Re: [PATCH v6 bpf-next 3/6] bpf: tcp: Handle BPF SYN Cookie in skb_steal_sock().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Eric Dumazet <edumazet@google.com>
Date: Thu, 14 Dec 2023 17:31:15 +0100
> On Thu, Dec 14, 2023 at 4:56 PM Kuniyuki Iwashima <kuniyu@amazon.com> wrote:
> >
> > We will support arbitrary SYN Cookie with BPF.
> >
> > If BPF prog validates ACK and kfunc allocates a reqsk, it will
> > be carried to TCP stack as skb->sk with req->syncookie 1.  Also,
> > the reqsk has its listener as req->rsk_listener with no refcnt
> > taken.
> >
> > When the TCP stack looks up a socket from the skb, we steal
> > inet_reqsk(skb->sk)->rsk_listener in skb_steal_sock() so that
> > the skb will be processed in cookie_v[46]_check() with the
> > listener.
> >
> > Note that we do not clear skb->sk and skb->destructor so that we
> > can carry the reqsk to cookie_v[46]_check().
> >
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> > ---
> >  include/net/request_sock.h | 15 +++++++++++++--
> >  1 file changed, 13 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/net/request_sock.h b/include/net/request_sock.h
> > index 26c630c40abb..8839133d6f6b 100644
> > --- a/include/net/request_sock.h
> > +++ b/include/net/request_sock.h
> > @@ -101,10 +101,21 @@ static inline struct sock *skb_steal_sock(struct sk_buff *skb,
> >         }
> >
> >         *prefetched = skb_sk_is_prefetched(skb);
> > -       if (*prefetched)
> > +       if (*prefetched) {
> > +#if IS_ENABLED(CONFIG_SYN_COOKIES)
> > +               if (sk->sk_state == TCP_NEW_SYN_RECV && inet_reqsk(sk)->syncookie) {
> > +                       struct request_sock *req = inet_reqsk(sk);
> > +
> > +                       *refcounted = false;
> > +                       sk = req->rsk_listener;
> > +                       req->rsk_listener = NULL;
> 
> I am not sure about interactions with MPTCP.
> 
> I would be nice to have their feedback.

Matthieu, Mat, Paolo, could you double check if the change
above is sane ?
https://lore.kernel.org/bpf/20231214155424.67136-4-kuniyu@amazon.com/


Short sumamry:

With this series, tc could allocate reqsk to skb->sk and set a
listener to reqsk->rsk_listener, then __inet_lookup_skb() returns
a listener in the same reuseport group, and skb is processed in the
listener function flow, especially cookie_v[46]_check().

The only difference here is that skb->sk has reqsk, which does not
have rsk_listener.


> 
> > +                       return sk;
> > +               }
> > +#endif
> >                 *refcounted = sk_is_refcounted(sk);
> > -       else
> > +       } else {
> >                 *refcounted = true;
> > +       }
> >
> >         skb->destructor = NULL;
> >         skb->sk = NULL;
> > --
> > 2.30.2