fix: Security- und Quality-Hardening aus Audit

- ipc.py: _recvall() Loop für fragmentierte Socket-Reads, 64 KiB
  Payload-Limit gegen OOM
- greeter.py: GREETD_SOCK Validierung (absoluter Pfad + S_ISSOCK),
  Socket-Leak behoben (_close_greetd_sock), shlex.split() statt
  str.split() für Exec-Befehle, greetd-Fehlermeldungen auf 200
  Zeichen begrenzt, get_style_context() durch get_color() ersetzt
  (GTK 4.10+ Deprecation), Socket-Timeout (10s)
- users.py: ValueError-Absicherung bei int(uid_str), Username-
  Sanitierung gegen Pfad-Traversal, Symlink-Check bei Avatar-Pfaden

tests/test_ipc.py

@@ -42,6 +42,27 @@ class FakeSocket:
         return cls(recv_data=data)
 
 
+class FragmentingSocket:
+    """A fake socket that delivers data in small chunks to simulate fragmentation."""
+
+    def __init__(self, data: bytes, chunk_size: int = 3):
+        self.sent = bytearray()
+        self._data = data
+        self._offset = 0
+        self._chunk_size = chunk_size
+
+    def sendall(self, data: bytes) -> None:
+        self.sent.extend(data)
+
+    def recv(self, n: int, flags: int = 0) -> bytes:
+        available = min(n, self._chunk_size, len(self._data) - self._offset)
+        if available <= 0:
+            return b""
+        chunk = self._data[self._offset : self._offset + available]
+        self._offset += available
+        return chunk
+
+
 class TestSendMessage:
     """Tests for encoding and sending length-prefixed JSON messages."""
 
@@ -107,6 +128,25 @@ class TestRecvMessage:
         with pytest.raises(ConnectionError):
             recv_message(sock)
 
+    def test_receives_fragmented_data(self) -> None:
+        """recv() may return fewer bytes than requested — must loop."""
+        response = {"type": "success"}
+        payload = json.dumps(response).encode("utf-8")
+        data = struct.pack("!I", len(payload)) + payload
+        sock = FragmentingSocket(data, chunk_size=3)
+
+        result = recv_message(sock)
+
+        assert result == response
+
+    def test_rejects_oversized_payload(self) -> None:
+        """Payloads exceeding the size limit must be rejected."""
+        header = struct.pack("!I", 10_000_000)
+        sock = FakeSocket(recv_data=header)
+
+        with pytest.raises(ConnectionError, match="too large"):
+            recv_message(sock)
+
 
 class TestCreateSession:
     """Tests for the create_session greetd request."""

tests/test_users.py

@@ -61,6 +61,32 @@ class TestGetUsers:
         assert users[0].gecos == ""
         assert users[0].display_name == "user"
 
+    def test_skips_invalid_uid(self, tmp_path: Path) -> None:
+        """Corrupt /etc/passwd with non-numeric UID should not crash."""
+        passwd = tmp_path / "passwd"
+        passwd.write_text(
+            "corrupt:x:NOTANUMBER:1000:Corrupt:/home/corrupt:/bin/bash\n"
+            "valid:x:1000:1000:Valid:/home/valid:/bin/bash\n"
+        )
+
+        users = get_users(passwd_path=passwd)
+
+        assert len(users) == 1
+        assert users[0].username == "valid"
+
+    def test_skips_username_with_slash(self, tmp_path: Path) -> None:
+        """Usernames containing path separators should be rejected."""
+        passwd = tmp_path / "passwd"
+        passwd.write_text(
+            "../evil:x:1000:1000:Evil:/home/evil:/bin/bash\n"
+            "normal:x:1001:1001:Normal:/home/normal:/bin/bash\n"
+        )
+
+        users = get_users(passwd_path=passwd)
+
+        assert len(users) == 1
+        assert users[0].username == "normal"
+
 
 class TestGetAvatarPath:
     """Tests for avatar file lookup."""
@@ -88,6 +114,22 @@ class TestGetAvatarPath:
 
         assert result == face
 
+    def test_ignores_symlinked_face(self, tmp_path: Path) -> None:
+        """~/.face as symlink should be ignored to prevent traversal."""
+        home = tmp_path / "home" / "attacker"
+        home.mkdir(parents=True)
+        target = tmp_path / "secret.txt"
+        target.write_text("sensitive data")
+        face = home / ".face"
+        face.symlink_to(target)
+        empty_icons = tmp_path / "no_icons"
+
+        result = get_avatar_path(
+            "attacker", accountsservice_dir=empty_icons, home_dir=home
+        )
+
+        assert result is None
+
     def test_returns_none_when_no_avatar(self, tmp_path: Path) -> None:
         empty_icons = tmp_path / "no_icons"
         home = tmp_path / "home" / "nobody"