netdev - Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous connect().

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Message-ID: <20240704173621.1804-1-kuniyu@amazon.com>
Date: Thu, 4 Jul 2024 10:36:21 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <edumazet@...gle.com>
CC: <brakmo@...com>, <davem@...emloft.net>, <dsahern@...nel.org>,
	<kuba@...nel.org>, <kuni1840@...il.com>, <kuniyu@...zon.com>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>
Subject: Re: [PATCH v1 net] tcp: Don't drop SYN+ACK for simultaneous connect().

From: Eric Dumazet <edumazet@...gle.com>
Date: Thu, 4 Jul 2024 10:44:55 +0200
> On Thu, Jul 4, 2024 at 5:57 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
> >
> > That said, this is no big deal because the Challenge ACK finally let the
> > connection state transition to TCP_ESTABLISHED in both directions.  If the
> > peer is not using Linux, there might be a small latency before ACK though.
> 
> I suggest removing these 3 lines. Removing a not needed challenge ACK is good
> regardless of the 'other peer' behavior.

I see, then should Fixes point to 0c24604b68fc ?

Also I noticed it still sends ACK in tcp_ack_snd_check() as if it's a
response to the normal 3WHS, so we need:

---8<---
@@ -6788,6 +6793,9 @@ tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 		tcp_fast_path_on(tp);
 		if (sk->sk_shutdown & SEND_SHUTDOWN)
 			tcp_shutdown(sk, SEND_SHUTDOWN);
+
+		if (!req)
+			goto consume;
 		break;
 
 	case TCP_FIN_WAIT1: {
---8<---

and I have a question regarding the consume: label.  Why do we use
__kfree_skb() there instead of consume_skb() ?  I guess it's because
skb_unref() is unnecessary and expensive and tracing is also expensive ?


> 
> >
> > 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.
> >
> > Link: https://www.rfc-editor.org/rfc/rfc9293.html#section-3.5-7 [0]
> > Fixes: 9872a4bde31b ("bpf: Add TCP connection BPF callbacks")
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > ---
> >  net/ipv4/tcp_input.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 77294fd5fd3e..70595009bb58 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -5980,6 +5980,11 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
> >          * RFC 5961 4.2 : Send a challenge ack
> >          */
> >         if (th->syn) {
> > +               if (sk->sk_state == TCP_SYN_RECV && !tp->syn_fastopen && 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);
> > @@ -5990,7 +5995,7 @@ static bool tcp_validate_incoming(struct sock *sk, struct sk_buff *skb,
> >         }
> >
> >         bpf_skops_parse_hdr(sk, skb);
> > -
> > +pass:
> 
> It is not clear to me why we do not call bpf_skops_parse_hdr(sk, skb)
> in this case ?

I skipped bpf_skops_parse_hdr() as it had this check.

        switch (sk->sk_state) {
        case TCP_SYN_RECV:
        case TCP_SYN_SENT:
        case TCP_LISTEN:
                return;
        }

Thanks!

> 
> 
> >         return true;
> >
> >  discard:
> > --
> > 2.30.2