Re: [PATCH net v2] tcp: Defer ts_recent changes until req is owned

[Eric Dumazet]

On Sat, Feb 22, 2025 at 11:41 AM Wang Hai <wanghai38@xxxxxxxxxx> wrote:
>
> The same 5-tuple packet may be processed by different CPUSs, so two
> CPUs may receive different ack packets at the same time when the
> state is TCP_NEW_SYN_RECV.
>
> In that case, req->ts_recent in tcp_check_req may be changed concurrently,
> which will probably cause the newsk's ts_recent to be incorrectly large.
> So that tcp_validate_incoming will fail.
>
> cpu1                                    cpu2
> tcp_check_req
>                                         tcp_check_req
>  req->ts_recent = rcv_tsval = t1
>                                          req->ts_recent = rcv_tsval = t2
>
>  syn_recv_sock
>   newsk->ts_recent = req->ts_recent = t2 // t1 < t2
> tcp_child_process
>  tcp_rcv_state_process
>   tcp_validate_incoming
>    tcp_paws_check
>     if ((s32)(rx_opt->ts_recent - rx_opt->rcv_tsval) <= paws_win)
>         // t2 - t1 > paws_win, failed
>
> In tcp_check_req, Defer ts_recent changes to this skb's to fix this bug.

I think this sentence is a bit misleading.

What your patch does is to no longer change req->ts_recent,
but conditionally update tcp_sk(child)->rx_opt.ts_recent

>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Wang Hai <wanghai38@xxxxxxxxxx>
> ---
> v1->v2: Modified the fix logic based on Eric's suggestion. Also modified the msg
>  net/ipv4/tcp_minisocks.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index b089b08e9617..53700206f498 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -815,12 +815,6 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
>
>         /* In sequence, PAWS is OK. */
>
> -       /* TODO: We probably should defer ts_recent change once
> -        * we take ownership of @req.
> -        */
> -       if (tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
> -               WRITE_ONCE(req->ts_recent, tmp_opt.rcv_tsval);
> -
>         if (TCP_SKB_CB(skb)->seq == tcp_rsk(req)->rcv_isn) {
>                 /* Truncate SYN, it is out of window starting
>                    at tcp_rsk(req)->rcv_isn + 1. */
> @@ -869,6 +863,9 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
>         if (!child)
>                 goto listen_overflow;
>
> +       if (own_req && tmp_opt.saw_tstamp && !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))
> +               tcp_sk(child)->rx_opt.ts_recent = tmp_opt.rcv_tsval;
> +

Please split this long line.

 if (own_req && tmp_opt.saw_tstamp &&
    !after(TCP_SKB_CB(skb)->seq, tcp_rsk(req)->rcv_nxt))