Backported of:

From c3c9db89273fabc62ea1b48389d9a3000c1c03ae Mon Sep 17 00:00:00 2001
From: Jay Bosamiya <jaybosamiya@gmail.com>
Date: Sun, 18 Jun 2017 22:11:03 +0530
Subject: [PATCH] [2.7] bpo-30657: Check & prevent integer overflow in
 PyString_DecodeEscape/PyBytes_DecodeEscape (#2174)
Index: python3.5-3.5.3/Objects/bytesobject.c
===================================================================
--- python3.5-3.5.3.orig/Objects/bytesobject.c
+++ python3.5-3.5.3/Objects/bytesobject.c
@@ -970,7 +970,13 @@ PyObject *PyBytes_DecodeEscape(const cha
     char *p, *buf;
     const char *end;
     PyObject *v;
-    Py_ssize_t newlen = recode_encoding ? 4*len:len;
+    Py_ssize_t newlen;
+    /* Check for integer overflow */
+    if (recode_encoding && (len > PY_SSIZE_T_MAX / 4)) {
+        PyErr_SetString(PyExc_OverflowError, "string is too large");
+        return NULL;
+    }
+    newlen = recode_encoding ? 4*len:len;
     v = PyBytes_FromStringAndSize((char *)NULL, newlen);
     if (v == NULL)
         return NULL;