gh-143308: fix UAF when PickleBuffer is concurrently mutated in a callback

Lib/test/test_pickle.py

@@ -388,6 +388,38 @@ class CPicklingErrorTests(PyPicklingErrorTests):
     class CPicklerTests(PyPicklerTests):
         pickler = _pickle.Pickler
         unpickler = _pickle.Unpickler
+        def test_release_in_callback_keepalive(self):
+                base = bytearray(b'A' * 16)
+                pb = pickle.PickleBuffer(base)
+
+                class Evil:
+                    def __bool__(self):
+                        pb.release()
+                        return True
+
+                def callback(p):
+                    return Evil()
+
+                for proto in range(5, pickle.HIGHEST_PROTOCOL + 1):
+                    result = self.dumps(pb, proto, buffer_callback=callback)
+                    self.assertIn(b'A' * 16, result)
+
+        def test_release_in_callback_complex_keepalive(self):
+                base = bytearray(b'A' * 32)
+                view = memoryview(base)[10:26]
+                pb = pickle.PickleBuffer(view)
+
+                class Evil:
+                    def __bool__(self):
+                        pb.release()
+                        return True
+
+                def callback(p):
+                    return Evil()
+
+                for proto in range(5, pickle.HIGHEST_PROTOCOL + 1):
+                    result = self.dumps(pb, proto, buffer_callback=callback)
+                    self.assertIn(b'A' * 16, result)
 
     class CPersPicklerTests(PyPersPicklerTests):
         pickler = _pickle.Pickler

Misc/NEWS.d/next/Core_and_Builtins/2025-12-31-17-38-33.gh-issue-143308.lY8UCR.rst

@@ -0,0 +1,2 @@
+Fixed Use-After-Free in _pickle.c by incrementing PickleBuffer exports
+during pickling.

Modules/_pickle.c

@@ -2646,43 +2646,58 @@ save_picklebuffer(PickleState *st, PicklerObject *self, PyObject *obj)
                         "pointing to a non-contiguous buffer");
         return -1;
     }
+
+    int rc = 0;
+    Py_buffer keepalive_view; // to ensure that 'view' is kept alive
+    if (PyObject_GetBuffer(view->obj, &keepalive_view, PyBUF_FULL_RO) != 0) {
+        return -1;
+    }
+
     int in_band = 1;
     if (self->buffer_callback != NULL) {
         PyObject *ret = PyObject_CallOneArg(self->buffer_callback, obj);
         if (ret == NULL) {
-            return -1;
+            goto error;
         }
         in_band = PyObject_IsTrue(ret);
         Py_DECREF(ret);
         if (in_band == -1) {
-            return -1;
+            goto error;
         }
     }
     if (in_band) {
         /* Write data in-band */
         if (view->readonly) {
-            return _save_bytes_data(st, self, obj, (const char *)view->buf,
+            rc = _save_bytes_data(st, self, obj, (const char *)view->buf,
                                     view->len);
         }
         else {
-            return _save_bytearray_data(st, self, obj, (const char *)view->buf,
+            rc = _save_bytearray_data(st, self, obj, (const char *)view->buf,
                                         view->len);
         }
+        goto exit;
     }
     else {
         /* Write data out-of-band */
         const char next_buffer_op = NEXT_BUFFER;
         if (_Pickler_Write(self, &next_buffer_op, 1) < 0) {
-            return -1;
+            goto error;
         }
         if (view->readonly) {
             const char readonly_buffer_op = READONLY_BUFFER;
             if (_Pickler_Write(self, &readonly_buffer_op, 1) < 0) {
-                return -1;
+                goto error;
             }
         }
     }
-    return 0;
+
+exit:
+    PyBuffer_Release(&keepalive_view);
+    return rc;
+
+error:
+    PyBuffer_Release(&keepalive_view);
+    return -1;
 }
 
 /* A copy of PyUnicode_AsRawUnicodeEscapeString() that also translates