bpo-36504: Fix signed integer overflow in _ctypes.c's PyCArrayType_new(). (GH-12660)

(cherry picked from commit 487b73a)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Author: miss-islington

Lib/ctypes/test/test_arrays.py

@@ -183,6 +183,12 @@ class T(Array):
                 _type_ = c_int
                 _length_ = 1.87
 
+    def test_bpo36504_signed_int_overflow(self):
+        # The overflow check in PyCArrayType_new() could cause signed integer
+        # overflow.
+        with self.assertRaises(OverflowError):
+            c_char * sys.maxsize * 2
+
     @unittest.skipUnless(sys.maxsize > 2**32, 'requires 64bit platform')
     @bigmemtest(size=_2G, memuse=1, dry_run=False)
     def test_large_array(self, size):

Misc/NEWS.d/next/Core and Builtins/2019-04-02-04-10-32.bpo-36504.k_V8Bm.rst

@@ -0,0 +1 @@
+Fix signed integer overflow in _ctypes.c's ``PyCArrayType_new()``.

Modules/_ctypes/_ctypes.c

@@ -1466,7 +1466,7 @@ PyCArrayType_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
     }
 
     itemsize = itemdict->size;
-    if (length * itemsize < 0) {
+    if (length > PY_SSIZE_T_MAX / itemsize) {
         PyErr_SetString(PyExc_OverflowError,
                         "array too large");
         goto error;