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

[Martin KaFai Lau <martin.lau@linux.dev>]

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? 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.

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.