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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Kui-Feng Lee <sinquersw@gmail.com>
Date: Wed, 18 Oct 2023 14:47:43 -0700
> 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?

Good point.

If we want not to skip other layers, maybe we can use tc ?

1) allocate ireq and set sack_ok etc with kfunc
2) bpf_sk_assign() to set ireq to skb (this could be done in kfunc above)
3) let inet_steal_sock() return req->sk_listener if not sk_fullsock(sk)
4) if skb->sk is reqsk in cookie_v[46]_check(), skip validation and
   req allocation and create full sk