Recently I was investigating quite an interesting issue - there is Ubuntu based VM our testers run some tests on. It was reported they’re unable to log into the virtual machine.
After a brief investigation it became clear the issue is not network or SSH key related.
These are records from auth log:
/var/log/auth.log
Feb 28 20:21:39 test-instance sshd[21954]: pam_unix(sshd:session): session opened for user ubuntu by (uid=0)
Feb 28 20:21:39 test-instance systemd-logind[756]: New session 75 of user ubuntu.
Feb 28 20:21:39 test-instance sshd[21954]: fatal: fork of unprivileged child failed
Feb 28 20:21:39 test-instance systemd-logind[756]: Removed session 75.
Quite an obscure error message, but smells like cgroup problem. Indeed: