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

@@ -281,6 +281,7 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
         name_e.textChanged.connect(on_filename)
         name_e.setText(os.path.basename(path))
 
+        def run_user_interaction_loop():
             while True:
                 if self.loop.exec_() != 2:  # 2 = next
                     raise UserCancelled
@@ -327,6 +328,11 @@ class InstallWizard(QDialog, MessageBoxMixin, BaseWizard):
                     else:
                         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)
 
     def run_upgrades(self, storage: WalletStorage, db: 'WalletDB') -> 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)
+        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):
+        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):
+        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