gh-144281: Fix crash on memoryview slice assignment with shared memory

Lib/test/_test_multiprocessing.py

@@ -4932,6 +4932,15 @@ def test_shared_memory_cleaned_after_process_termination(self):
                     "resource_tracker: There appear to be 1 leaked "
                     "shared_memory objects to clean up at shutdown", err)
 
+    def test_shared_memory_slice_assignment_no_crash(self):
+        from multiprocessing import shared_memory
+        shm = shared_memory.SharedMemory(create=True, size=10)
+        mv = shm.buf
+        shm.close()
+        shm.unlink()
+        with self.assertRaises((BufferError, ValueError)):
+            mv[:5] = b'hello'
+
     @unittest.skipIf(os.name != "posix", "resource_tracker is posix only")
     @resource_tracker_format_subtests
     def test_shared_memory_untracking(self):

Misc/NEWS.d/next/Core_and_Builtins/2026-01-27-23-35-19.gh-issue-144281.IiakEV.rst

@@ -0,0 +1,2 @@
+Fix a possible interpreter crash during memoryview slice assignment when the
+underlying buffer is backed by shared memory.

Objects/memoryobject.c

@@ -2656,6 +2656,11 @@ memory_ass_sub(PyObject *_self, PyObject *key, PyObject *value)
 
     CHECK_RELEASED_INT(self);
 
+    if (view->buf == NULL) {
+        PyErr_SetString(PyExc_BufferError, "memoryview: underlying buffer is no longer valid");
+        return -1;
+    }
+
     fmt = adjust_fmt(view);
     if (fmt == NULL)
         return -1;