Pravin Shelar <pshe...@ovn.org> writes: > On Thu, Jan 18, 2018 at 5:28 PM, Daniel Axtens <d...@axtens.net> wrote: >> Pravin Shelar <pshe...@ovn.org> writes: >> >>> On Thu, Jan 18, 2018 at 5:08 AM, Daniel Axtens <d...@axtens.net> wrote: >>>> Pravin Shelar <pshe...@ovn.org> writes: >>>> >>>>> On Mon, Jan 15, 2018 at 6:09 PM, Daniel Axtens <d...@axtens.net> 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
