netdev - Re: [PATCH net-next] tcp: avoid RST in 3-way shakehands due to failure in tcp_timewait_state

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Message-ID: <20241107054309.91543-1-kuniyu@amazon.com>
Date: Wed, 6 Nov 2024 21:43:09 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kerneljasonxing@...il.com>
CC: <davem@...emloft.net>, <dsahern@...nel.org>, <edumazet@...gle.com>,
	<horms@...nel.org>, <kernelxing@...cent.com>, <kuba@...nel.org>,
	<kuniyu@...zon.com>, <netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH net-next] tcp: avoid RST in 3-way shakehands due to failure in tcp_timewait_state_process

From: Jason Xing <kerneljasonxing@...il.com>
Date: Thu, 7 Nov 2024 13:23:50 +0800
> On Thu, Nov 7, 2024 at 12:15 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From: Jason Xing <kerneljasonxing@...il.com>
> > Date: Thu, 7 Nov 2024 11:16:04 +0800
> > > > Here is how things happen in production:
> > > > Time        Client(A)        Server(B)
> > > > 0s          SYN-->
> > > > ...
> > > > 132s                         <-- FIN
> > > > ...
> > > > 169s        FIN-->
> > > > 169s                         <-- ACK
> > > > 169s        SYN-->
> > > > 169s                         <-- ACK
> > >
> > > I noticed the above ACK doesn't adhere to RFC 6191. It says:
> > > "If the previous incarnation of the connection used Timestamps, then:
> > >      if ...
> > >      ...
> > >      * Otherwise, silently drop the incoming SYN segment, thus leaving
> > >          the previous incarnation of the connection in the TIME-WAIT
> > >          state.
> > > "
> > > But the timewait socket sends an ACK because of this code snippet:
> > > tcp_timewait_state_process()
> > >     -> // the checks of SYN packet failed.
> > >     -> if (!th->rst) {
> > >         -> return TCP_TW_ACK; // this line can be traced back to 2005
> >
> > This is a challenge ACK following RFC 5961.
> 
> Please note the idea of challenge ack was proposed in 2010. But this
> code snippet has already existed before 2005. If it is a challenge
> ack, then at least we need to count it (by using NET_INC_STATS(net,
> LINUX_MIB_TCPCHALLENGEACK);).

The word was not accurate, the behaviour is compliant with RFC 5961.
RFC is often formalised based on real implementations.

Incrementing the count makes sense to me.

> 
> >
> > If SYN is returned here, the client may lose the chance to RST the
> > previous connection in TIME_WAIT.
> >
> > https://www.rfc-editor.org/rfc/rfc9293.html#section-3.10.7.4-2.4.1
> > ---8<---
> >       -  TIME-WAIT STATE
> >
> >          o  If the SYN bit is set in these synchronized states, it may
> >             be either a legitimate new connection attempt (e.g., in the
> >             case of TIME-WAIT), an error where the connection should be
> >             reset, or the result of an attack attempt, as described in
> >             RFC 5961 [9].  For the TIME-WAIT state, new connections can
> >             be accepted if the Timestamp Option is used and meets
> >             expectations (per [40]).  For all other cases, RFC 5961
> >             provides a mitigation with applicability to some situations,
> >             though there are also alternatives that offer cryptographic
> >             protection (see Section 7).  RFC 5961 recommends that in
> >             these synchronized states, if the SYN bit is set,
> >             irrespective of the sequence number, TCP endpoints MUST send
> >             a "challenge ACK" to the remote peer:
> >
> >             <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
> > ---8<---
> >
> > https://datatracker.ietf.org/doc/html/rfc5961#section-4
> > ---8<---
> >    1) If the SYN bit is set, irrespective of the sequence number, TCP
> >       MUST send an ACK (also referred to as challenge ACK) to the remote
> >       peer:
> >
> >       <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK>
> >
> >       After sending the acknowledgment, TCP MUST drop the unacceptable
> >       segment and stop processing further.
> > ---8<---
> 
> The RFC 5961 4.2 was implemented in tcp_validate_incoming():
>         /* step 4: Check for a SYN
>          * RFC 5961 4.2 : Send a challenge ack
>          */
>         if (th->syn) {
>                 if (sk->sk_state == TCP_SYN_RECV && sk->sk_socket && th->ack &&
>                     TCP_SKB_CB(skb)->seq + 1 == TCP_SKB_CB(skb)->end_seq &&
>                     TCP_SKB_CB(skb)->seq + 1 == tp->rcv_nxt &&
>                     TCP_SKB_CB(skb)->ack_seq == tp->snd_nxt)
>                         goto pass;
> syn_challenge:
>                 if (syn_inerr)
>                         TCP_INC_STATS(sock_net(sk), TCP_MIB_INERRS);
>                 NET_INC_STATS(sock_net(sk),
> LINUX_MIB_TCPSYNCHALLENGE);
>                 tcp_send_challenge_ack(sk);
>                 SKB_DR_SET(reason, TCP_INVALID_SYN);
>                 goto discard;
>         }
> 
> Also, this quotation you mentioned obviously doesn't match the kernel
> implementation:
> "If the SYN bit is set, irrespective of the sequence number, TCP MUST
> send an ACK"
> The tcp_timewait_state_process() does care about the seq number, or
> else timewait socket would refuse every SYN packet.

That's why I pasted RFC 9293 first that clearly states that we
should check seq number and then return ACK for all other cases.