pythongh-106092: Fix use-after-free crash in frame_dealloc (python#10…

…6875) It was possible for the trashcan to delay the deallocation of a PyFrameObject until after its corresponding _PyInterpreterFrame has already been freed. So frame_dealloc needs to avoid dereferencing the f_frame pointer unless it first checks that the pointer still points to the interpreter frame within the frame object. Signed-off-by: Anders Kaseorg <[email protected]>
miss-islington · Aug 1, 2023 · 557b05c · 557b05c
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commit 557b05c
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diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-07-18-16-13-51.gh-issue-106092.bObgRM.rst b/Misc/NEWS.d/next/Core and Builtins/2023-07-18-16-13-51.gh-issue-106092.bObgRM.rst
@@ -0,0 +1,2 @@
+Fix a segmentation fault caused by a use-after-free bug in ``frame_dealloc``
+when the trashcan delays the deallocation of a ``PyFrameObject``.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
@@ -879,20 +879,21 @@ frame_dealloc(PyFrameObject *f)
     /* It is the responsibility of the owning generator/coroutine
      * to have cleared the generator pointer */
 
-    assert(f->f_frame->owner != FRAME_OWNED_BY_GENERATOR ||
-        _PyFrame_GetGenerator(f->f_frame)->gi_frame_state == FRAME_CLEARED);
-
     if (_PyObject_GC_IS_TRACKED(f)) {
         _PyObject_GC_UNTRACK(f);
     }
 
     Py_TRASHCAN_BEGIN(f, frame_dealloc);
     PyObject *co = NULL;
 
+    /* GH-106092: If f->f_frame was on the stack and we reached the maximum
+     * nesting depth for deallocations, the trashcan may have delayed this
+     * deallocation until after f->f_frame is freed. Avoid dereferencing
+     * f->f_frame unless we know it still points to valid memory. */
+    _PyInterpreterFrame *frame = (_PyInterpreterFrame *)f->_f_frame_data;
+
     /* Kill all local variables including specials, if we own them */
-    if (f->f_frame->owner == FRAME_OWNED_BY_FRAME_OBJECT) {
-        assert(f->f_frame == (_PyInterpreterFrame *)f->_f_frame_data);
-        _PyInterpreterFrame *frame = (_PyInterpreterFrame *)f->_f_frame_data;
+    if (f->f_frame == frame && frame->owner == FRAME_OWNED_BY_FRAME_OBJECT) {
         /* Don't clear code object until the end */
         co = frame->f_executable;
         frame->f_executable = NULL;