Re: [PATCH 19/31] net/tcp: Add TCP-AO SNE support

[Francesco Ruggeri]

On Tue, Aug 23, 2022 at 7:50 AM Leonard Crestez <cdleonard@xxxxxxxxx> wrote:
>
> On 8/18/22 19:59, Dmitry Safonov wrote:
> > Add Sequence Number Extension (SNE) extension for TCP-AO.
> > This is needed to protect long-living TCP-AO connections from replaying
> > attacks after sequence number roll-over, see RFC5925 (6.2).
>
> > +#ifdef CONFIG_TCP_AO
> > +     ao = rcu_dereference_protected(tp->ao_info,
> > +                                    lockdep_sock_is_held((struct sock *)tp));
> > +     if (ao) {
> > +             if (ack < ao->snd_sne_seq)
> > +                     ao->snd_sne++;
> > +             ao->snd_sne_seq = ack;
> > +     }
> > +#endif
> >       tp->snd_una = ack;
> >   }
>
> ... snip ...
>
> > +#ifdef CONFIG_TCP_AO
> > +     ao = rcu_dereference_protected(tp->ao_info,
> > +                                    lockdep_sock_is_held((struct sock *)tp));
> > +     if (ao) {
> > +             if (seq < ao->rcv_sne_seq)
> > +                     ao->rcv_sne++;
> > +             ao->rcv_sne_seq = seq;
> > +     }
> > +#endif
> >       WRITE_ONCE(tp->rcv_nxt, seq);
>
> It should always be the case that (rcv_nxt == rcv_sne_seq) and (snd_una
> == snd_sne_seq) so the _sne_seq fields are redundant. It's possible to
> avoid those extra fields.

There are cases where rcv_nxt and snd_una are set outside of
tcp_rcv_nxt_update() and tcp_snd_una_update(), mostly during the
initial handshake, so those cases would have to be taken care of
explicitly, especially wrt rollovers.
I see your point though, there may be a potential for some cleaning
up here.

Francesco