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

Nikolay Aleksandrov nikolay at cumulusnetworks.com
Fri Jul 31 17:51:25 UTC 2020


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 at 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.




More information about the Bridge mailing list