Fixes issue #14234: CVE-2012-0876: Randomize hashes of xml attributes

in the hash table internal to the pyexpat module's copy of the expat
library to avoid a denial of service due to hash collisions.
Patch by David Malcolm with some modifications by the expat project.

Misc/NEWS

@@ -15,6 +15,11 @@ Core and Builtins
   service attacks due to hash collisions within the dict and set types.  Patch
   by David Malcolm, based on work by Victor Stinner.
 
+- Issue #14234: CVE-2012-0876: Randomize hashes of xml attributes in the hash
+  table internal to the pyexpat module's copy of the expat library to avoid a
+  denial of service due to hash collisions.  Patch by David Malcolm with some
+  modifications by the expat project.
+
 Library
 -------
 

Modules/expat/expat.h

@@ -883,6 +883,15 @@ XMLPARSEAPI(int)
 XML_SetParamEntityParsing(XML_Parser parser,
                           enum XML_ParamEntityParsing parsing);
 
+/* Sets the hash salt to use for internal hash calculations.
+   Helps in preventing DoS attacks based on predicting hash
+   function behavior. This must be called before parsing is started.
+   Returns 1 if successful, 0 when called after parsing has started.
+*/
+XMLPARSEAPI(int)
+XML_SetHashSalt(XML_Parser parser,
+                unsigned long hash_salt);
+
 /* If XML_Parse or XML_ParseBuffer have returned XML_STATUS_ERROR, then
    XML_GetErrorCode returns information about the error.
 */

Modules/expat/pyexpatns.h

@@ -97,6 +97,7 @@
 #define XML_SetEntityDeclHandler        PyExpat_XML_SetEntityDeclHandler
 #define XML_SetExternalEntityRefHandler PyExpat_XML_SetExternalEntityRefHandler
 #define XML_SetExternalEntityRefHandlerArg  PyExpat_XML_SetExternalEntityRefHandlerArg
+#define XML_SetHashSalt                 PyExpat_XML_SetHashSalt
 #define XML_SetNamespaceDeclHandler     PyExpat_XML_SetNamespaceDeclHandler
 #define XML_SetNotationDeclHandler      PyExpat_XML_SetNotationDeclHandler
 #define XML_SetNotStandaloneHandler     PyExpat_XML_SetNotStandaloneHandler

Modules/pyexpat.c

@@ -1245,6 +1245,8 @@ newxmlparseobject(char *encoding, char *namespace_separator, PyObject *intern)
     else {
         self->itself = XML_ParserCreate(encoding);
     }
+    XML_SetHashSalt(self->itself,
+                    (unsigned long)_Py_HashSecret.prefix);
     self->intern = intern;
     Py_XINCREF(self->intern);
 #ifdef Py_TPFLAGS_HAVE_GC