gh-142663: Fix use-after-free in memoryview comparison

When comparing two memoryview objects with different formats, `memory_richcompare` uses the `struct` module to unpack elements. A custom `struct.Struct.unpack_from` implementation could releases and resizes underlying buffer, which invalidates the buffer pointer, during iteration. This leads to a use-after-free when the comparison loop continued accessing the freed memory.

The fix increments the `exports` count of the memoryview objects before performing the comparison, effectively locking the buffers. This mirrors the protection already provided for non-memoryview objects via `PyObject_GetBuffer`.

Author: superboy-zjc

Lib/test/test_memoryview.py

@@ -228,6 +228,67 @@ def test_compare(self):
                 self.assertRaises(TypeError, lambda: m >= c)
                 self.assertRaises(TypeError, lambda: c > m)
 
+    def test_compare_use_after_free(self):
+        # Prevent crash in comparisons of memoryview objs with re-entrant struct.unpack_from.
+        # Regression test for https://github.com/python/cpython/issues/142663.
+
+        class ST(struct.Struct):
+            # Context set by the subtests
+            view = None
+            source = None
+
+            def unpack_from(self, buf, /, offset=0):
+                # Attempt to release the buffer while it's being used in comparison loop.
+                if self.view is not None:
+                    self.view.release()
+
+                # array resize invalidates the buffer pointer used by the comparison loop.
+                if self.source is not None:
+                    self.source.append(3.14)
+
+                return (1,)
+
+        with support.swap_attr(struct, "Struct", ST):
+            # Case 1: 1-D comparison (uses cmp_base optimized loop)
+            # Use mixed types ('d' vs 'l') to force struct unpacking path.
+            with self.subTest(ndim=1):
+                a = array.array("d", [1.0, 2.0])
+                b = array.array("l", [1, 2])
+                mv_a = memoryview(a)
+                mv_b = memoryview(b)
+
+                ST.view = mv_a
+                ST.source = a
+                try:
+                    with self.assertRaises(BufferError):
+                        # Expect BufferError because the memoryview is locked during comparison
+                        mv_a == mv_b
+                finally:
+                    ST.view = None
+                    ST.source = None
+                    mv_a.release()
+                    mv_b.release()
+
+            # Case 2: N-D comparison (uses cmp_rec recursive function)
+            # Use mixed types ('d' vs 'l') to force struct unpacking path.
+            with self.subTest(ndim=2):
+                a = array.array("d", [1.0, 2.0])
+                b = array.array("l", [1, 2])
+                mv_a = memoryview(a).cast("B").cast("d", shape=(1, 2))
+                mv_b = memoryview(b).cast("B").cast("l", shape=(1, 2))
+
+                ST.view = mv_a
+                ST.source = a
+                try:
+                    with self.assertRaises(BufferError):
+                        # Expect BufferError because the memoryview is locked during comparison
+                        mv_a == mv_b
+                finally:
+                    ST.view = None
+                    ST.source = None
+                    mv_a.release()
+                    mv_b.release()
+
     def check_attributes_with_type(self, tp):
         m = self._view(tp(self._source))
         self.assertEqual(m.format, self.format)

Misc/NEWS.d/next/Library/2025-12-30-22-12-27.gh-issue-142663.gq7iIf.rst

@@ -0,0 +1,3 @@
+:class:`memoryview`: Fix a use-after-free crash during comparison when an
+overridden :meth:`struct.Struct.unpack_from` releases and resizes the
+underlying buffer.

Objects/memoryobject.c

@@ -3165,6 +3165,13 @@ memory_richcompare(PyObject *v, PyObject *w, int op)
             goto result;
         }
     }
+    /* Prevent memoryview object from being released and its underlying buffer
+       reshaped during a mixed format comparison loop. */
+    // See https://github.com/python/cpython/issues/142663.
+    ((PyMemoryViewObject *)v)->exports++;
+    if (PyMemoryView_Check(w)) {
+        ((PyMemoryViewObject *)w)->exports++;
+    }
 
     if (vv->ndim == 0) {
         equal = unpack_cmp(vv->buf, ww->buf,
@@ -3183,6 +3190,11 @@ memory_richcompare(PyObject *v, PyObject *w, int op)
                         vfmt, unpack_v, unpack_w);
     }
 
+    ((PyMemoryViewObject *)v)->exports--;
+    if (PyMemoryView_Check(w)) {
+        ((PyMemoryViewObject *)w)->exports--;
+    }
+
 result:
     if (equal < 0) {
         if (equal == MV_COMPARE_NOT_IMPL)