Patch for security issue CVE-2021-3541

This is relapted to parameter entities expansion and following
the line of the billion laugh attack. Somehow in that path the
counting of parameters was missed and the normal algorithm based
on entities "density" was useless.

parser.c

@@ -140,6 +140,7 @@ xmlParserEntityCheck(xmlParserCtxtPtr ctxt, size_t size,
                      xmlEntityPtr ent, size_t replacement)
 {
     size_t consumed = 0;
+    int i;
 
     if ((ctxt == NULL) || (ctxt->options & XML_PARSE_HUGE))
         return (0);
@@ -177,6 +178,28 @@ xmlParserEntityCheck(xmlParserCtxtPtr ctxt, size_t size,
 	    rep = NULL;
 	}
     }
+
+    /*
+     * Prevent entity exponential check, not just replacement while
+     * parsing the DTD
+     * The check is potentially costly so do that only once in a thousand
+     */
+    if ((ctxt->instate == XML_PARSER_DTD) && (ctxt->nbentities > 10000) &&
+        (ctxt->nbentities % 1024 == 0)) {
+	for (i = 0;i < ctxt->inputNr;i++) {
+	    consumed += ctxt->inputTab[i]->consumed +
+	               (ctxt->inputTab[i]->cur - ctxt->inputTab[i]->base);
+	}
+	if (ctxt->nbentities > consumed * XML_PARSER_NON_LINEAR) {
+	    xmlFatalErr(ctxt, XML_ERR_ENTITY_LOOP, NULL);
+	    ctxt->instate = XML_PARSER_EOF;
+	    return (1);
+	}
+	consumed = 0;
+    }
+
+
+
     if (replacement != 0) {
 	if (replacement < XML_MAX_TEXT_LENGTH)
 	    return(0);
@@ -7963,6 +7986,9 @@ xmlParsePEReference(xmlParserCtxtPtr ctxt)
             xmlChar start[4];
             xmlCharEncoding enc;
 
+	    if (xmlParserEntityCheck(ctxt, 0, entity, 0))
+	        return;
+
 	    if ((entity->etype == XML_EXTERNAL_PARAMETER_ENTITY) &&
 	        ((ctxt->options & XML_PARSE_NOENT) == 0) &&
 		((ctxt->options & XML_PARSE_DTDVALID) == 0) &&