Re: [PATCH net-next v4 2/3] virtio_net: add optional flow dissection in virtio_net_hdr_to_skb

[Jason Wang <jasowang@redhat.com>]

在 2021/6/17 下午10:43, Willem de Bruijn 写道:
> On Wed, Jun 16, 2021 at 6:22 AM Jason Wang <jasowang@redhat.com> wrote:
>>
>> 在 2021/6/15 下午10:47, Willem de Bruijn 写道:
>>>>>> Isn't virtio_net_hdr a virito-net specific metadata?
>>>>> I don't think it is ever since it was also adopted for tun, tap,
>>>>> pf_packet and uml. Basically, all callers of virtio_net_hdr_to_skb.
>>>> For tun/tap it was used to serve the backend of the virtio-net datapath
>>> The purpose of this patch series is to protect the kernel against packets
>>> inserted from userspace, by adding validation at the entry point.
>>>
>>> Agreed that BPF programs can do unspeakable things to packets, but
>>> that is a different issue (with a different trust model), and out of scope.
>>
>> Ok, I think I understand your point, so basically we had two types of
>> untrusted sources for virtio_net_hdr_to_skb():
>>
>> 1) packet injected from userspace: tun, tap, packet
>> 2) packet received from a NIC: virtio-net, uml
>>
>> If I understand correctly, you only care about case 1). But the method
>> could also be used by case 2).
>>
>> For 1) the proposal should work, we only need to care about csum/gso
>> stuffs instead of virtio specific attributes like num_buffers.
>>
>> And 2) is the one that I want to make sure it works as expected since it
>> requires more context to validate and we have other untrusted NICs
> Yes. I did not fully appreciate the two different use cases of this
> interface. For packets entering the the receive stack from a network
> device, I agree that XDP is a suitable drop filter in principle. No
> need for another layer.


So strictly speaking, tuntap is also the path that the packet entering 
the receive stack.


> In practice, the single page constraint is a
> large constraint today. But as you point out multi-buffer is a work in
> progress.


Another advantage is that XDP is per device, but it looks to me flow 
dissector is per ns.


>
>> Ideally, I think XDP is probably the best. It is designed to do such
>> early packet dropping per device. But it misses some important features
>> which makes it can't work today.
>>
>> The method proposed in this patch should work for userspace injected
>> packets, but it may not work very well in the case of XDP in the future.
>> Using another bpf program type may only solve the issue of vnet header
>> coupling with vnet header.
>>
>> I wonder whether or not we can do something in the middle:
>>
>> 1) make sure the flow dissector works even for the case of XDP (note
>> that tun support XDP)
> I think that wherever an XDP program exists in the datapath, that can
> do the filtering, so there is no need for additional flow dissection.


I agree.


>
> If restricting this patch series to the subset of callers that inject
> into the egress path *and* one of them has an XDP hook, the scope for
> this feature set is narrower.


Right, if we don't restrict, we can't prevent user from using this in 2).


>
>> 2) use some general fields instead of virtio-net specific fields, e.g
>> using device header instead of vnet header in the flow keys strcuture
> Can you give an example of what would be in the device header?
>
> Specific for GSO, we have two sets of constants: VIRTIO_NET_HDR_GSO_..
> and SKB_GSO_.. Is the suggestion to replace the current use of the
> first in field flow_keys->virtio_net_hdr.gso_type with the second in
> flow_keys->gso_type?


No, I meant using a general fields like flow_keys->device_hdr. And use 
bpf helpers to access the field.

I think for mitigation, we don't care too much about the performance lose.

Thanks


>