ekklesiaandClaude Opus 5
23dff0b7b4
Ctrl+Alt+Delete opened Plasma's session screen and a teacher hibernated a classroom computer from it. A hibernated room cannot be recovered by anybody standing in front of it, which on a Sunday morning is everybody. logout=false was already set and did not stop it. That restriction governs Plasma's own actions — what plasmashell draws in a menu or an applet. The chord is a KGlobalAccel shortcut registered by ksmserver and it asks D-Bus for the greeter without consulting a Plasma action at all. A restriction on a menu cannot see a key. Three layers, because each one alone has a way past it. The key. kglobalshortcutsrc unbinds every ksmserver session action and PowerDevil's Sleep, Hibernate, PowerDown and PowerOff, each entry marked immutable. It is deliberately not in the xdg directory with the other settings: kglobalacceld opens that file as a SimpleConfig, so it reads one file in the teacher's home and no system directory at all. Measured — with it in the cascade, kreadconfig6 reported the chord unbound while the shortcut server went on answering Ctrl+Alt+Del. classroom-desktop-reset copies the package's copy into place before the shortcut server starts, the same way the panel is copied out of the package at every login. The policy. A polkit rule refuses login1.suspend* and .hibernate* for the classroom user and for nobody else. This is the layer that matters: a key can be unbound and anything reaching logind can still ask. Refusing here rather than masking sleep.target keeps a technician's own account working, and is also what tells Plasma to stop drawing the button — CanSuspend and CanHibernate come back no. Hybrid sleep and suspend-then-hibernate have no action of their own in systemd 257; both are authorised through hibernate, so denying it denies all three. The greeter. Its D-Bus activation is shadowed with /bin/false, the same mechanism already used for the old notification daemon. Layer one removes the caller, this removes the callee. Measured in the lab, from the package, after a reboot: the escape reproduced first with the shadow removed — the greeter really does start — and then refused, exited 1, with it back. The chord reports nothing bound. systemctl suspend as the classroom user is denied. The maintenance chord and the display chord both still answer, by key code, after the shortcut file is replaced wholesale. A root process may still suspend, hibernate, reboot and power off. 182 checks pass across a logout and back. Screen locking stays disabled, the power button still does nothing, and reboot and shut down are not newly denied to anybody. Co-Authored-By: Claude Opus 5 <[email protected]>
5.3 KiB
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, andaction/lock_screen=falseis 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.