From 2b541a2a045dd52a6a0f735c1714e9f0e6533f9e Mon Sep 17 00:00:00 2001
From: Nico-Posada <102486290+Nico-Posada@users.noreply.github.com>
Date: Sun, 18 May 2025 03:11:38 -0400
Subject: [PATCH] gh-134100: Fix use-after-free in
 `PyImport_ImportModuleLevelObject` (GH-134117) (cherry picked from commit
 4e9005d32ff466925f40af410f2ea6bf2329bcf8)

Co-authored-by: Nico-Posada <102486290+Nico-Posada@users.noreply.github.com>
---
 .../import_/test_relative_imports.py              | 15 +++++++++++++++
 ...2025-05-16-17-25-52.gh-issue-134100.5-FbLK.rst |  2 ++
 Python/import.c                                   |  4 +++-
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-17-25-52.gh-issue-134100.5-FbLK.rst

diff --git a/Lib/test/test_importlib/import_/test_relative_imports.py b/Lib/test/test_importlib/import_/test_relative_imports.py
index e535d119763148..1549cbe96ce2d1 100644
--- a/Lib/test/test_importlib/import_/test_relative_imports.py
+++ b/Lib/test/test_importlib/import_/test_relative_imports.py
@@ -223,6 +223,21 @@ def test_relative_import_no_package_exists_absolute(self):
             self.__import__('sys', {'__package__': '', '__spec__': None},
                             level=1)
 
+    def test_malicious_relative_import(self):
+        # https://github.com/python/cpython/issues/134100
+        # Test to make sure UAF bug with error msg doesn't come back to life
+        import sys
+        loooong = "".ljust(0x23000, "b")
+        name = f"a.{loooong}.c"
+
+        with util.uncache(name):
+            sys.modules[name] = {}
+            with self.assertRaisesRegex(
+                KeyError,
+                r"'a\.b+' not in sys\.modules as expected"
+            ):
+                __import__(f"{loooong}.c", {"__package__": "a"}, level=1)
+
 
 (Frozen_RelativeImports,
  Source_RelativeImports
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-17-25-52.gh-issue-134100.5-FbLK.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-17-25-52.gh-issue-134100.5-FbLK.rst
new file mode 100644
index 00000000000000..d672347f9ad246
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-05-16-17-25-52.gh-issue-134100.5-FbLK.rst
@@ -0,0 +1,2 @@
+Fix a use-after-free bug that occurs when an imported module isn't
+in :data:`sys.modules` after its initial import. Patch by Nico-Posada.
diff --git a/Python/import.c b/Python/import.c
index 23399f26fe2a93..64048a4ef91ec3 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -3830,15 +3830,17 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
                 }
 
                 final_mod = import_get_module(tstate, to_return);
-                Py_DECREF(to_return);
                 if (final_mod == NULL) {
                     if (!_PyErr_Occurred(tstate)) {
                         _PyErr_Format(tstate, PyExc_KeyError,
                                       "%R not in sys.modules as expected",
                                       to_return);
                     }
+                    Py_DECREF(to_return);
                     goto error;
                 }
+
+                Py_DECREF(to_return);
             }
         }
         else {