netdev - Re: [PATCH v2 net-next] tcp: be less liberal in TSEcr received while in SYN

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Message-ID: <20250226224753.48208-1-kuniyu@amazon.com>
Date: Wed, 26 Feb 2025 14:47:53 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <davem@...emloft.net>, <eric.dumazet@...il.com>, <horms@...nel.org>,
	<kuba@...nel.org>, <kuniyu@...zon.com>, <matttbe@...nel.org>,
	<ncardwell@...gle.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>,
	<yonghaoz1994@...il.com>
Subject: Re: [PATCH v2 net-next] tcp: be less liberal in TSEcr received while in SYN_RECV state

From: Eric Dumazet <edumazet@...gle.com>
Date: Tue, 25 Feb 2025 17:10:48 +0000
> 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 TSEcrRejected
> TcpExtTSEcrRejected            0                  0.0
> 
> Due to TCP fastopen implementation, do not apply yet these checks
> for fastopen flows.
> 
> v2: No longer use req->num_timeout, but treq->snt_tsval_first
>     to detect when first SYNACK is prepared. This means
>     we make sure to not send an initial zero TSval.
>     Make sure MPTCP and TCP selftests are passing.
>     Change MIB name to TcpExtTSEcrRejected
> 
> v1: https://lore.kernel.org/netdev/CADVnQykD8i4ArpSZaPKaoNxLJ2if2ts9m4As+=Jvdkrgx1qMHw@mail.gmail.com/T/
> 
> Reported-by: Yong-Hao Zou <yonghaoz1994@...il.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>