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

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Message-ID: <40fcf43d-b9c2-439a-9375-d2ff78be203f@kernel.org>
Date: Tue, 25 Feb 2025 11:33:14 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
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 25/02/2025 11:21, Eric Dumazet wrote:
> 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.

Good point. But in the tests, it is not suppose to clear the timestamps.

(Of course, when I take a capture, I cannot reproduce the issue :) )

> 
> 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;
Thank you for the suggestion. It doesn't look to be that, I can still
reproduce the issue.

If I print the different TS (rcv, snt first, snt last) when tsecr_reject
is set, I get this:

[  227.984292] mattt: 2776726299 2776727335 2776727335
[  227.984684] mattt: 2776726299 2776727335 2776727335
[  227.984771] mattt: 3603918977 3603920020 3603920020
[  227.984896] mattt: 3603918977 3603920020 3603920020
[  230.031921] mattt: 3603918977 3603920020 3603922068
[  230.032283] mattt: 2776726299 2776727335 2776729383
[  230.032554] mattt: 2776729384 2776727335 2776729383
      ack rx                [FAIL] got 0 JOIN[s] ack rx expected 2

So not 0 or uninit values.

Cheers,
Matt
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