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[3.10] gh-92112: Fix crash triggered by an evil custom mro() (GH-92113) #92370

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Merged
merged 2 commits into from
May 7, 2022
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[3.10] gh-92112: Fix crash triggered by an evil custom mro() (GH-92113) #92370

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[3.10] gh-92112: Fix crash triggered by an evil custom `mro()` (GH-92…
May 6, 2022
58f5d6a
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JelleZijlstra May 6, 2022
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17 changes: 17 additions & 0 deletions Lib/test/test_descr.py
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Original file line number Diff line number Diff line change
Expand Up @@ -5737,6 +5737,23 @@ def mro(cls):
class A(metaclass=M):
pass

def test_disappearing_custom_mro(self):
"""
gh-92112: A custom mro() returning a result conflicting with
__bases__ and deleting itself caused a double free.
"""
class B:
pass

class M(DebugHelperMeta):
def mro(cls):
del M.mro
return (B,)

with self.assertRaises(TypeError):
class A(metaclass=M):
pass


if __name__ == "__main__":
unittest.main()
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core and Builtins/2022-05-01-10-58-38.gh-issue-92112.lLJemu.rst
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@@ -0,0 +1 @@
Fix crash triggered by an evil custom ``mro()`` on a metaclass.
20 changes: 11 additions & 9 deletions Objects/typeobject.c
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Expand Up @@ -369,22 +369,26 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
Py_ssize_t i, n;
int custom = !Py_IS_TYPE(type, &PyType_Type);
int unbound;
PyObject *mro_meth = NULL;
PyObject *type_mro_meth = NULL;

if (custom) {
PyObject *mro_meth, *type_mro_meth;
mro_meth = lookup_maybe_method(
(PyObject *)type, &PyId_mro, &unbound);
if (mro_meth == NULL)
if (mro_meth == NULL) {
goto clear;
}
type_mro_meth = lookup_maybe_method(
(PyObject *)&PyType_Type, &PyId_mro, &unbound);
if (type_mro_meth == NULL)
if (type_mro_meth == NULL) {
Py_DECREF(mro_meth);
goto clear;
if (mro_meth != type_mro_meth)
}
int custom_mro = (mro_meth != type_mro_meth);
Py_DECREF(mro_meth);
Py_DECREF(type_mro_meth);
if (custom_mro) {
goto clear;
Py_XDECREF(mro_meth);
Py_XDECREF(type_mro_meth);
}
}
n = PyTuple_GET_SIZE(bases);
for (i = 0; i < n; i++) {
Expand All @@ -400,8 +404,6 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
}
return;
clear:
Py_XDECREF(mro_meth);
Py_XDECREF(type_mro_meth);
type->tp_flags &= ~Py_TPFLAGS_VALID_VERSION_TAG;
type->tp_version_tag = 0; /* 0 is not a valid version tag */
}
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