Issue #15736: Fix overflow in _PySequence_BytesToCharpArray().

Lib/test/test_capi.py

@@ -67,6 +67,14 @@ class CAPITest(unittest.TestCase):
                 return 1
         self.assertRaises(TypeError, _posixsubprocess.fork_exec,
                           1,Z(),3,[1, 2],5,6,7,8,9,10,11,12,13,14,15,16,17)
+        # Issue #15736: overflow in _PySequence_BytesToCharpArray()
+        class Z(object):
+            def __len__(self):
+                return sys.maxsize
+            def __getitem__(self, i):
+                return b'x'
+        self.assertRaises(MemoryError, _posixsubprocess.fork_exec,
+                          1,Z(),3,[1, 2],5,6,7,8,9,10,11,12,13,14,15,16,17)
 
     @unittest.skipUnless(_posixsubprocess, '_posixsubprocess required for this test.')
     def test_subprocess_fork_exec(self):

Objects/abstract.c

@@ -2728,6 +2728,13 @@ _PySequence_BytesToCharpArray(PyObject* self)
     if (argc == -1)
         return NULL;
 
+    assert(argc >= 0);
+
+    if ((size_t)argc > (PY_SSIZE_T_MAX-sizeof(char *)) / sizeof(char *)) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+
     array = malloc((argc + 1) * sizeof(char *));
     if (array == NULL) {
         PyErr_NoMemory();