[Python-checkins] bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure m_self is kept alive long enough (GH-22670)

[Miss Skeleton (bot)]

https://github.com/python/cpython/commit/8a12503b4532e33d590ecea7eb94cd0e6b0f1488
commit: 8a12503b4532e33d590ecea7eb94cd0e6b0f1488
branch: 3.9
author: Miss Skeleton (bot) <31488909+miss-islington at users.noreply.github.com>
committer: GitHub <noreply at github.com>
date: 2020-10-12T14:29:01-07:00
summary:

bpo-42015: Reorder dereferencing calls in meth_dealloc, to make sure m_self is kept alive long enough (GH-22670)

(cherry picked from commit 04b8631d84a870dda456ef86039c1baf34d08500)

Co-authored-by: Yannick Jadoul <yannick.jadoul at belgacom.net>

files:
A Misc/NEWS.d/next/C API/2020-10-12-20-13-58.bpo-42015.X4H2_V.rst
M Objects/methodobject.c

diff --git a/Misc/NEWS.d/next/C API/2020-10-12-20-13-58.bpo-42015.X4H2_V.rst b/Misc/NEWS.d/next/C API/2020-10-12-20-13-58.bpo-42015.X4H2_V.rst
new file mode 100644
index 0000000000000..d77619f64bb17
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2020-10-12-20-13-58.bpo-42015.X4H2_V.rst	
@@ -0,0 +1,3 @@
+Fix potential crash in deallocating method objects when dynamically
+allocated `PyMethodDef`'s lifetime is managed through the ``self``
+argument of a `PyCFunction`.
diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index 5659f2143d182..7b430416c5a04 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -164,9 +164,11 @@ meth_dealloc(PyCFunctionObject *m)
     if (m->m_weakreflist != NULL) {
         PyObject_ClearWeakRefs((PyObject*) m);
     }
+    // Dereference class before m_self: PyCFunction_GET_CLASS accesses
+    // PyMethodDef m_ml, which could be kept alive by m_self
+    Py_XDECREF(PyCFunction_GET_CLASS(m));
     Py_XDECREF(m->m_self);
     Py_XDECREF(m->m_module);
-    Py_XDECREF(PyCFunction_GET_CLASS(m));
     PyObject_GC_Del(m);
 }
 
@@ -243,9 +245,9 @@ meth_get__qualname__(PyCFunctionObject *m, void *closure)
 static int
 meth_traverse(PyCFunctionObject *m, visitproc visit, void *arg)
 {
+    Py_VISIT(PyCFunction_GET_CLASS(m));
     Py_VISIT(m->m_self);
     Py_VISIT(m->m_module);
-    Py_VISIT(PyCFunction_GET_CLASS(m));
     return 0;
 }