Fix PAM conversation segfault causing unrecoverable red lockscreen

ctypes auto-converts c_char_p return values to Python bytes, losing
the original malloc'd pointer from strdup(). When PAM called free()
on the response, it hit a ctypes-internal buffer instead — segfault.
Use c_void_p for PamResponse.resp and strdup restype to preserve raw
pointers. Also use calloc/strdup for proper malloc'd memory that PAM
can safely free().

Add try/except in auth thread so UI stays interactive on PAM errors.

src/moonlock/auth.py

@@ -23,8 +23,10 @@ class PamMessage(Structure):
 class PamResponse(Structure):
     """PAM response structure (pam_response)."""
 
+    # resp is c_void_p (not c_char_p) to preserve raw malloc'd pointers —
+    # ctypes auto-converts c_char_p returns to Python bytes, losing the pointer.
     _fields_ = [
-        ("resp", c_char_p),
+        ("resp", c_void_p),
         ("resp_retcode", c_int),
     ]
 
@@ -56,6 +58,19 @@ def _get_libpam() -> ctypes.CDLL:
     return ctypes.CDLL(pam_path)
 
 
+def _get_libc() -> ctypes.CDLL:
+    """Load and return the libc shared library."""
+    libc_path = ctypes.util.find_library("c")
+    if not libc_path:
+        raise OSError("libc not found")
+    libc = ctypes.CDLL(libc_path)
+    libc.calloc.restype = c_void_p
+    libc.calloc.argtypes = [ctypes.c_size_t, ctypes.c_size_t]
+    libc.strdup.restype = c_void_p
+    libc.strdup.argtypes = [c_char_p]
+    return libc
+
+
 def _make_conv_func(password: str) -> PamConvFunc:
     """Create a PAM conversation callback that provides the password."""
 
@@ -65,13 +80,19 @@ def _make_conv_func(password: str) -> PamConvFunc:
         resp: POINTER(POINTER(PamResponse)),
         appdata_ptr: c_void_p,
     ) -> int:
-        # Allocate response array
-        response = (PamResponse * num_msg)()
+        # PAM expects malloc'd memory — it will free() the responses and resp strings
+        libc = _get_libc()
+        resp_array = libc.calloc(num_msg, ctypes.sizeof(PamResponse))
+        if not resp_array:
+            return PAM_AUTH_ERR
+
+        resp_ptr = ctypes.cast(resp_array, POINTER(PamResponse))
         for i in range(num_msg):
-            response[i].resp = password.encode("utf-8")
-            response[i].resp_retcode = 0
-        # PAM expects malloc'd memory; ctypes handles this via the array
-        resp[0] = ctypes.cast(response, POINTER(PamResponse))
+            # strdup allocates with malloc, which PAM can safely free()
+            resp_ptr[i].resp = libc.strdup(password.encode("utf-8"))
+            resp_ptr[i].resp_retcode = 0
+
+        resp[0] = resp_ptr
         return PAM_SUCCESS
 
     return PamConvFunc(_conv)

src/moonlock/lockscreen.py

@@ -181,7 +181,10 @@ class LockscreenWindow(Gtk.ApplicationWindow):
 
         # Use GLib thread pool to avoid blocking GTK mainloop
         def _auth_thread() -> bool:
-            result = _do_auth()
+            try:
+                result = _do_auth()
+            except Exception:
+                result = False
             GLib.idle_add(_on_auth_done, result)
             return GLib.SOURCE_REMOVE