Re: [PATCH 6/7] net/tcp: ACCESS_ONCE() on snd/rcv SNEs

[Eric Dumazet]

On Tue, Nov 21, 2023 at 3:01 AM Dmitry Safonov <dima@xxxxxxxxxx> wrote:
>
> SNEs need READ_ONCE()/WRITE_ONCE() for access as they can be written and
> read at the same time.
>
> This is actually a shame: I planned to send it in TCP-AO patches, but
> it seems I've chosen a wrong commit to git-commit-fixup some time ago.
> It ended up in a commit that adds a selftest. Human factor.
>
> Fixes: 64382c71a557 ("net/tcp: Add TCP-AO SNE support")
> Signed-off-by: Dmitry Safonov <dima@xxxxxxxxxx>
> ---
>  net/ipv4/tcp_ao.c    | 4 ++--
>  net/ipv4/tcp_input.c | 4 ++--
>  2 files changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/net/ipv4/tcp_ao.c b/net/ipv4/tcp_ao.c
> index 122ff58168ee..9b7f1970c2e9 100644
> --- a/net/ipv4/tcp_ao.c
> +++ b/net/ipv4/tcp_ao.c
> @@ -956,8 +956,8 @@ tcp_inbound_ao_hash(struct sock *sk, const struct sk_buff *skb,
>                 if (unlikely(th->syn && !th->ack))
>                         goto verify_hash;
>
> -               sne = tcp_ao_compute_sne(info->rcv_sne, tcp_sk(sk)->rcv_nxt,
> -                                        ntohl(th->seq));
> +               sne = tcp_ao_compute_sne(READ_ONCE(info->rcv_sne),
> +                                        tcp_sk(sk)->rcv_nxt, ntohl(th->seq));


I think this is a wrong fix. Something is definitely fishy here.

Update side should only happen for an established socket ?

And the read side should have locked the socket before calling
tcp_inbound_ao_hash(),
otherwise reading other fields (like tcp_sk(sk)->rcv_nxt) would be racy anyway.


>                 /* Established socket, traffic key are cached */
>                 traffic_key = rcv_other_key(key);
>                 err = tcp_ao_verify_hash(sk, skb, family, info, aoh, key,
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index bcb55d98004c..78896c8be0d4 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c

tcp_snd_sne_update() definitely only deals with full sockets
(TCP_AO_ESTABLISHED)

> @@ -3583,7 +3583,7 @@ static void tcp_snd_sne_update(struct tcp_sock *tp, u32 ack)
>         ao = rcu_dereference_protected(tp->ao_info,
>                                        lockdep_sock_is_held((struct sock *)tp));
>         if (ao && ack < tp->snd_una)
> -               ao->snd_sne++;
> +               WRITE_ONCE(ao->snd_sne, ao->snd_sne + 1);
>  #endif
>  }
>
> @@ -3609,7 +3609,7 @@ static void tcp_rcv_sne_update(struct tcp_sock *tp, u32 seq)
>         ao = rcu_dereference_protected(tp->ao_info,
>                                        lockdep_sock_is_held((struct sock *)tp));
>         if (ao && seq < tp->rcv_nxt)
> -               ao->rcv_sne++;
> +               WRITE_ONCE(ao->rcv_sne, ao->rcv_sne + 1);
>  #endif
>  }
>
> --
> 2.42.0
>