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

[Philo Lu <lulie@linux.alibaba.com>]

Hi Jason,

On 2024/11/5 10:55, Jason Xing wrote:
> From: Jason Xing <kernelxing@tencent.com>
> 
> We found there are rare chances that some RST packets appear during
> the shakehands because the timewait socket cannot accept the SYN and
> doesn't return TCP_TW_SYN in tcp_timewait_state_process().
> 
> Here is how things happen in production:
> Time        Client(A)        Server(B)
> 0s          SYN-->
> ...
> 132s                         <-- FIN
> ...
> 169s        FIN-->
> 169s                         <-- ACK
> 169s        SYN-->
> 169s                         <-- ACK
> 169s        RST-->
> As above picture shows, the two flows have a start time difference
> of 169 seconds. B starts to send FIN so it will finally enter into
> TIMEWAIT state. Nearly at the same time A launches a new connection
> that soon is reset by itself due to receiving a ACK.
> 
> There are two key checks in tcp_timewait_state_process() when timewait
> socket in B receives the SYN packet:
> 1) after(TCP_SKB_CB(skb)->seq, rcv_nxt)
> 2) (s32)(READ_ONCE(tcptw->tw_ts_recent) - tmp_opt.rcv_tsval) < 0)
> 
> Regarding the first rule, it fails as expected because in the first
> connection the seq of SYN sent from A is 1892994276, then 169s have
> passed, the second SYN has 239034613 (caused by overflow of s32).
> 
> Then how about the second rule?
> It fails again!
> Let's take a look at how the tsval comes out:
> __tcp_transmit_skb()
>      -> tcp_syn_options()
>          -> opts->tsval = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) + tp->tsoffset;
> The timestamp depends on two things, one is skb->skb_mstamp_ns, the
> other is tp->tsoffset. The latter value is fixed, so we don't need
> to care about it. If both operations (sending FIN and then starting
> sending SYN) from A happen in 1ms, then the tsval would be the same.
> It can be clearly seen in the tcpdump log. Notice that the tsval is
> with millisecond precision.
> 
> Based on the above analysis, I decided to make a small change to
> the check in tcp_timewait_state_process() so that the second flow
> would not fail.
> 

I wonder what a bad result the RST causes. As far as I know, the client 
will not close the connect and return. Instead, it re-sends an SYN in 
TCP_TIMEOUT_MIN(2) jiffies (implemented in 
tcp_rcv_synsent_state_process). So the second connection could still be 
established successfully, at the cost of a bit more delay. Like:

  Time        Client(A)        Server(B)
  0s          SYN-->
  ...
  132s                         <-- FIN
  ...
  169s        FIN-->
  169s                         <-- ACK
  169s        SYN-->
  169s                         <-- ACK
  169s        RST-->
~2jiffies    SYN-->
                               <-- SYN,ACK

Thanks.

-- 
Philo