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gh-126703: Fix possible use after free in pycfunction freelist (GH-132319)

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Ken Jin 2025-04-09 22:49:33 +08:00 • committed by GitHub
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2 changed files with 6 additions and 1 deletions
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Misc/NEWS.d/next/Core_and_Builtins/2025-04-09-13-47-33.gh-issue-126703.kXiQHj.rst Normal file
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@ -0,0 +1 @@
Fix possible use after free in cases where a method's definition has the same lifetime as its ``self``.

6
Objects/methodobject.c
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@ -173,12 +173,16 @@ meth_dealloc(PyObject *self)
if (m->m_weakreflist != NULL) {
PyObject_ClearWeakRefs((PyObject*) m);
}
// We need to access ml_flags here rather than later.
// `m->m_ml` might have the same lifetime
// as `m_self` when it's dynamically allocated.
int ml_flags = m->m_ml->ml_flags;
// 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);
if (m->m_ml->ml_flags & METH_METHOD) {
if (ml_flags & METH_METHOD) {
assert(Py_IS_TYPE(self, &PyCMethod_Type));
_Py_FREELIST_FREE(pycmethodobject, m, PyObject_GC_Del);
}