum(skb, ETH_HLEN);

As I said before, it would be safer to remember mac_len and use it later.

	__u16 mac_len = skb->mac_len;
...
	err = skb_vlan_push(...)
...
	if (skb_at_tc_ingress(skb))
		skb_pull_rcsum(skb, mac_len);


-- 
Toshiaki Makita