[Bug 2156] New: Fix oom_adj on Linux after sshd reload

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=2156

            Bug ID: 2156
           Summary: Fix oom_adj on Linux after sshd reload
           Product: Portable OpenSSH
           Version: 6.2p1
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P5
         Component: sshd
          Assignee: unassigned-bugs at mindrot.org
          Reporter: deleriux1 at gmail.com

Created attachment 2342
  --> https://bugzilla.mindrot.org/attachment.cgi?id=2342&action=edit
Make oom_adj reset before re-execing sshd

Currently, on linux sshd attempts to remove itself from the influence
of oom-killer by modifying the oom_adj parameter for itself in proc to
-17.

This is controlled via two functions; oom_adjust_setup() and
oom_adjust_restore().

Setup saves the old score (typically zero on initialization) and sets
sshd to -17 whilst oom_adjust_restore places the saved value from
initialization back into the running sshd process.

The principle here being that whilst we want the main sshd listener
process never to be oom-killed, we want any subsequent children that
sshd spawns to be oom-killed if necessary.

The problem that occurs however is on a SIGHUP, sshd re-execs itself
and inherits the oom_adj score from its protege. When sshd goes through
its initialization steps it calls oom_adjust_setup() and saves the now
-17 value. On oom_adjust_restore() sshd then restores -17 to all child
processes sshd spawns.

This has the effect of after performing a reload causing all sshd
processes (and by the nature of inheritance) all descendant pids having
-17 as their oom_adj value which causes all sshd sessions after the
reload to never be candidates for oom-killer.

Clearly this is not desirable as it can lead to unrecoverable out of
memory conditions on linux.

This is very simple to test on linux. Restart sshd on a linux host.
Login to it via ssh and then run cat /proc/self/oom_adj. How reload
ssh, relogin again via ssh and recheck cat /proc/self/oom_adj.

I have created a patch, which I believe is honourable to the design
philosophy being utilized by openssh.

This patch adds an additional function to platform.h called
platform_pre_restart() that (if defined) will perform
oom_adjust_restore(). This is then called inside of sighup_restart()
prior to performing the re-exec of ssh.

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