Re: Sending 802.1Q packets using AF_PACKET socket on filtered bridge forwards with wrong MAC addresses

[Toshiaki Makita]

Hi,
(CC: Vlad)

On 2017/11/30 7:01, Brandon Carpenter wrote:
> I narrowed the search to a memmove() called from
> skb_reorder_vlan_header() in net/core/skbuff.c.
> 
>>     memmove(skb->data - ETH_HLEN, skb->data - skb->mac_len - VLAN_HLEN,
>>        2 * ETH_ALEN);
> 
> Calling skb_reset_mac_len() after skb_reset_mac_header() before
> calling br_allowed_ingress() in net/bridge/br_device.c fixes the
> problem.
> 
> diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
> index af5b8c87f590..e10131e2f68f 100644
> --- a/net/bridge/br_device.c
> +++ b/net/bridge/br_device.c
> @@ -58,6 +58,7 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct
> net_device *dev)
>         BR_INPUT_SKB_CB(skb)->brdev = dev;
> 
>         skb_reset_mac_header(skb);
> +       skb_reset_mac_len(skb);
>         eth = eth_hdr(skb);
>         skb_pull(skb, ETH_HLEN);

Thanks for debugging this problem.
It seems this has been broken since a6e18ff11170 ("vlan: Fix untag
operations of stacked vlans with REORDER_HEADER off").

Unfortunately this does not always work correctly, since in tx path
drivers assume network header to be set to L3 protocol header offset.
Packet socket (packet_snd()) determines network header by
dev_hard_header which is ETH_HLEN in bridge devices, so this works for
packet socket, but with vlan devices on top of bridge device with
tx-vlan hwaccel disabled we get ETH_HLEN + VLAN_HLEN or longer by mac_len.

Since mac_len can be arbitrarily long if we stack vlan devices on bridge
devices, and since we want to untag the outermost tag, using mac_len to
untag in tx path is probably no longer correct.

I'll think deeper about how to fix it.

> I'll put together an official patch  and submit it. Should I use
> another email account? Are my emails being ignored because of that
> stupid disclaimer my employer attaches to my messages (outside my
> control)?
> 
> Brandon
> 

-- 
Toshiaki Makita