Re: [PATCH v1 bpf-next 00/11] bpf: tcp: Add SYN Cookie generation/validation SOCK_OPS hooks.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 10/18/23 10:20, Kuniyuki Iwashima wrote:
> From: Eric Dumazet <edumazet@google.com>
> Date: Wed, 18 Oct 2023 10:02:51 +0200
>> On Wed, Oct 18, 2023 at 8:19 AM Martin KaFai Lau <martin.lau@linux.dev> wrote:
>>>
>>> On 10/17/23 9:48 AM, Kuniyuki Iwashima wrote:
>>>> From: Martin KaFai Lau <martin.lau@linux.dev>
>>>> Date: Mon, 16 Oct 2023 22:53:15 -0700
>>>>> On 10/13/23 3:04 PM, Kuniyuki Iwashima wrote:
>>>>>> Under SYN Flood, the TCP stack generates SYN Cookie to remain stateless
>>>>>> After 3WHS, the proxy restores SYN and forwards it and ACK to the backend
>>>>>> server.  Our kernel module works at Netfilter input/output hooks and first
>>>>>> feeds SYN to the TCP stack to initiate 3WHS.  When the module is triggered
>>>>>> for SYN+ACK, it looks up the corresponding request socket and overwrites
>>>>>> tcp_rsk(req)->snt_isn with the proxy's cookie.  Then, the module can
>>>>>> complete 3WHS with the original ACK as is.
>>>>>
>>>>> Does the current kernel module also use the timestamp bits differently?
>>>>> (something like patch 8 and patch 10 trying to do)
>>>>
>>>> Our SYN Proxy uses TS as is.  The proxy nodes generate a random number
>>>> if TS is in SYN.
>>>>
>>>> But I thought someone would suggest making TS available so that we can
>>>> mock the default behaviour at least, and it would be more acceptable.
>>>>
>>>> The selftest uses TS just to strengthen security by validating 32-bits
>>>> hash.  Dropping a part of hash makes collision easier to happen, but
>>>> 24-bits were sufficient for us to reduce SYN flood to the managable
>>>> level at the backend.
>>>
>>> While enabling bpf to customize the syncookie (and timestamp), I want to explore
>>> where can this also be done other than at the tcp layer.
>>>
>>> Have you thought about directly sending the SYNACK back at a lower layer like
>>> tc/xdp after receiving the SYN?
> 
> Yes.  Actually, at netconf I mentioned the cookie generation hook will not
> be necessary and should be replaced with XDP.
> 
> 
>>> There are already bpf_tcp_{gen,check}_syncookie
>>> helper that allows to do this for the performance reason to absorb synflood. It
>>> will be natural to extend it to handle the customized syncookie also.
> 
> Maybe we even need not extend it and can use XDP as said below.
> 
> 
>>>
>>> I think it should already be doable to send a SYNACK back with customized
>>> syncookie (and timestamp) at tc/xdp today.
>>>
>>> When ack is received, the prog@tc/xdp can verify the cookie. It will probably
>>> need some new kfuncs to create the ireq and queue the child socket. The bpf prog
>>> can change the ireq->{snd_wscale, sack_ok...} if needed. The details of the
>>> kfuncs need some more thoughts. I think most of the bpf-side infra is ready,
>>> e.g. acquire/release/ref-tracking...etc.
>>>
>>
>> I think I mostly agree with this.
> 
> I didn't come up with kfunc to create ireq and queue it to listener, so
> cookie_v[46]_check() were best place for me to extend easily, but now it
> sounds like kfunc would be the way to go.
> 
> Maybe we can move the core part of cookie_v[46]_check() except for kernel
> cookie's validation to __cookie_v[46]_check() and expose a wrapper of it
> as kfunc ?
> 
> Then, we can look up sk and pass the listener, skb, and flags (for sack_ok,
> etc) to the kfunc.  (It could still introduce some conflicts with Eric's
> patch though...)

Does that mean the packets handled in this way (in XDP) will skip all 
netfilter at all?


> 
> 
>>
>> I am rebasing  a patch adding usec resolution to TCP TS,
>> that we used for about 10 years at Google, because it is time to upstream it.
>>
>> I am worried about more changes/conflicts caused by Kuniyuki patch set...
>