island: Refactor get_file_descriptor_for_new_secure_file as contextmgr

get_file_descriptor_for_new_secure_file() has been refactored as a
contextmanager. Additionally, it has been renamed to
open_new_securely_permissioned_file(). The function can now be used
similarly to open().

Example:

with open_new_securely_permissioned_file(file_path, "wb") as f:
    f.write(data)

monkey/monkey_island/cc/server_utils/encryptor.py

@@ -6,7 +6,7 @@ import os
 from Crypto import Random  # noqa: DUO133  # nosec: B413
 from Crypto.Cipher import AES  # noqa: DUO133  # nosec: B413
 
-from monkey_island.cc.server_utils.file_utils import get_file_descriptor_for_new_secure_file
+from monkey_island.cc.server_utils.file_utils import open_new_securely_permissioned_file
 
 __author__ = "itay.mizeretz"
 
@@ -27,8 +27,7 @@ class Encryptor:
 
     def _init_key(self, password_file_path: str):
         self._cipher_key = Random.new().read(self._BLOCK_SIZE)
-        get_file_descriptor_for_new_secure_file(path=password_file_path)
-        with open(password_file_path, "wb") as f:
+        with open_new_securely_permissioned_file(password_file_path, "wb") as f:
             f.write(self._cipher_key)
 
     def _load_existing_key(self, password_file):

monkey/monkey_island/cc/server_utils/file_utils.py

@@ -2,6 +2,8 @@ import logging
 import os
 import platform
 import stat
+from contextlib import contextmanager
+from typing import Generator
 
 LOG = logging.getLogger(__name__)
 
@@ -54,11 +56,15 @@ def _create_secure_directory_windows(path: str):
         raise ex
 
 
-def get_file_descriptor_for_new_secure_file(path: str) -> int:
+@contextmanager
+def open_new_securely_permissioned_file(path: str, mode: str = "w") -> Generator:
     if is_windows_os():
-        return _get_file_descriptor_for_new_secure_file_windows(path)
+        fd = _get_file_descriptor_for_new_secure_file_windows(path)
     else:
-        return _get_file_descriptor_for_new_secure_file_linux(path)
+        fd = _get_file_descriptor_for_new_secure_file_linux(path)
+
+    with open(fd, mode) as f:
+        yield f
 
 
 def _get_file_descriptor_for_new_secure_file_linux(path: str) -> int:

monkey/tests/unit_tests/monkey_island/cc/server_utils/test_file_utils.py

@@ -6,8 +6,8 @@ import pytest
 from monkey_island.cc.server_utils.file_utils import (
     create_secure_directory,
     expand_path,
-    get_file_descriptor_for_new_secure_file,
     is_windows_os,
+    open_new_securely_permissioned_file,
 )
 
 if is_windows_os():
@@ -98,22 +98,26 @@ def test_create_secure_directory__perm_windows(test_path):
     assert ace_inheritance == ACE_INHERIT_OBJECT_AND_CONTAINER
 
 
-def test_get_file_descriptor_for_new_secure_file__already_exists(test_path):
+def test_open_new_securely_permissioned_file__already_exists(test_path):
     os.close(os.open(test_path, os.O_CREAT, stat.S_IRWXU))
     assert os.path.isfile(test_path)
 
     with pytest.raises(Exception):
-        get_file_descriptor_for_new_secure_file(test_path)
+        with open_new_securely_permissioned_file(test_path):
+            pass
 
 
-def test_get_file_descriptor_for_new_secure_file__no_parent_dir(test_path_nested):
+def test_open_new_securely_permissioned_file__no_parent_dir(test_path_nested):
     with pytest.raises(Exception):
-        get_file_descriptor_for_new_secure_file(test_path_nested)
+        with open_new_securely_permissioned_file(test_path_nested):
+            pass
 
 
 @pytest.mark.skipif(is_windows_os(), reason="Tests Posix (not Windows) permissions.")
-def test_get_file_descriptor_for_new_secure_file__perm_linux(test_path):
-    os.close(get_file_descriptor_for_new_secure_file(test_path))
+def test_open_new_securely_permissioned_file__perm_linux(test_path):
+    with open_new_securely_permissioned_file(test_path):
+        pass
+
     st = os.stat(test_path)
 
     expected_mode = stat.S_IRUSR | stat.S_IWUSR
@@ -123,8 +127,9 @@ def test_get_file_descriptor_for_new_secure_file__perm_linux(test_path):
 
 
 @pytest.mark.skipif(not is_windows_os(), reason="Tests Windows (not Posix) permissions.")
-def test_get_file_descriptor_for_new_secure_file__perm_windows(test_path):
-    os.close(get_file_descriptor_for_new_secure_file(test_path))
+def test_open_new_securely_permissioned_file__perm_windows(test_path):
+    with open_new_securely_permissioned_file(test_path):
+        pass
 
     acl, user_sid = _get_acl_and_sid_from_path(test_path)
 
@@ -140,3 +145,12 @@ def test_get_file_descriptor_for_new_secure_file__perm_windows(test_path):
     assert ace_sid == user_sid
     assert ace_permissions == FULL_CONTROL and ace_access_mode == ACE_ACCESS_MODE_GRANT_ACCESS
     assert ace_inheritance == ACE_INHERIT_OBJECT_AND_CONTAINER
+
+
+def test_open_new_securely_permissioned_file__write(test_path):
+    TEST_STR = b"Hello World"
+    with open_new_securely_permissioned_file(test_path, "wb") as f:
+        f.write(TEST_STR)
+
+    with open(test_path, "rb") as f:
+        assert f.read() == TEST_STR