Re: [PATCH net] net: bridge: clear bridge's private skb space on xmit

[Nikolay Aleksandrov <nikolay@cumulusnetworks.com>]

On 31/07/2020 20:37, Nikolay Aleksandrov wrote:
> On 31/07/2020 20:27, David Ahern wrote:
>> On 7/31/20 10:26 AM, Nikolay Aleksandrov wrote:
>>> We need to clear all of the bridge private skb variables as they can be
>>> stale due to the packet being recirculated through the stack and then
>>> transmitted through the bridge device. Similar memset is already done on
>>> bridge's input. We've seen cases where proxyarp_replied was 1 on routed
>>> multicast packets transmitted through the bridge to ports with neigh
>>> suppress which were getting dropped. Same thing can in theory happen with
>>> the port isolation bit as well.
>>>
>>> Fixes: 821f1b21cabb ("bridge: add new BR_NEIGH_SUPPRESS port flag to suppress arp and nd flood")
>>> Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
>>> ---
>>>  net/bridge/br_device.c | 2 ++
>>>  1 file changed, 2 insertions(+)
>>>
>>> diff --git a/net/bridge/br_device.c b/net/bridge/br_device.c
>>> index 8c7b78f8bc23..9a2fb4aa1a10 100644
>>> --- a/net/bridge/br_device.c
>>> +++ b/net/bridge/br_device.c
>>> @@ -36,6 +36,8 @@ netdev_tx_t br_dev_xmit(struct sk_buff *skb, struct net_device *dev)
>>>  	const unsigned char *dest;
>>>  	u16 vid = 0;
>>>  
>>> +	memset(skb->cb, 0, sizeof(struct br_input_skb_cb));
>>> +
>>>  	rcu_read_lock();
>>>  	nf_ops = rcu_dereference(nf_br_ops);
>>>  	if (nf_ops && nf_ops->br_dev_xmit_hook(skb)) {
>>>
>>
>> What's the performance hit of doing this on every packet?
>>
>> Can you just set a flag that tells the code to reset on recirculation?
>> Seems like br_input_skb_cb has space for that.
>>
> 
> Virtually non-existent, we had a patch that turned that field into a 16 byte
> field so that is really 2 8 byte stores. It is already cache hot, we could
> initialize each individual field separately as br_input does.
> 
> I don't want to waste flags on such thing, this makes it future-proof 
> and I'll remove the individual field zeroing later which will alleviate
> the cost further.
> 

Also note that we already do this on input for each packet since the
struct was reduced to 16 bytes. It's the safest way since every different
sub-part of the bridge uses some set of these private variables and
we've had many similar bugs where they were used stale or unintentionally
were not initialized for some path.