core: fix security and concurrency issues found in a backend audit (#2805)

* core: fix security and concurrency issues found in backend audit

Security:
- privesc: pipe the sudo password via stdin (sudo -S) instead of
  embedding it in the command string, so it no longer appears in argv
  (readable by any local user via /proc/<pid>/cmdline or ps)
- greeter: tokenize a session .desktop Exec= line into argv and execve
  directly instead of running it through /bin/sh -c, closing a command-
  injection path via user-writable ~/.local/share/wayland-sessions
- plugins: reject path-separator/.. in plugin id/name before joining
  into a filesystem path, closing an arbitrary-directory-delete in the
  uninstall/update fallback
- keybinds/hyprland: always quote unrecognized bind actions/keys when
  writing generated Lua; only re-emit genuine round-tripped custom Lua
  verbatim (tracked via an explicit flag), closing a Lua-injection path
- desktop/mimeapps: reject newline/bracket in mime/desktop-id fields so
  they can't inject fake sections into the shared mimeapps.list

Robustness / concurrency:
- server: recover panics in the request-dispatch path so one bad
  handler can't crash the daemon and drop every client
- go-wayland: recover panics in the shared dispatch choke point so a
  malformed compositor event can't crash CLI tools / the daemon
- server: per-connection D-Bus client ID instead of a shared constant,
  fixing cross-client signal delivery and subscription teardown
- network: guard the NetworkManager device maps with a mutex (a
  concurrent map read/write here is an unrecoverable fatal error)
- cups: close the event channel on Stop() so Unsubscribe() of the last
  subscriber no longer deadlocks; allocate the fresh channel in Start()
- freedesktop: reuse the shared session conn for the settings watcher
  and tear it down in Close(), fixing a per-Manager conn+goroutine leak
- clipboard: mutex-guard lazy dbusConn creation
- geolocation: use WithMatchMember for the GeoClue2 LocationUpdated
  signal (was WithMatchSender with an interface.member string, so the
  match never fired and live location updates never arrived)
- screenshot: set failed=true on buffer/pool creation errors so the
  dispatch loop doesn't wait forever for a ready/failed that never comes

* apply code review comments

---------

Co-authored-by: bbedward <bbedward@gmail.com>

core/internal/server/freedesktop/manager.go

@@ -137,22 +137,25 @@ func (m *Manager) consumeSelfEcho(value uint32) bool {
 }
 
 func (m *Manager) watchSettingsChanges() {
-	conn, err := dbus.ConnectSessionBus()
-	if err != nil {
-		log.Warnf("color-scheme watcher: session bus connect: %v", err)
+	// reuse the shared session connection; a dedicated one was unreachable
+	// from Close() and leaked with this goroutine
+	if m.sessionConn == nil {
 		return
 	}
+	conn := m.sessionConn
 
 	if err := conn.AddMatchSignal(
 		dbus.WithMatchInterface(dbusPortalSettingsInterface),
 		dbus.WithMatchMember("SettingChanged"),
 	); err != nil {
 		log.Warnf("Failed to watch portal settings changes: %v", err)
-		conn.Close()
 		return
 	}
 
 	signals := make(chan *dbus.Signal, 64)
+	m.stateMutex.Lock()
+	m.settingsSignals = signals
+	m.stateMutex.Unlock()
 	conn.Signal(signals)
 
 	for sig := range signals {
@@ -309,6 +312,18 @@ func (m *Manager) Close() {
 		m.systemConn.Close()
 	}
 	if m.sessionConn != nil {
+		m.sessionConn.RemoveMatchSignal(
+			dbus.WithMatchInterface(dbusPortalSettingsInterface),
+			dbus.WithMatchMember("SettingChanged"),
+		)
+		m.stateMutex.Lock()
+		signals := m.settingsSignals
+		m.settingsSignals = nil
+		m.stateMutex.Unlock()
+		if signals != nil {
+			m.sessionConn.RemoveSignal(signals)
+			close(signals)
+		}
 		m.sessionConn.Close()
 	}
 }

core/internal/server/freedesktop/types.go

@@ -71,4 +71,6 @@ type Manager struct {
 	screensaverGnomeClaimed       bool
 	selfEchoMu                    sync.Mutex
 	selfEchoes                    []colorSchemeEcho
+	// registered on sessionConn by watchSettingsChanges; guarded by stateMutex
+	settingsSignals chan *dbus.Signal
 }