From: Benjamin Johnson <ben332004@gmail.com>
Date: Wed, 17 Dec 2025 15:09:57 +0000 (-0800)
Subject: gh-112127: Fix possible use-after-free in atexit.unregister() (GH-114092)
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=2b466c47c333106dc9522ab77898e6972e25a2c6;p=thirdparty%2FPython%2Fcpython.git

gh-112127: Fix possible use-after-free in atexit.unregister() (GH-114092)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
---

diff --git a/Lib/test/_test_atexit.py b/Lib/test/_test_atexit.py
index f618c1fcbca5..490b0686a0c1 100644
--- a/Lib/test/_test_atexit.py
+++ b/Lib/test/_test_atexit.py
@@ -135,6 +135,19 @@ class GeneralTest(unittest.TestCase):
         finally:
             atexit.unregister(func)
 
+    def test_eq_unregister_clear(self):
+        # Issue #112127: callback's __eq__ may call unregister or _clear
+        class Evil:
+            def __eq__(self, other):
+                action(other)
+                return NotImplemented
+
+        for action in atexit.unregister, lambda o: atexit._clear():
+            with self.subTest(action=action):
+                atexit.register(lambda: None)
+                atexit.unregister(Evil())
+                atexit._clear()
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/ACKS b/Misc/ACKS
index e3927ff0b336..a14089a39cce 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -908,6 +908,7 @@ Jim Jewett
 Pedro Diaz Jimenez
 Orjan Johansen
 Fredrik Johansson
+Benjamin Johnson
 Benjamin K. Johnson
 Gregory K. Johnson
 Kent Johnson
diff --git a/Misc/NEWS.d/next/Library/2025-12-17-14-41-09.gh-issue-112127.13OHQk.rst b/Misc/NEWS.d/next/Library/2025-12-17-14-41-09.gh-issue-112127.13OHQk.rst
new file mode 100644
index 000000000000..c983683ebd55
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-17-14-41-09.gh-issue-112127.13OHQk.rst
@@ -0,0 +1,2 @@
+Fix possible use-after-free in :func:`atexit.unregister` when the callback
+is unregistered during comparison.
diff --git a/Modules/atexitmodule.c b/Modules/atexitmodule.c
index 4536b03fbc4d..f81f0b572479 100644
--- a/Modules/atexitmodule.c
+++ b/Modules/atexitmodule.c
@@ -257,10 +257,11 @@ static int
 atexit_unregister_locked(PyObject *callbacks, PyObject *func)
 {
     for (Py_ssize_t i = 0; i < PyList_GET_SIZE(callbacks); ++i) {
-        PyObject *tuple = PyList_GET_ITEM(callbacks, i);
+        PyObject *tuple = Py_NewRef(PyList_GET_ITEM(callbacks, i));
         assert(PyTuple_CheckExact(tuple));
         PyObject *to_compare = PyTuple_GET_ITEM(tuple, 0);
         int cmp = PyObject_RichCompareBool(func, to_compare, Py_EQ);
+        Py_DECREF(tuple);
         if (cmp < 0)
         {
             return -1;