Re: [PATCH v2 11/25] tcp: authopt: Implement Sequence Number Extension

[Francesco Ruggeri]

On Tue, Nov 2, 2021 at 3:03 AM Leonard Crestez <cdleonard@xxxxxxxxx> wrote:
>
> On 11/1/21 9:22 PM, Francesco Ruggeri wrote:
> >> +/* Compute SNE for a specific packet (by seq). */
> >> +static int compute_packet_sne(struct sock *sk, struct tcp_authopt_info *info,
> >> +                             u32 seq, bool input, __be32 *sne)
> >> +{
> >> +       u32 rcv_nxt, snd_nxt;
> >> +
> >> +       // We can't use normal SNE computation before reaching TCP_ESTABLISHED
> >> +       // For TCP_SYN_SENT the dst_isn field is initialized only after we
> >> +       // validate the remote SYN/ACK
> >> +       // For TCP_NEW_SYN_RECV there is no tcp_authopt_info at all
> >> +       if (sk->sk_state == TCP_SYN_SENT ||
> >> +           sk->sk_state == TCP_NEW_SYN_RECV ||
> >> +           sk->sk_state == TCP_LISTEN)
> >> +               return 0;
> >> +
> >
> > In case of TCP_NEW_SYN_RECV, if our SYNACK had sequence number
> > 0xffffffff, we will receive an ACK sequence number of 0, which
> > should have sne = 1.
> >
> > In a somewhat similar corner case, when we receive a SYNACK to
> > our SYN in tcp_rcv_synsent_state_process, if the SYNACK has
> > sequence number 0xffffffff, we set tp->rcv_nxt to 0, and we
> > should set sne to 1.
> >
> > There may be more similar corner cases related to a wraparound
> > during the handshake.
> >
> > Since as you pointed out all we need is "recent" valid <sne, seq>
> > pairs as reference, rather than relying on rcv_sne being paired
> > with tp->rcv_nxt (and similarly for snd_sne and tp->snd_nxt),
> > would it be easier to maintain reference <sne, seq> pairs for send
> > and receive in tcp_authopt_info, appropriately handle the different
> > handshake cases and initialize the pairs, and only then track them
> > in tcp_rcv_nxt_update and tcp_rcv_snd_update?
>
> For TCP_NEW_SYN_RECV there is no struct tcp_authopt_info, only a request
> minisock. I think those are deliberately kept small save resources on
> SYN floods so I'd rather not increase their size.
>
> For all the handshake cases we can just rely on SNE=0 for ISN and we
> already need to keep track of ISNs because they're part of the signature.
>

Exactly. But the current code, when setting rcv_sne and snd_sne,
always compares the sequence number with the <info->rcv_sne, tp->rcv_nxt>
(or <info->snd_sne, tp->snd_nxt>) pair, where info->rcv_sne and
info->snd_sne are initialized to 0 at the time of info creation.
In other words, the code assumes that rcv_sne always corresponds to
tp->rcv_nxt, and snd_sne to tp->snd_nxt. But that may not be true
when info is created, on account of rollovers during a handshake.
So it is not just a matter of what to use for SNE before info is
created and used, but also how SNEs are initialized in info.
That is why I was suggesting of saving valid <sne, seq> pairs
(initialized with <0, ISN>) in tcp_authopt_info rather than just SNEs,
and then always compare seq to those pairs if info is available.
The pairs could then be updated in tcp_rcv_nxt_update and
tcp_snd_una_update.

Regards,
Francesco