Subject: Re: [net-next PATCH] net: bridge: fix for bridging 802.1Q without REORDER_HDR

From: Vlad Yasevich <[email-masked]>
Date: Mon, 14 Sep 2015 14:21:10 -0400

On 09/11/2015 04:20 PM, Phil Sutter wrote:
> On Fri, Sep 11, 2015 at 12:24:45PM -0700, Stephen Hemminger wrote:
>> On Fri, 11 Sep 2015 21:22:03 +0200
>> Phil Sutter <[email-masked]> wrote:
>>
>>> When forwarding packets from an 802.1Q interface with REORDER_HDR set to
>>> zero, the VLAN header previously inserted by vlan_do_receive() needs to
>>> be stripped from the packet and the mac_header adjustment undone,
>>> otherwise a tagged frame with first four bytes missing will be
>>> transmitted.
>>>
>>> Signed-off-by: Phil Sutter <[email-masked]>
>>> ---
>>>  net/bridge/br_input.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>>
>>> diff --git a/net/bridge/br_input.c b/net/bridge/br_input.c
>>> index f921a5d..e4e3fc7 100644
>>> --- a/net/bridge/br_input.c
>>> +++ b/net/bridge/br_input.c
>>> @@ -288,6 +288,16 @@ rx_handler_result_t br_handle_frame(struct sk_buff **pskb)
>>>  	}
>>>  
>>>  forward:
>>> +	if (is_vlan_dev(skb->dev) &&
>>> +	    !(vlan_dev_priv(skb->dev)->flags & VLAN_FLAG_REORDER_HDR)) {
>>> +		unsigned int offset = skb->data - skb_mac_header(skb);
>>> +
>>> +		skb_push(skb, offset);
>>> +		memmove(skb->data + VLAN_HLEN, skb->data, 2 * ETH_ALEN);
>>> +		skb->mac_header += VLAN_HLEN;
>>> +		skb_pull(skb, offset);
>>> +		skb_reset_mac_len(skb);
>>> +	}
>>>  	switch (p->state) {
>>>  	case BR_STATE_FORWARDING:
>>>  		rhook = rcu_dereference(br_should_route_hook);
>>
>> Thanks for finding this. Is this a new thing or has it always been there?
> 
> Sorry, I didn't check if this is a regression or not. Seen initially
> with RHEL7's kernel-3.10.0-229.7.2, which due to the massive backporting
> is by far not as old as it might seem. But it's surely not a brand new
> problem of net-next or so.
> 
> Since nowadays no sane mind touches REORDER_HDR (there was originally a
> bug in NetworkManager which defaulted this to 0), it may very well be
> there for a long time already.
> 
>> Sorry, this looks so special case it doesn't seem like a good idea.
>> Something is broken in VLAN handling if this is required.
> 
> It is so ugly, I wish I had found a better way to fix the problem. Well,
> maybe I miss something:
> 
> - packet enters __netif_receive_skb_core():
>   - skb->protocol is set to ETH_P_8021Q, so:
>     - packet is untagged
>     - skb->vlan_tci set
>     - skb->protocol set to 'real' protocol
>   - skb_vlan_tag_present(skb) == true, so:
>     - vlan_do_receive() is called:
>       - tags the packet again
>       - zeroes vlan_tci
>     - goto another_round
> - __netif_receive_skb_core(), round 2:
>   - skb->protocol is not ETH_P_8021Q -> no untagging
>   - skb_vlan_tag_present(skb) == false -> no vlan_do_receive()
>   - rx_handler handler (== br_handle_frame) is called
> 
> IMO the root of all evil is the existence of REORDER_HDR itself. It
> causes an skb which should have been untagged to being passed along with
> VLAN header present and code dealing with it needs to clean up the mess.

So the problem here appears the be the code the in br_dev_queue_push_xmit().
It assumes that MAC_HLEN worth of data has been removed from the skb,
which is normal in case of normal VLAN processing.  However, without
REORDER_HEADER set this is no longer the case.  In this case, the ethernet
header is shifted 4 bytes, and when we push the it back we miss the 4 bytes
of the destination mac address...

I wonder if it would be safe to just use skb->mac_len.

Of course, looks like vlan filtering also makes this assumption and
could be really broken.  And God forbid, someone creates a bunch of
nested encapsulated vlans (Q-in-Q-in...) with REORDER_HEADER == 0.
We could end up completely leaving the ethernet header out.

Looks like it's been there for a very long while.

-vlad

> 
> Cheers, Phil
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