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

Nikolay Aleksandrov nikolay at cumulusnetworks.com
Fri Jul 31 18:10:09 UTC 2020

On 31/07/2020 20:51, Nikolay Aleksandrov wrote:
> 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.

In addition this doesn't need to be a recirculation, in theory it could happen
by a routed packet to svi on the bridge which got its skb->cb initialized before
hitting the bridge's xmit function. So a flag can't catch all possible cases.

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