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GH-159101: Fix UAF when tp_finalize changes the base class #159888

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GH-159101: Fix UAF when tp_finalize changes the base class #159888

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4041aae
Set GC_FINALIZED before calling tp_finalize
sergey-miryanov Oct 18, 2026
e0e5535
Re-extract the type of self after calling tp_finalize
sergey-miryanov Oct 20, 2026
7a24d33
Add news
sergey-miryanov Oct 20, 2026
5dfc9ea
Update Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-20-38-38.gh-issu…
sergey-miryanov Oct 20, 2026
cafc48a
Merge branch 'main' into bug/159101-uaf-after-finalize
sergey-miryanov Oct 26, 2026
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-20-38-38.gh-issue-159101.vFwrZY.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix use-after-free when :meth:`~object.__del__` changes the base class. Patch by
Sergey Miryanov.
8 changes: 4 additions & 4 deletions Objects/object.c
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Expand Up @@ -583,13 +583,13 @@ PyObject_CallFinalizer(PyObject *self)
if (tp->tp_finalize == NULL)
return;
/* tp_finalize should only be called once. */
if (_PyType_IS_GC(tp) && _PyGC_FINALIZED(self))
return;

tp->tp_finalize(self);
if (_PyType_IS_GC(tp)) {
if (_PyGC_FINALIZED(self))
return;

_PyGC_SET_FINALIZED(self);
}
tp->tp_finalize(self);
}

int
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4 changes: 4 additions & 0 deletions Objects/typeobject.c
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Expand Up @@ -2771,6 +2771,10 @@ subtype_dealloc(PyObject *self)
}
_PyObject_GC_UNTRACK(self);
}

/* Extract the type again; tp_finalize may have changed it */
type = Py_TYPE(self);

/*
If we added a weaklist, we clear it. Do this *before* calling tp_del,
clearing slots, or clearing the instance dict.
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