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

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Message-ID: <CAL+tcoByx13C1Bk1E33C_TqhpXydNNMe=PF93-5daRQeUC=V7A@mail.gmail.com>
Date: Wed, 19 Feb 2025 11:31:19 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Wang Hai <wanghai38@...wei.com>
Cc: Eric Dumazet <edumazet@...gle.com>, 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 Wed, Feb 19, 2025 at 10:16 AM Wang Hai <wanghai38@...wei.com> wrote:
>
>
>
> On 2025/2/18 21:35, Eric Dumazet wrote:
> > 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) ?
> >
> Hi Eric,
>
> Currently we have a real problem, so we want to solve it. This bug
> causes the upper layers to be unable to be notified to call accept after
> the successful three-way handshake.
>
> Skb from cpu1 that fails at tcp_paws_check (which it could have
> succeeded) will not be able to enter the TCP_ESTABLISHED state, and
> therefore parent->sk_data_ready(parent) will not be triggered, and skb
> from cpu2 can complete the three-way handshake, but there is also no way
> to call parent->sk_data_ready(parent) to notify the upper layer, which
> will result
> in the upper layer not being able to sense and call accept to obtain the
> nsk.
>
> 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 // failed
>   parent->sk_data_ready(parent); // will not be called
>                                      tcp_v4_do_rcv
>                                      tcp_rcv_state_process // Complete the three-way handshake
>                                                                                                         // missing parent->sk_data_ready(parent);

IIUC, the ack received from cpu1 triggered calling
inet_csk_complete_hashdance() so its state transited from
TCP_NEW_SYN_RECV to TCP_SYN_RECV, right? If so, the reason why not
call sk_data_ready() if the skb entered into tcp_child_process() is
that its state failed to transit to TCP_ESTABLISHED?

Here is another question. How did the skb on the right side enter into
tcp_v4_do_rcv() after entering tcp_check_req() if the state of sk
which the skb belongs to is TCP_NEW_SYN_RECV? Could you elaborate more
on this point?

Thanks,
Jason