Index: Lib/test/test_mmap.py
===================================================================
--- Lib/test/test_mmap.py	(revision 78823)
+++ Lib/test/test_mmap.py	(working copy)
@@ -1,6 +1,6 @@
 from test.test_support import TESTFN, run_unittest, import_module
 import unittest
-import os, re, itertools
+import os, re, itertools, socket
 
 mmap = import_module('mmap')
 
@@ -585,6 +585,17 @@
             except:
                 pass
             m.close()
+        
+        def test_invalid_descriptor(self):
+            # socket file descriptors are valid, but out of range
+            # for _get_osfhandle, causing a crash when validating the
+            #parameters to _get_osfhandle.
+            s = socket.socket()
+            try:
+                with self.assertRaises(mmap.error):
+                    m = mmap.mmap(s.fileno(), 10)
+            finally:
+                s.close()
 
 
 def test_main():
Index: Modules/mmapmodule.c
===================================================================
--- Modules/mmapmodule.c	(revision 78823)
+++ Modules/mmapmodule.c	(working copy)
@@ -1280,6 +1280,11 @@
 			   "don't use 0 for anonymous memory");
 	 */
 	if (fileno != -1 && fileno != 0) {
+		/* Ensure that fileno is within the CRT's valid range */
+		if (_PyVerify_fd(fileno) == 0) {
+			PyErr_SetFromErrno(mmap_module_error);
+			return NULL;
+		}
 		fh = (HANDLE)_get_osfhandle(fileno);
 		if (fh==(HANDLE)-1) {
 			PyErr_SetFromErrno(mmap_module_error);