Subject: Re: [PATCH 0/3] Check gso_size of packets when forwarding

From: Daniel Axtens <[email-masked]>
Date: Fri, 19 Jan 2018 22:58:42 +1100

Pravin Shelar <[email-masked]> writes:

> On Thu, Jan 18, 2018 at 5:28 PM, Daniel Axtens <[email-masked]> wrote:
>> Pravin Shelar <[email-masked]> writes:
>>
>>> On Thu, Jan 18, 2018 at 5:08 AM, Daniel Axtens <[email-masked]> wrote:
>>>> Pravin Shelar <[email-masked]> writes:
>>>>
>>>>> On Mon, Jan 15, 2018 at 6:09 PM, Daniel Axtens <[email-masked]> wrote:
>>>>>> When regular packets are forwarded, we validate their size against the
>>>>>> MTU of the destination device. However, when GSO packets are
>>>>>> forwarded, we do not validate their size against the MTU. We
>>>>>> implicitly assume that when they are segmented, the resultant packets
>>>>>> will be correctly sized.
>>>>>>
>>>>>> This is not always the case.
>>>>>>
>>>>>> We observed a case where a packet received on an ibmveth device had a
>>>>>> GSO size of around 10kB. This was forwarded by Open vSwitch to a bnx2x
>>>>>> device, where it caused a firmware assert. This is described in detail
>>>>>> at [0] and was the genesis of this series. Rather than fixing it in
>>>>>> the driver, this series fixes the forwarding path.
>>>>>>
>>>>> Are there any other possible forwarding path in networking stack? TC
>>>>> is one subsystem that could forward such a packet to the bnx2x device,
>>>>> how is that handled ?
>>>>
>>>> So far I have only looked at bridges, openvswitch and macvlan. In
>>>> general, if the code uses dev_forward_skb() it should automatically be
>>>> fine as that invokes is_skb_forwardable(), which we patch.
>>>>
>>> But there are other ways to forward packets, e.g tc-mirred or bpf
>>> redirect. We need to handle all these cases rather than fixing one at
>>> a time. As Jason suggested netif_needs_gso() looks like good function
>>> to validate if a device is capable of handling given GSO packet.
>>
>> I am not entirely sure this is a better solution.
>>
>> The biggest reason I am uncomfortable with this is that if
>> netif_needs_gso() returns true, the skb will be segmented. The segment
>> sizes will be based on gso_size. Since gso_size is greater than the MTU,
>> the resulting segments will themselves be over-MTU. Those over-MTU
>> segments will then be passed to the network card. I think we should not
>> be creating over-MTU segments; we should instead be dropping the packet
>> and logging.
>>
>
> Would this oversized segment cause the firmware assert?
> If this solves the assert issue then I do not see much value in adding
> checks in fast-path just for logging.

No - I tested this (or rather, as I don't have direct access to a bnx2x
card, this was tested on my behalf): as long as the packet is not a GSO
packet, it doesn't cause the crash. So we *could* segment them, I just
think that knowingly creating invalid segments is not particularly
pleasant.

Do you think my approach in a later email which checks and drops without
logging is sufficient? It is the same GSO check, and also checks non-GSO
packets against the MTU.

Regards,
Daniel