8000 gh-126703: Fix possible use after free in pycfunction freelist by Fidget-Spinner · Pull Request #132319 · python/cpython · GitHub
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gh-126703: Fix possible use after free in pycfunction freelist #132319

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Merged
merged 4 commits into from
Apr 9, 2025
Merged

gh-126703: Fix possible use after free in pycfunction freelist #132319

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c68d4f1
Fix possible use after free in pycfunction freelist
Fidget-Spinner Apr 9, 2025
4fa1daf
📜🤖 Added by blurb_it.
blurb-it[bot] Apr 9, 2025
088d3a4
Address review
Fidget-Spinner Apr 9, 2025
7125b6a
Merge branch 'freelist-fix' of github.com:Fidget-Spinner/cpython into…
Fidget-Spinner Apr 9, 2025
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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-04-09-13-47-33.gh-issue-126703.kXiQHj.rst
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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 changes: 5 additions & 1 deletion Objects/methodobject.c
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Expand Up @@ -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` might have the same lifetime
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@hawkinsp hawkinsp Apr 9, 2025 •
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m->m_ml I think here, but yes.

You are also right that probably we should update the API documentation to clarify this is an acceptable thing to do.

// 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);
}
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