[Bug 1690485] Re: openssh-server SIGSYS with 'UsePrivilegeSeparation sandbox'

[Luke A. Perkins]

See also Ticket #1831765 and #1832110 regarding the path of the
privilege separation directory (aka: /run/sshd).

This path is hard-coded into sshd and there is no means of changing its
location. I have found that if the directory is missing, all ssh
communication unceremoniously stops (iff UsePrivilegeSeparation yes).
The original OpenSSH 7.6p1 assigns the privilege separation directory to
"/var/empty" (see man sshd at openssh.com). When Ubuntu changed to
systemd from Upstart, the privilege separation directory was changed
from "/var/empty" to "/run/sshd". This is also supported by reviewing
the value of /lib/systemd/system/ssh.service and look at the
RuntimeDirectory=sshd.

My work-around is to create a service that does nothing other than
create the "/run/sshd" directory and define the
RuntimeDirectory=(anything but sshd) in your .service file.

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https://bugs.launchpad.net/bugs/1690485

Title:
  openssh-server SIGSYS with 'UsePrivilegeSeparation sandbox'

Status in openssh package in Ubuntu:
  New

Bug description:
  The 'sshd' process gets 'authentication failure' and refuses to allow
  any login.

  dmesg indicates that the problem is SIGSYS on a call to 'socket'
  (syscall #41, signal #31).

  On a hunch, I decided to test whether the problem is related to
  'seccomp' and changed /etc/ssh/sshd_config from the default

  # UsePrivilegeSeparation sandbox

  to the former standard value

  UsePrivilegeSeparation yes

  and logins started to work again.

  Obviously, I'd like to have the additional protection that sandboxing
  would give me.

  ProblemType: Bug
  DistroRelease: Ubuntu 17.04
  Package: openssh-server 1:7.4p1-10
  ProcVersionSignature: Ubuntu 4.10.0-20.22-generic 4.10.8
  Uname: Linux 4.10.0-20-generic x86_64
  ApportVersion: 2.20.4-0ubuntu4
  Architecture: amd64
  CurrentDesktop: XFCE
  Date: Fri May 12 21:06:20 2017
  InstallationDate: Installed on 2017-04-08 (35 days ago)
  InstallationMedia:
   
  SourcePackage: openssh
  UpgradeStatus: Upgraded to zesty on 2017-04-24 (19 days ago)

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