gh-142665: fix UAF when accessing a memoryview concurrently mutates the underlying buffer

Lib/test/test_memoryview.py

@@ -685,13 +685,17 @@ def __bool__(self):
         with self.assertRaises(ValueError):
             m[MyIndex()]
 
+        # Other exceptions can be raised when working on a released buffer.
+        # See https://github.com/python/cpython/issues/142665.
         ba = None
         m = memoryview(bytearray(b'\xff'*size))
-        self.assertEqual(list(m[:MyIndex()]), [255] * 4)
+        with self.assertRaises(BufferError):
+            m[:MyIndex()]
 
         ba = None
         m = memoryview(bytearray(b'\xff'*size))
-        self.assertEqual(list(m[MyIndex():8]), [255] * 4)
+        with self.assertRaises(BufferError):
+            m[MyIndex():8]
 
         ba = None
         m = memoryview(bytearray(b'\xff'*size)).cast('B', (64, 2))

Misc/NEWS.d/next/Core_and_Builtins/2026-01-01-12-22-35.gh-issue-142665.7YCUBF.rst

@@ -0,0 +1,3 @@
+Fix use-after-free crashes when slicing a :class:`memoryview` or
+accessing the elements of a sliced view concurrently mutates the
+underlying buffer. Patch by Bénédikt Tran.

Objects/memoryobject.c

@@ -2623,7 +2623,10 @@ memory_subscript(PyObject *_self, PyObject *key)
         if (sliced == NULL)
             return NULL;
 
-        if (init_slice(&sliced->view, key, 0) < 0) {
+        self->exports++;
+        int rc = init_slice(&sliced->view, key, 0);
+        self->exports--;
+        if (rc < 0) {
             Py_DECREF(sliced);
             return NULL;
         }