Skip to content

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

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
merged 4 commits into from
Apr 9, 2025
Merged

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

Show file tree
Hide file tree
Changes from 2 commits
Commits
Show all changes
4 commits
Select commit Hold shift + click to select a range
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
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2025-04-09-13-47-33.gh-issue-126703.kXiQHj.rst
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -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
View file
Open in desktop
Original file line number Diff line number Diff line change
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
Copy link
Contributor

@hawkinsp hawkinsp Apr 9, 2025 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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);
}
Expand Down
Loading