[Bug 1564451] Re: User processes are counted towards systemd limit for sshd processes

[Seth Arnold]

I'm having trouble reproducing this. I started a few thousand /bin/sleep
commands and was able to log in via ssh as another user; the error
message when logging in again as my normal user account showed an error
message that looked appropriate.

In one ssh:
sarnold at sec-xenial-amd64:~$ for i in `seq 1 6000` ; do /bin/sleep 10 & done
...
[15813] 24043
[15814] 24044
[15815] 24045
-bash: fork: retry: Resource temporarily unavailable
-bash: fork: retry: Resource temporarily unavailable

In another terminal:
$ ssh -oControlPath=none  root at sec-xenial-amd64
Welcome to Ubuntu Xenial Xerus (development branch) (GNU/Linux 4.4.0-16-generic x86_64)

 * Documentation:  https://help.ubuntu.com/

0 packages can be updated.
0 updates are security updates.

Last login: Thu Mar 31 15:42:23 2016 from 192.168.122.1
root at sec-xenial-amd64:~# 

$ ssh -oControlPath=none  sec-xenial-amd64
shell request failed on channel 0

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https://bugs.launchpad.net/bugs/1564451

Title:
  User processes are counted towards systemd limit for sshd processes

Status in openssh package in Ubuntu:
  New

Bug description:
  When running Xenial, user processes are counted towards the limit for
  the ssh.service, with a limit of 512. So if I login as a normal user
  via ssh and start 512 processes, nobody will be able to login any more
  and even all other users currently logged in will not be able to start
  any new tasks. I'm not certain whether this behaviour is by design,
  but to me it looks like a critical DOS possibility, so tagging as
  security bug.

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