Classroom_Desktop/docs/kiosk.md

Lockdown

What a teacher can and cannot do to the machine, and where each of those answers lives.

Most of it is KDE's own Kiosk mechanism, in /etc/ekklesia-classroom/xdg/kdeglobals: no shell, no command line, no lock screen, no editable panel. That file is the first place to look, and lifting a restriction for maintenance is a one-line edit there followed by a session restart.

This page is about the part that is not in that file: ending or suspending the session.

The escape this closed

A teacher pressed Ctrl+Alt+Delete, got Plasma's session screen, and hibernated the classroom computer from it. A hibernated room cannot be recovered by anybody standing in front of it who does not know where the power button is, which on a Sunday morning is everybody.

[KDE Action Restrictions] logout=false was already set and did not stop it, and the reason is worth keeping written down: that restriction governs Plasma's own actions — the entries plasmashell draws in a menu or an applet. Ctrl+Alt+Delete is not one of them. It is a KGlobalAccel shortcut registered by ksmserver, and pressing it asks D-Bus for the logout greeter without ever consulting a Plasma action. A restriction on a menu cannot see a key.

Three layers

1. The key

/etc/ekklesia-classroom/kglobalshortcutsrc unbinds every ksmserver session action and PowerDevil's Sleep, Hibernate, PowerDown and PowerOff keys, with each entry marked [$i] so nothing writes them back.

It is not in /etc/ekklesia-classroom/xdg with the other settings, and that is deliberate. kglobalacceld opens kglobalshortcutsrc as a KConfig SimpleConfig: no cascade, no system directories. Measured — with the file in the cascade, kreadconfig6 reported the chord unbound while the shortcut server went on answering Ctrl+Alt+Del.

So classroom-desktop-reset copies the package's copy into ~/.config/kglobalshortcutsrc before the session's shortcut server starts, exactly as it does with the panel. Replacing the whole file costs nothing: KGlobalAccel puts every shortcut the file does not name back from each component's own defaults, and the entries the package does name are immutable, which is what stops it putting those back too.

The room's own chords are registered from their desktop entries and are unaffected — converged-check asserts both of them by key code after the replace.

2. The policy

/usr/share/polkit-1/rules.d/49-ekklesia-classroom-sleep.rules refuses org.freedesktop.login1.suspend* and .hibernate* for the classroom user only.

This is the layer that matters, because a key can be unbound and anything that reaches logind can still ask. PowerDevil, the greeter, systemctl suspend from the maintenance terminal and any future applet all go through these actions.

Hybrid sleep and suspend-then-hibernate have no polkit action of their own in systemd 257: logind authorises both through the hibernate action, so denying hibernate denies all three. The rule names the other two spellings anyway, in case a future systemd splits them out.

Refusing here rather than masking sleep.target is the point. Masking is machine-wide and would take suspend away from a technician's account too; worse, it would be invisible to Plasma, which would go on drawing a Hibernate button that silently failed. logind asks polkit whether this user may sleep, so a NO is both the enforcement and the supported way to tell the desktop not to offer it — CanSuspend() and CanHibernate() come back no.

Nothing here touches power-off or reboot, and nothing here applies to any other account.

3. The greeter

/usr/local/share/dbus-1/services/org.kde.LogoutPrompt.service shadows plasma-workspace's own activation file with Exec=/bin/false, the same mechanism the classroom already uses for the old notification daemon.

Ctrl+Alt+Delete does not draw anything itself; it asks D-Bus to start ksmserver-logout-greeter. Layer 1 removes the caller, this removes the callee, so any other caller finds nothing to start either.

What is deliberately still there

• Screen locking stays disabled. None of this touches kscreenlockerrc, and action/lock_screen=false is unchanged. A classroom must never present a password prompt.
• The power button still does nothing on a short press, at both the logind and PowerDevil layers, and a long firmware hold still cuts the power. That is the only recovery a person standing at a wedged machine has.
• Meta+Ctrl+Alt+Shift+K still opens the maintenance terminal, and Meta+Ctrl+Alt+Shift+D still opens the display dialog.
• Reboot and shut down are not newly denied to anybody. A teacher asking for either already lands on an administrator's password prompt, which they do not have; that behaviour is unchanged. A technician on SSH or at the maintenance terminal, with their own account, can still do all of it.

Lifting it

For a technician who needs a classroom machine to suspend — a laptop on a bench, say — drop a file of the same name into /etc/polkit-1/rules.d/, which polkit reads ahead of the package's copy. Nothing needs rebuilding and nothing needs reinstalling.

To put the session screen back, remove the D-Bus shadow and the kglobalshortcutsrc entries; both are package files, so an update will restore them, which is the intended direction.

Settings the teacher must not reach (0.2.6.2)

During testing the kernel's memory killer raised Plasma's Memory Shortage Avoided notice, and its Configure button opened KDE's Notifications settings under the teacher's account — every application and system service, with Apply. Reproduced in the lab: the button runs kcmshell6 notifications.

Plasma draws that button on any notification whose application has KDE notification settings, without asking the Kiosk policy. What does ask is the thing it launches: kcmshell6 and System Settings refuse a module listed under [KDE Control Module Restrictions]. KDE has no wildcard for that group, so every settings module Debian 13's Plasma 6.3 installs — 90, found on the lab machine — is listed, immutable, in the classroom's kdeglobals. Measured afterwards:

Entry point	Before	After
A notification's Configure button	Notifications settings, editable	The module kcm_notifications is disabled … by the system administrator
That window's Help button	KDE Help Center: menus, man pages, file dialogs	not installed (Conflicts: khelpcenter; plasma-desktop only recommends it)
Volume: right-click	Configure Audio Devices…	entry gone — the applet asks the policy
Clock: right-click	Adjust Date and Time…, Configure Digital Clock…	Configure gone (plasma/allow_configure_when_locked=false); Adjust opens a System Settings with nothing in it
Bell, empty panel: right-click	no menu	no menu
Network	already refused (0.2.4)	unchanged

The button itself still shows on such a notification, and still opens the small "disabled" window: hiding it would mean replacing Plasma's notification applet, which is exactly what this appliance does not do. Notifications themselves are untouched — Parent Call, the room's messages, Wonder Ink's "Sent to the printer" all still appear.

The maintenance route is unaffected. classroom-admin-terminal starts Konsole with XDG_CONFIG_DIRS=/etc/xdg, outside this file, so anything a technician starts from it — systemsettings, kcmshell6 kcm_notifications — is the whole of it. Verified in the lab: the same module refused to the teacher opened fully, editable, from the maintenance terminal.

tools/lab/converged-check fails if any settings module on the machine is not refused in the classroom's cascade, so a Plasma update that adds one cannot quietly reopen this; and it checks the maintenance cascade still refuses nothing.