qt PasswordLineEdit: try to clear password from memory

If an attacker has access to the process' memory, it's probably already game over,
still we can make their life a bit harder.

I really tried but failed to encapsulate this logic inside PasswordLineEdit.
The destroyed signal arrives too late.
deleteLater is not called.
__del__ gets called too late.

electrum/gui/qt/installwizard.py

@@ -281,51 +281,57 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
         name_e.textChanged.connect(on_filename)
         name_e.setText(os.path.basename(path))
 
-        while True:
-            if self.loop.exec_() != 2:  # 2 = next
-                raise UserCancelled
-            assert temp_storage
-            if temp_storage.file_exists() and not temp_storage.is_encrypted():
-                break
-            if not temp_storage.file_exists():
-                break
-            wallet_from_memory = get_wallet_from_daemon(temp_storage.path)
-            if wallet_from_memory:
-                raise WalletAlreadyOpenInMemory(wallet_from_memory)
-            if temp_storage.file_exists() and temp_storage.is_encrypted():
-                if temp_storage.is_encrypted_with_user_pw():
-                    password = pw_e.text()
-                    try:
-                        temp_storage.decrypt(password)
-                        break
-                    except InvalidPassword as e:
-                        self.show_message(title=_('Error'), msg=str(e))
-                        continue
-                    except BaseException as e:
-                        self.logger.exception('')
-                        self.show_message(title=_('Error'), msg=repr(e))
-                        raise UserCancelled()
-                elif temp_storage.is_encrypted_with_hw_device():
-                    try:
-                        self.run('choose_hw_device', HWD_SETUP_DECRYPT_WALLET, storage=temp_storage)
-                    except InvalidPassword as e:
-                        self.show_message(title=_('Error'),
-                                          msg=_('Failed to decrypt using this hardware device.') + '\n' +
-                                              _('If you use a passphrase, make sure it is correct.'))
-                        self.reset_stack()
-                        return self.select_storage(path, get_wallet_from_daemon)
-                    except (UserCancelled, GoBack):
-                        raise
-                    except BaseException as e:
-                        self.logger.exception('')
-                        self.show_message(title=_('Error'), msg=repr(e))
-                        raise UserCancelled()
-                    if temp_storage.is_past_initial_decryption():
-                        break
+        def run_user_interaction_loop():
+            while True:
+                if self.loop.exec_() != 2:  # 2 = next
+                    raise UserCancelled
+                assert temp_storage
+                if temp_storage.file_exists() and not temp_storage.is_encrypted():
+                    break
+                if not temp_storage.file_exists():
+                    break
+                wallet_from_memory = get_wallet_from_daemon(temp_storage.path)
+                if wallet_from_memory:
+                    raise WalletAlreadyOpenInMemory(wallet_from_memory)
+                if temp_storage.file_exists() and temp_storage.is_encrypted():
+                    if temp_storage.is_encrypted_with_user_pw():
+                        password = pw_e.text()
+                        try:
+                            temp_storage.decrypt(password)
+                            break
+                        except InvalidPassword as e:
+                            self.show_message(title=_('Error'), msg=str(e))
+                            continue
+                        except BaseException as e:
+                            self.logger.exception('')
+                            self.show_message(title=_('Error'), msg=repr(e))
+                            raise UserCancelled()
+                    elif temp_storage.is_encrypted_with_hw_device():
+                        try:
+                            self.run('choose_hw_device', HWD_SETUP_DECRYPT_WALLET, storage=temp_storage)
+                        except InvalidPassword as e:
+                            self.show_message(title=_('Error'),
+                                              msg=_('Failed to decrypt using this hardware device.') + '\n' +
+                                                  _('If you use a passphrase, make sure it is correct.'))
+                            self.reset_stack()
+                            return self.select_storage(path, get_wallet_from_daemon)
+                        except (UserCancelled, GoBack):
+                            raise
+                        except BaseException as e:
+                            self.logger.exception('')
+                            self.show_message(title=_('Error'), msg=repr(e))
+                            raise UserCancelled()
+                        if temp_storage.is_past_initial_decryption():
+                            break
+                        else:
+                            raise UserCancelled()
                     else:
-                        raise UserCancelled()
-                else:
-                    raise Exception('Unexpected encryption version')
+                        raise Exception('Unexpected encryption version')
+
+        try:
+            run_user_interaction_loop()
+        finally:
+            pw_e.clear()
 
         return temp_storage.path, (temp_storage if temp_storage.file_exists() else None)
 
@@ -482,8 +488,11 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
         playout = PasswordLayout(msg=msg, kind=kind, OK_button=self.next_button,
                                  force_disable_encrypt_cb=force_disable_encrypt_cb)
         playout.encrypt_cb.setChecked(True)
-        self.exec_layout(playout.layout())
-        return playout.new_password(), playout.encrypt_cb.isChecked()
+        try:
+            self.exec_layout(playout.layout())
+            return playout.new_password(), playout.encrypt_cb.isChecked()
+        finally:
+            playout.clear_password_fields()
 
     @wizard_dialog
     def request_password(self, run_next, force_disable_encrypt_cb=False):

electrum/gui/qt/password_dialog.py

@@ -25,6 +25,7 @@
 
 import re
 import math
+from functools import partial
 
 from PyQt5.QtCore import Qt
 from PyQt5.QtGui import QPixmap
@@ -165,6 +166,10 @@ class PasswordLayout(object):
             pw = None
         return pw
 
+    def clear_password_fields(self):
+        for field in [self.pw, self.new_pw, self.conf_pw]:
+            field.clear()
+
 
 class PasswordLayoutForHW(object):
 
@@ -258,9 +263,12 @@ class ChangePasswordDialogForSW(ChangePasswordDialogBase):
                                       force_disable_encrypt_cb=not wallet.can_have_keystore_encryption())
 
     def run(self):
-        if not self.exec_():
-            return False, None, None, None
-        return True, self.playout.old_password(), self.playout.new_password(), self.playout.encrypt_cb.isChecked()
+        try:
+            if not self.exec_():
+                return False, None, None, None
+            return True, self.playout.old_password(), self.playout.new_password(), self.playout.encrypt_cb.isChecked()
+        finally:
+            self.playout.clear_password_fields()
 
 
 class ChangePasswordDialogForHW(ChangePasswordDialogBase):
@@ -301,6 +309,9 @@ class PasswordDialog(WindowModalDialog):
         run_hook('password_dialog', pw, grid, 1)
 
     def run(self):
-        if not self.exec_():
-            return
-        return self.pw.text()
+        try:
+            if not self.exec_():
+                return
+            return self.pw.text()
+        finally:
+            self.pw.clear()

electrum/gui/qt/util.py

@@ -753,6 +753,12 @@ class PasswordLineEdit(QLineEdit):
         QLineEdit.__init__(self, *args, **kwargs)
         self.setEchoMode(QLineEdit.Password)
 
+    def clear(self):
+        # Try to actually overwrite the memory.
+        # This is really just a best-effort thing...
+        self.setText(len(self.text()) * " ")
+        super().clear()
+
 
 class TaskThread(QThread):
     '''Thread that runs background tasks.  Callbacks are guaranteed