Screen lock can be bypassed when using LightDM
Step to reproduce:
- On a a fresh install of Ubuntu 19.10 (Gnome-Shell 3.34.0, Gnome-session 3.34.1)
- Install the lightdm package, reboot to verify it is indeed used instead of GDM
- After logging-in to Gnome-Shell, lock the screen
- On the LightDM lock screen, switch to TTY 1 then back to 7
- No password is asked and I'm now back, fully logged-in, in the session.
This issue was originally reported in bugzilla (I updated the above steps): https://bugzilla.gnome.org/show_bug.cgi?id=666860
My understanding is that gnome-session trust the DM to fully handle the screen lock, when it should not. User switching may be affected too.