[Bug 2917] New: keepalive packets are sent twice each interval if connection is interrupted

[bugzilla-daemon at bugzilla.mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=2917

            Bug ID: 2917
           Summary: keepalive packets are sent twice each interval if
                    connection is interrupted
           Product: Portable OpenSSH
           Version: -current
          Hardware: amd64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: sshd
          Assignee: unassigned-bugs at mindrot.org
          Reporter: dev at sapalski.de

Created attachment 3193
  --> https://bugzilla.mindrot.org/attachment.cgi?id=3193&action=edit
Update last_client_time on timeout

I've discovered a bug in
serverloop.c(function=wait_until_can_do_something)!
With latest openssh (7.8p1 as well as current master) sshd disconnects
a non-responding client after approximately: ((ClientAliveCountMax / 2)
* ClientAliveInterval) seconds

I did a bisect which showed that the fix introduced for bz#2756 causes
this behavior: https://bugzilla.mindrot.org/show_bug.cgi?id=2756

How to reproduce:
 1. server #> /sbin/sshd -p 2020 -ddd -f ${sshd_config} 2>&1 | ts
 2. client $> ssh $IP -p2020
 3. close e.g. notebook of the client connection and wait for the
timeout to happen (maybe killing the client process is already enough,
didn't test it).

${sshd_config}
----
TCPKeepAlive no
ClientAliveInterval 15
ClientAliveCountMax 8
----

The debug log of sshd shows:
----
...
[2018-04-26 11:59:35] debug3: /tmp/sshd_config:94 setting TCPKeepAlive
no
[2018-04-26 11:59:35] debug3: /tmp/sshd_config:98 setting
ClientAliveInterval 15
[2018-04-26 11:59:35] debug3: /tmp/sshd_config:99 setting
ClientAliveCountMax 8
...
[2018-04-26 12:00:16] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:00:16] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:00:31] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:00:31] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:00:46] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:00:46] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:01:01] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:01:01] debug2: channel 0: request keepalive at openssh.com
confirm 1
[2018-04-26 12:01:16] Timeout, client not responding from user $USER
x.x.x.x port xxxxx
----

keepalive packets are sent twice on every interval. I think the problem
is that if a timeout of the select call in
function=wait_until_can_do_something happens the next select call will
return immediately with data contained in 'writesetp' which causes to
send another keepalive packet since variable=last_client_time wasn't
updated.

A very naive patch which solves the problem for me is attached.
However, there might a better solution for this problem!

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