Commit message: Another variation of overflow in Conditional sections

Which happen after the previous fix to
https://bugzilla.gnome.org/show_bug.cgi?id=756456

But stopping the parser and exiting we didn't pop the intermediary entities
and doing the SKIP there applies on an input which may be too small
CVE: CVE-2015-7942
CWE: CWE-119
Vulnerability description: The xmlParseConditionalSections function in parser.c in libxml2 does not properly skip intermediary entities when it stops parsing invalid input, which allows context-dependent attackers to cause a denial of service (out-of-bounds read and crash) via crafted XML data, a different vulnerability than CVE-2015-7941.

Project: libxml2
File: parser.c

xmlParseConditionalSections(xmlParserCtxtPtr ctxt) {
    int id = ctxt->input->id;

    SKIP(3);
    SKIP_BLANKS;
    if (CMP7(CUR_PTR, 'I', 'N', 'C', 'L', 'U', 'D', 'E')) {
	SKIP(7);
	SKIP_BLANKS;
	if (RAW != '[') {
	    xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID, NULL);
	    xmlStopParser(ctxt);
	    return;
	} else {
	    if (ctxt->input->id != id) {
		xmlValidityError(ctxt, XML_ERR_ENTITY_BOUNDARY,
	    "All markup of the conditional section is not in the same entity\n",
				     NULL, NULL);
	    }
	    NEXT;
	}
	if (xmlParserDebugEntities) {
	    if ((ctxt->input != NULL) && (ctxt->input->filename))
		xmlGenericError(xmlGenericErrorContext,
			"%s(%d): ", ctxt->input->filename,
			ctxt->input->line);
	    xmlGenericError(xmlGenericErrorContext,
		    "Entering INCLUDE Conditional Section\n");
	}

	while (((RAW != 0) && ((RAW != ']') || (NXT(1) != ']') ||
	        (NXT(2) != '>'))) && (ctxt->instate != XML_PARSER_EOF)) {
	    const xmlChar *check = CUR_PTR;
	    unsigned int cons = ctxt->input->consumed;

	    if ((RAW == '<') && (NXT(1) == '!') && (NXT(2) == '[')) {
		xmlParseConditionalSections(ctxt);
	    } else if (IS_BLANK_CH(CUR)) {
		NEXT;
	    } else if (RAW == '%') {
		xmlParsePEReference(ctxt);
	    } else
		xmlParseMarkupDecl(ctxt);

	    /*
	     * Pop-up of finished entities.
	     */
	    while ((RAW == 0) && (ctxt->inputNr > 1))
		xmlPopInput(ctxt);

	    if ((CUR_PTR == check) && (cons == ctxt->input->consumed)) {
		xmlFatalErr(ctxt, XML_ERR_EXT_SUBSET_NOT_FINISHED, NULL);
		break;
	    }
	}
	if (xmlParserDebugEntities) {
	    if ((ctxt->input != NULL) && (ctxt->input->filename))
		xmlGenericError(xmlGenericErrorContext,
			"%s(%d): ", ctxt->input->filename,
			ctxt->input->line);
	    xmlGenericError(xmlGenericErrorContext,
		    "Leaving INCLUDE Conditional Section\n");
	}

    } else if (CMP6(CUR_PTR, 'I', 'G', 'N', 'O', 'R', 'E')) {
	int state;
	xmlParserInputState instate;
	int depth = 0;

	SKIP(6);
	SKIP_BLANKS;
	if (RAW != '[') {
	    xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID, NULL);
	    xmlStopParser(ctxt);
	    return;
	} else {
	    if (ctxt->input->id != id) {
		xmlValidityError(ctxt, XML_ERR_ENTITY_BOUNDARY,
	    "All markup of the conditional section is not in the same entity\n",
				     NULL, NULL);
	    }
	    NEXT;
	}
	if (xmlParserDebugEntities) {
	    if ((ctxt->input != NULL) && (ctxt->input->filename))
		xmlGenericError(xmlGenericErrorContext,
			"%s(%d): ", ctxt->input->filename,
			ctxt->input->line);
	    xmlGenericError(xmlGenericErrorContext,
		    "Entering IGNORE Conditional Section\n");
	}

	/*
	 * Parse up to the end of the conditional section
	 * But disable SAX event generating DTD building in the meantime
	 */
	state = ctxt->disableSAX;
	instate = ctxt->instate;
	if (ctxt->recovery == 0) ctxt->disableSAX = 1;
	ctxt->instate = XML_PARSER_IGNORE;

	while (((depth >= 0) && (RAW != 0)) &&
               (ctxt->instate != XML_PARSER_EOF)) {
	  if ((RAW == '<') && (NXT(1) == '!') && (NXT(2) == '[')) {
	    depth++;
	    SKIP(3);
	    continue;
	  }
	  if ((RAW == ']') && (NXT(1) == ']') && (NXT(2) == '>')) {
	    if (--depth >= 0) SKIP(3);
	    continue;
	  }
	  NEXT;
	  continue;
	}

	ctxt->disableSAX = state;
	ctxt->instate = instate;

	if (xmlParserDebugEntities) {
	    if ((ctxt->input != NULL) && (ctxt->input->filename))
		xmlGenericError(xmlGenericErrorContext,
			"%s(%d): ", ctxt->input->filename,
			ctxt->input->line);
	    xmlGenericError(xmlGenericErrorContext,
		    "Leaving IGNORE Conditional Section\n");
	}

    } else {
	xmlFatalErr(ctxt, XML_ERR_CONDSEC_INVALID_KEYWORD, NULL);
	xmlStopParser(ctxt);
	return;
    }

    if (RAW == 0)
        SHRINK;

    if (RAW == 0) {
	xmlFatalErr(ctxt, XML_ERR_CONDSEC_NOT_FINISHED, NULL);
    } else {
	if (ctxt->input->id != id) {
	    xmlValidityError(ctxt, XML_ERR_ENTITY_BOUNDARY,
	"All markup of the conditional section is not in the same entity\n",
				 NULL, NULL);
	}
        SKIP(3);
    }
}