Merge pull request #650 from libexpat/issue-649-fix-overeager-dtd-des…

…truction

[CVE-2022-43680] Fix overeager DTD destruction (fixes #649)

expat/Changes

@@ -3,6 +3,11 @@ NOTE: We are looking for help with a few things:
       If you can help, please get in touch.  Thanks!
 
 Release x.x.x xxx xxxxxxxxxxxx xx xxxx
+        Security fixes:
+  #616 #649 #650  CVE-2022-43680 -- Fix heap use-after-free after overeager
+                    destruction of a shared DTD in function
+                    XML_ExternalEntityParserCreate in out-of-memory situations
+
         Bug fixes:
        #612 #645  Fix curruption from undefined entities
        #613 #654  Fix case when parsing was suspended while processing nested

expat/lib/xmlparse.c

@@ -1068,6 +1068,14 @@ parserCreate(const XML_Char *encodingName,
   parserInit(parser, encodingName);
 
   if (encodingName && ! parser->m_protocolEncodingName) {
+    if (dtd) {
+      // We need to stop the upcoming call to XML_ParserFree from happily
+      // destroying parser->m_dtd because the DTD is shared with the parent
+      // parser and the only guard that keeps XML_ParserFree from destroying
+      // parser->m_dtd is parser->m_isParamEntity but it will be set to
+      // XML_TRUE only later in XML_ExternalEntityParserCreate (or not at all).
+      parser->m_dtd = NULL;
+    }
     XML_ParserFree(parser);
     return NULL;
   }

expat/tests/runtests.c

@@ -10208,6 +10208,53 @@ START_TEST(test_alloc_long_notation) {
 }
 END_TEST
 
+static int XMLCALL
+external_entity_parser_create_alloc_fail_handler(XML_Parser parser,
+                                                 const XML_Char *context,
+                                                 const XML_Char *base,
+                                                 const XML_Char *systemId,
+                                                 const XML_Char *publicId) {
+  UNUSED_P(base);
+  UNUSED_P(systemId);
+  UNUSED_P(publicId);
+
+  if (context != NULL)
+    fail("Unexpected non-NULL context");
+
+  // The following number intends to fail the upcoming allocation in line
+  // "parser->m_protocolEncodingName = copyString(encodingName,
+  // &(parser->m_mem));" in function parserInit.
+  allocation_count = 3;
+
+  const XML_Char *const encodingName = XCS("UTF-8"); // needs something non-NULL
+  const XML_Parser ext_parser
+      = XML_ExternalEntityParserCreate(parser, context, encodingName);
+  if (ext_parser != NULL)
+    fail(
+        "Call to XML_ExternalEntityParserCreate was expected to fail out-of-memory");
+
+  allocation_count = ALLOC_ALWAYS_SUCCEED;
+  return XML_STATUS_ERROR;
+}
+
+START_TEST(test_alloc_reset_after_external_entity_parser_create_fail) {
+  const char *const text = "<!DOCTYPE doc SYSTEM 'foo'><doc/>";
+
+  XML_SetExternalEntityRefHandler(
+      g_parser, external_entity_parser_create_alloc_fail_handler);
+  XML_SetParamEntityParsing(g_parser, XML_PARAM_ENTITY_PARSING_ALWAYS);
+
+  if (XML_Parse(g_parser, text, (int)strlen(text), XML_TRUE)
+      != XML_STATUS_ERROR)
+    fail("Call to parse was expected to fail");
+
+  if (XML_GetErrorCode(g_parser) != XML_ERROR_EXTERNAL_ENTITY_HANDLING)
+    fail("Call to parse was expected to fail from the external entity handler");
+
+  XML_ParserReset(g_parser, NULL);
+}
+END_TEST
+
 static void
 nsalloc_setup(void) {
   XML_Memory_Handling_Suite memsuite = {duff_allocator, duff_reallocator, free};
@@ -12401,6 +12448,8 @@ make_suite(void) {
   tcase_add_test(tc_alloc, test_alloc_long_public_id);
   tcase_add_test(tc_alloc, test_alloc_long_entity_value);
   tcase_add_test(tc_alloc, test_alloc_long_notation);
+  tcase_add_test__ifdef_xml_dtd(
+      tc_alloc, test_alloc_reset_after_external_entity_parser_create_fail);
 
   suite_add_tcase(s, tc_nsalloc);
   tcase_add_checked_fixture(tc_nsalloc, nsalloc_setup, nsalloc_teardown);