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/desktop/mimeapps.go

@@ -133,6 +133,11 @@ func mergedAssociations() *MimeAssociations {
 	return merged
 }
 
+// isSafeIniField rejects values that would corrupt a key=value line in mimeapps.list
+func isSafeIniField(s string) bool {
+	return !strings.ContainsAny(s, "\n\r[]")
+}
+
 func writeUserMimeapps(update func(*MimeAssociations)) error {
 	mimeappsWriteMu.Lock()
 	defer mimeappsWriteMu.Unlock()
@@ -152,6 +157,7 @@ func writeUserMimeapps(update func(*MimeAssociations)) error {
 	var buf bytes.Buffer
 	w := bufio.NewWriter(&buf)
 
+	var writeErr error
 	writeSection := func(name string, entries map[string]string) {
 		fmt.Fprintf(w, "[%s]\n", name)
 		keys := make([]string, 0, len(entries))
@@ -160,7 +166,14 @@ func writeUserMimeapps(update func(*MimeAssociations)) error {
 		}
 		sort.Strings(keys)
 		for _, k := range keys {
-			fmt.Fprintf(w, "%s=%s\n", k, entries[k])
+			v := entries[k]
+			if !isSafeIniField(k) || !isSafeIniField(v) {
+				if writeErr == nil {
+					writeErr = fmt.Errorf("invalid mimeapps.list field %q=%q", k, v)
+				}
+				continue
+			}
+			fmt.Fprintf(w, "%s=%s\n", k, v)
 		}
 		fmt.Fprintln(w)
 	}
@@ -177,6 +190,10 @@ func writeUserMimeapps(update func(*MimeAssociations)) error {
 	writeSection(groupAdded, flatten(assoc.Added))
 	writeSection(groupRemoved, flatten(assoc.Removed))
 
+	if writeErr != nil {
+		return writeErr
+	}
+
 	if err := w.Flush(); err != nil {
 		return err
 	}