netdev - Re: [PATCH v2 net-next 1/2] tcp: Don't drop SYN+ACK for simultaneous connect().

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Message-ID: <CANn89iLtDQ7edCAkmswFtHeQFA9ptwe8jCotaF0bdvhGaLj=2A@mail.gmail.com>
Date: Mon, 8 Jul 2024 11:42:48 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, David Ahern <dsahern@...nel.org>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, Dmitry Safonov <dima@...sta.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 1/2] tcp: Don't drop SYN+ACK for simultaneous connect().

On Mon, Jul 8, 2024 at 11:09 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> RFC 9293 states that in the case of simultaneous connect(), the connection
> gets established when SYN+ACK is received. [0]
>
>       TCP Peer A                                       TCP Peer B
>
>   1.  CLOSED                                           CLOSED
>   2.  SYN-SENT     --> <SEQ=100><CTL=SYN>              ...
>   3.  SYN-RECEIVED <-- <SEQ=300><CTL=SYN>              <-- SYN-SENT
>   4.               ... <SEQ=100><CTL=SYN>              --> SYN-RECEIVED
>   5.  SYN-RECEIVED --> <SEQ=100><ACK=301><CTL=SYN,ACK> ...
>   6.  ESTABLISHED  <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED
>   7.               ... <SEQ=100><ACK=301><CTL=SYN,ACK> --> ESTABLISHED
>
> However, since commit 0c24604b68fc ("tcp: implement RFC 5961 4.2"), such a
> SYN+ACK is dropped in tcp_validate_incoming() and responded with Challenge
> ACK.
>
> For example, the write() syscall in the following packetdrill script fails
> with -EAGAIN, and wrong SNMP stats get incremented.
>
>    0 socket(..., SOCK_STREAM|SOCK_NONBLOCK, IPPROTO_TCP) = 3
>   +0 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
>
>   +0 > S  0:0(0) <mss 1460,sackOK,TS val 1000 ecr 0,nop,wscale 8>
>   +0 < S  0:0(0) win 1000 <mss 1000>
>   +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 3308134035 ecr 0,nop,wscale 8>
>   +0 < S. 0:0(0) ack 1 win 1000
>
>   +0 write(3, ..., 100) = 100
>   +0 > P. 1:101(100) ack 1
>
>   --
>
>   # packetdrill cross-synack.pkt
>   cross-synack.pkt:13: runtime error in write call: Expected result 100 but got -1 with errno 11 (Resource temporarily unavailable)
>   # nstat
>   ...
>   TcpExtTCPChallengeACK           1                  0.0
>   TcpExtTCPSYNChallenge           1                  0.0
>
> The problem is that bpf_skops_established() is triggered by the Challenge
> ACK instead of SYN+ACK.  This causes the bpf prog to miss the chance to
> check if the peer supports a TCP option that is expected to be exchanged
> in SYN and SYN+ACK.
>
> Let's accept a bare SYN+ACK for non-TFO TCP_SYN_RECV sockets to avoid such
> a situation.
>
> Note that tcp_ack_snd_check() in tcp_rcv_state_process() is skipped not to
> send an unnecessary ACK, but this could be a bit risky for net.git, so this
> targets for net-next.
>
> Link: https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>