bpo-30657: Fix CVE-2017-1000158

Fixes possible integer overflow in PyBytes_DecodeEscape.

Co-Authored-By: Jay Bosamiya <jaybosamiya@gmail.com>

Author: hroncok

Misc/ACKS

@@ -167,6 +167,7 @@ Médéric Boquien
 Matias Bordese
 Jonas Borgström
 Jurjen Bos
+Jay Bosamiya
 Peter Bosch
 Dan Boswell
 Eric Bouck
@@ -651,6 +652,7 @@ Ken Howard
 Brad Howes
 Mike Hoy
 Ben Hoyt
+Miro Hrončok
 Chiu-Hsiang Hsu
 Chih-Hao Huang
 Christian Hudon

Misc/NEWS.d/next/Security/2017-12-01-18-51-03.bpo-30657.Fd8kId.rst

@@ -0,0 +1,2 @@
+Fixed possible integer overflow in PyBytes_DecodeEscape, CVE-2017-1000158.
+Original patch by Jay Bosamiya; rebased to Python 3 by Miro Hrončok.

Objects/bytesobject.c

@@ -970,7 +970,13 @@ PyObject *PyBytes_DecodeEscape(const char *s,
     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;