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

[Wang Hai]

On 2025/2/18 21:35, Eric Dumazet wrote:
> On Tue, Feb 18, 2025 at 12:00 PM Wang Hai <wanghai38@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >   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,

According to the plan, can we fix it like this?

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b089b08e9617..1210d4967b94 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -814,13 +814,6 @@ 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. */
@@ -878,6 +871,9 @@ 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 && 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;
+
        return inet_csk_complete_hashdance(sk, child, req, own_req);

 listen_overflow:
>