Running sshd with Privilege Seperation drops connection on password change

[Nasim, Kam]

Hi folks,

I came across this issue on both stock CentOS(v6.4) and Ubuntu(14.04 LTS) and was wondering if any of you have seen it.
As far as I can tell this seems like a day-1 bug to me.

PROBLEM:
If I expire a linux user's password (passwd -e <user>) and then log in via ssh, it will prompt you for a password change.
On changing the password successfully, sshd will drop the connection and client has to reconnect.


ANALYSIS:

Looking at sshd debug logs, it would appear that the child process that runs passwd SIGCHLD's to the parent which appears to be treating that signal as a SIGTERM:

Dec 3 18:36:17 knasim-ubuntu1 passwd[3152]: pam_unix(passwd:chauthtok): password changed for wrs <<<<<<<
Dec 3 18:36:17 knasim-ubuntu1 sshd[3151]: debug1: Received SIGCHLD. <<<<<<<<<<<<<<<<<<
Dec 3 18:36:17 knasim-ubuntu1 sshd[3151]: debug1: session_by_pid: pid 3152
Dec 3 18:36:17 knasim-ubuntu1 sshd[3151]: debug1: session_exit_message: session 0 channel 0 pid 3152
Dec 3 18:36:17 knasim-ubuntu1 sshd[3151]: debug2: channel 0: request exit-status confirm 0 <<<<<<<<<<<


By default, in  sshd "Privilege Separation"  is enabled.  What does this mean?

It means that sshd spawns an unprivileged process that does basic authentication and that guy spawns a child process running as root to run passwd (in this case).

After password change, the child SIGCHLD's the parent which instead of handling it properly treats it like a SIGTERM and closes the socket.

The following process tree should give a better idea of the process nestings:

With Privilege Separation:
ubuntu at knasim-ubuntu1:~$ pstree
init─┬─acpid
     ├─atd
     ├─cron
     ├─dbus-daemon
     ├─dhclient
     ├─7*[getty]
     ├─rsyslogd───3*[{rsyslogd}]
     ├─sshd───bash───ssh
     ├─sshd───bash───pstree
     ├─sshd───sshd───sshd───passwd
     ├─systemd-logind
     ├─systemd-udevd
     ├─upstart-file-br
     ├─upstart-socket-
     └─upstart-udev-br

Without Privilege Separation:
init─┬─acpid
     ├─atd
     ├─cron
     ├─dbus-daemon
     ├─dhclient
     ├─7*[getty]
     ├─rsyslogd───3*[{rsyslogd}]
     ├─sshd─┬─sshd───bash───ssh
     │      ├─sshd───bash───pstree
     │      └─sshd───sshd
     ├─systemd-logind
     ├─systemd-udevd
     ├─upstart-file-br
     ├─upstart-socket-
     └─upstart-udev-br

If I disable Privilege Seperation ("UsePrivilegeSeperation no") in sshd config then the problem goes away but that opens up a security loophole where the process is running at root privilege even prior to authentication.


What do you guys think? Have others come across this? Is there a patch available for this?

Thanks,
Kam