linux-kernel - Re: [PATCH net] tcp: Fix error ts_recent time during three-way handshake

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Message-ID: <CANn89iKF+LC_isruAAd+nyxgytr4LPeFTe9=ey0j=Xy5URMvkg@mail.gmail.com>
Date: Tue, 18 Feb 2025 14:35:57 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Wang Hai <wanghai38@...wei.com>
Cc: ncardwell@...gle.com, kuniyu@...zon.com, davem@...emloft.net, 
	dsahern@...nel.org, kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] tcp: Fix error ts_recent time during three-way handshake

On Tue, Feb 18, 2025 at 12:00 PM Wang Hai <wanghai38@...wei.com> wrote:
>
> If two ack packets from a connection enter tcp_check_req at the same time
> through different cpu, it may happen that req->ts_recent is updated with
> with a more recent time and the skb with an older time creates a new sock,
> which will cause the tcp_validate_incoming check to fail.
>
> cpu1                                cpu2
> tcp_check_req
>                                     tcp_check_req
> req->ts_recent = tmp_opt.rcv_tsval = t1
>                                     req->ts_recent = tmp_opt.rcv_tsval = t2
>
> 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) // failed
>
> In tcp_check_req, restore ts_recent to this skb's to fix this bug.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Wang Hai <wanghai38@...wei.com>
> ---
>  net/ipv4/tcp_minisocks.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
> index b089b08e9617..0208455f9eb8 100644
> --- a/net/ipv4/tcp_minisocks.c
> +++ b/net/ipv4/tcp_minisocks.c
> @@ -878,6 +878,10 @@ struct sock *tcp_check_req(struct sock *sk, struct sk_buff *skb,
>         sock_rps_save_rxhash(child, skb);
>         tcp_synack_rtt_meas(child, req);
>         *req_stolen = !own_req;
> +       if (own_req && tcp_sk(child)->rx_opt.tstamp_ok &&
> +           unlikely(tcp_sk(child)->rx_opt.ts_recent != tmp_opt.rcv_tsval))
> +               tcp_sk(child)->rx_opt.ts_recent = tmp_opt.rcv_tsval;
> +
>         return inet_csk_complete_hashdance(sk, child, req, own_req);

Have you seen the comment at line 818 ?

/* TODO: We probably should defer ts_recent change once
 * we take ownership of @req.
 */

Plan was clear and explained. Why implement something else (and buggy) ?