Comment 7 for bug 1556447

Yes, I'm running lightdm.

$ cat /etc/pam.d/common-session
session [default=1] pam_permit.so
session requisite pam_deny.so
session required pam_permit.so
session optional pam_umask.so
session required pam_unix.so
session optional pam_systemd.so
session optional pam_cgfs.so -c freezer,memory,name=systemd

(comments all stripped for brevity

That file should be unmodified relative to the package defaults, or put another way, I haven't fiddled with it.