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

[Seth Arnold]

Hello Kevin, I don't want to step on Colin's toes here, but I couldn't
sort out anything from the logs or files so far. I've had success before
with perf to grab call graph information. The usual process is along the
lines of:

- enable the ddebs.ubuntu.com repository to get access to debug symbols:
https://wiki.ubuntu.com/Debug%20Symbol%20Packages

- install packages openssh-server-dbgsym libc6-dbgsym .. as needed

- Record perf results during testing with something like:
  sudo perf record -e syscalls:sys_enter_socket -R -a --call-graph dwarf sleep 20

- test

- Check results with something like:
  sudo perf script


With luck this will spit out the function names of the call chain at the time of the socket() syscall that is being killed (probably alongside a lot of unrelated instances -- it's possible to tune the perf line to narrow it down if necessary).

I hope this helps,
Thanks

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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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