[5.1.x] Fixed CVE-2024-39330 -- Added extra file name validation in Storage's save method.

Thanks to Josh Schneier for the report, and to Carlton Gibson and Sarah
Boyce for the reviews.

tests/file_storage/test_base.py

@@ -0,0 +1,72 @@
+import os
+from unittest import mock
+
+from django.core.exceptions import SuspiciousFileOperation
+from django.core.files.storage import Storage
+from django.test import SimpleTestCase
+
+
+class CustomStorage(Storage):
+    """Simple Storage subclass implementing the bare minimum for testing."""
+
+    def exists(self, name):
+        return False
+
+    def _save(self, name):
+        return name
+
+
+class StorageValidateFileNameTests(SimpleTestCase):
+
+    invalid_file_names = [
+        os.path.join("path", "to", os.pardir, "test.file"),
+        os.path.join(os.path.sep, "path", "to", "test.file"),
+    ]
+    error_msg = "Detected path traversal attempt in '%s'"
+
+    def test_validate_before_get_available_name(self):
+        s = CustomStorage()
+        # The initial name passed to `save` is not valid nor safe, fail early.
+        for name in self.invalid_file_names:
+            with (
+                self.subTest(name=name),
+                mock.patch.object(s, "get_available_name") as mock_get_available_name,
+                mock.patch.object(s, "_save") as mock_internal_save,
+            ):
+                with self.assertRaisesMessage(
+                    SuspiciousFileOperation, self.error_msg % name
+                ):
+                    s.save(name, content="irrelevant")
+                self.assertEqual(mock_get_available_name.mock_calls, [])
+                self.assertEqual(mock_internal_save.mock_calls, [])
+
+    def test_validate_after_get_available_name(self):
+        s = CustomStorage()
+        # The initial name passed to `save` is valid and safe, but the returned
+        # name from `get_available_name` is not.
+        for name in self.invalid_file_names:
+            with (
+                self.subTest(name=name),
+                mock.patch.object(s, "get_available_name", return_value=name),
+                mock.patch.object(s, "_save") as mock_internal_save,
+            ):
+                with self.assertRaisesMessage(
+                    SuspiciousFileOperation, self.error_msg % name
+                ):
+                    s.save("valid-file-name.txt", content="irrelevant")
+                self.assertEqual(mock_internal_save.mock_calls, [])
+
+    def test_validate_after_internal_save(self):
+        s = CustomStorage()
+        # The initial name passed to `save` is valid and safe, but the result
+        # from `_save` is not (this is achieved by monkeypatching _save).
+        for name in self.invalid_file_names:
+            with (
+                self.subTest(name=name),
+                mock.patch.object(s, "_save", return_value=name),
+            ):
+
+                with self.assertRaisesMessage(
+                    SuspiciousFileOperation, self.error_msg % name
+                ):
+                    s.save("valid-file-name.txt", content="irrelevant")