gh-148653: Fix SIGSEGV in marshal.loads for self-referencing tuples

Lib/test/test_marshal.py

@@ -731,5 +731,25 @@ def test_read_object_from_file(self):
             os_helper.unlink(os_helper.TESTFN)
 
 
+class SelfRefTupleTest(unittest.TestCase):
+    """Regression test for gh-148653: TYPE_TUPLE with FLAG_REF back-reference.
+
+    R_REF registered the tuple in p->refs before its slots were populated.
+    A TYPE_REF back-reference to the partial tuple could reach a hashing
+    site (PySet_Add) with NULL slots, crashing with SIGSEGV.
+
+    The fix uses the two-phase r_ref_reserve/r_ref_insert pattern so the
+    Py_None placeholder is detected by the TYPE_REF handler, raising
+    ValueError instead.
+    """
+
+    def test_self_ref_tuple(self):
+        # TYPE_TUPLE|FLAG_REF n=2; NONE; TYPE_SET n=1; TYPE_REF(0)
+        payload = (b'\xa8\x02\x00\x00\x00N'
+                   b'<\x01\x00\x00\x00r\x00\x00\x00\x00')
+        with self.assertRaises(ValueError):
+            marshal.loads(payload)
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2026-04-16-12-51-32.gh-issue-148653.G0_AMQ.rst

@@ -0,0 +1,2 @@
+Fix SIGSEGV in :func:`marshal.loads` for self-referencing tuples. Such
+payloads will now raise :exc:`ValueError` instead of crashing.

Python/marshal.c

@@ -1385,7 +1385,9 @@ r_object(RFILE *p)
         }
     _read_tuple:
         v = PyTuple_New(n);
-        R_REF(v);
+        idx = r_ref_reserve(flag, p);
+        if (idx < 0)
+            Py_CLEAR(v);
         if (v == NULL)
             break;
 
@@ -1400,6 +1402,7 @@ r_object(RFILE *p)
             }
             PyTuple_SET_ITEM(v, i, v2);
         }
+        v = r_ref_insert(v, idx, flag, p);
         retval = v;
         break;