netdev - Re: [PATCH net-next] tcp: be less liberal in tsecr received while in SYN

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Message-ID: <CANn89iJu5dPMF3BFN7bbNZR-zZF_xjxGqstHucmBc3EvcKZXJw@mail.gmail.com>
Date: Tue, 25 Feb 2025 11:21:14 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Matthieu Baerts <matttbe@...nel.org>
Cc: Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuniyu@...zon.com>, 
	Simon Horman <horms@...nel.org>, Florian Westphal <fw@...len.de>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Jakub Kicinski <kuba@...nel.org>, 
	Yong-Hao Zou <yonghaoz1994@...il.com>, "David S . Miller" <davem@...emloft.net>, 
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: be less liberal in tsecr received while in
 SYN_RECV state

On Tue, Feb 25, 2025 at 11:19 AM Matthieu Baerts <matttbe@...nel.org> wrote:
>
> Hi Eric,
>
> On 25/02/2025 11:11, Eric Dumazet wrote:
> > On Tue, Feb 25, 2025 at 11:09 AM Matthieu Baerts <matttbe@...nel.org> wrote:
> >>
> >> Hi Paolo, Eric,
> >>
> >> On 25/02/2025 10:59, Paolo Abeni wrote:
> >>> On 2/24/25 12:06 PM, Eric Dumazet wrote:
> >>>> Yong-Hao Zou mentioned that linux was not strict as other OS in 3WHS,
> >>>> for flows using TCP TS option (RFC 7323)
> >>>>
> >>>> As hinted by an old comment in tcp_check_req(),
> >>>> we can check the TSecr value in the incoming packet corresponds
> >>>> to one of the SYNACK TSval values we have sent.
> >>>>
> >>>> In this patch, I record the oldest and most recent values
> >>>> that SYNACK packets have used.
> >>>>
> >>>> Send a challenge ACK if we receive a TSecr outside
> >>>> of this range, and increase a new SNMP counter.
> >>>>
> >>>> nstat -az | grep TcpExtTSECR_Rejected
> >>>> TcpExtTSECR_Rejected            0                  0.0
> >>
> >> (...)
> >>
> >>> It looks like this change causes mptcp self-test failures:
> >>>
> >>> https://netdev-3.bots.linux.dev/vmksft-mptcp/results/6642/1-mptcp-join-sh/stdout
> >>>
> >>> ipv6 subflows creation fails due to the added check:
> >>>
> >>> # TcpExtTSECR_Rejected            3                  0.0
> >>
> >> You have been faster to report the issue :-)
> >>
> >>> (for unknown reasons the ipv4 variant of the test is successful)
> >>
> >> Please note that it is not the first time the MPTCP test suite caught
> >> issues with the IPv6 stack. It is likely possible the IPv6 stack is less
> >> covered than the v4 one in the net selftests. (Even if I guess here the
> >> issue is only on MPTCP side.)
> >
> >
> > subflow_prep_synack() does :
> >
> >  /* clear tstamp_ok, as needed depending on cookie */
> > if (foc && foc->len > -1)
> >      ireq->tstamp_ok = 0;
> >
> > I will double check fastopen code then.
>
> Fastopen is not used in the failing tests. To be honest, it is not clear
> to me why only the two tests I mentioned are failing, they are many
> other tests using IPv6 in the MP_JOIN.

Yet, clearing tstamp_ok might be key here.

Apparently tcp_check_req() can get a non zero tmp_opt.rcv_tsecr even
if tstamp_ok has been cleared at SYNACK generation.

I would test :

diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index a87ab5c693b524aa6a324afe5bf5ff0498e528cc..0ed27f5c923edafdf48919600491eb1cb50bc913
100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -674,7 +674,8 @@ struct sock *tcp_check_req(struct sock *sk, struct
sk_buff *skb,
                if (tmp_opt.saw_tstamp) {
                        tmp_opt.ts_recent = READ_ONCE(req->ts_recent);
                        if (tmp_opt.rcv_tsecr) {
-                               tsecr_reject = !between(tmp_opt.rcv_tsecr,
+                               if (inet_rsk(req)->tstamp_ok)
+                                       tsecr_reject =
!between(tmp_opt.rcv_tsecr,

tcp_rsk(req)->snt_tsval_first,

READ_ONCE(tcp_rsk(req)->snt_tsval_last));
                                tmp_opt.rcv_tsecr -= tcp_rsk(req)->ts_off;