[CVE-2022-23852] Prevent XML_GetBuffer signed integer overflow

expat/Changes

@@ -2,6 +2,18 @@ NOTE: We are looking for help with a few things:
       https://github.com/libexpat/libexpat/labels/help%20wanted
       If you can help, please get in touch.  Thanks!
 
+Release x.x.x xxx xxxxxxx xx xxxx
+        Security fixes:
+            #550  CVE-2022-23852 -- Fix signed integer overflow
+                    (undefined behavior) in function XML_GetBuffer
+                    (that is also called by function XML_Parse internally)
+                    for when XML_CONTEXT_BYTES is defined to >0 (which is both
+                    common and default).
+                    Impact is denial of service or more.
+
+        Special thanks to:
+            Samanta Navarro
+
 Release 2.4.3 Sun January 16 2022
         Security fixes:
        #531 #534  CVE-2021-45960 -- Fix issues with left shifts by >=29 places

expat/lib/xmlparse.c

@@ -2067,6 +2067,11 @@ XML_GetBuffer(XML_Parser parser, int len) {
     keep = (int)EXPAT_SAFE_PTR_DIFF(parser->m_bufferPtr, parser->m_buffer);
     if (keep > XML_CONTEXT_BYTES)
       keep = XML_CONTEXT_BYTES;
+    /* Detect and prevent integer overflow */
+    if (keep > INT_MAX - neededSize) {
+      parser->m_errorCode = XML_ERROR_NO_MEMORY;
+      return NULL;
+    }
     neededSize += keep;
 #endif /* defined XML_CONTEXT_BYTES */
     if (neededSize

expat/tests/runtests.c

@@ -3847,6 +3847,30 @@ START_TEST(test_get_buffer_2) {
 }
 END_TEST
 
+/* Test for signed integer overflow CVE-2022-23852 */
+#if defined(XML_CONTEXT_BYTES)
+START_TEST(test_get_buffer_3_overflow) {
+  XML_Parser parser = XML_ParserCreate(NULL);
+  assert(parser != NULL);
+
+  const char *const text = "\n";
+  const int expectedKeepValue = (int)strlen(text);
+
+  // After this call, variable "keep" in XML_GetBuffer will
+  // have value expectedKeepValue
+  if (XML_Parse(parser, text, (int)strlen(text), XML_FALSE /* isFinal */)
+      == XML_STATUS_ERROR)
+    xml_failure(parser);
+
+  assert(expectedKeepValue > 0);
+  if (XML_GetBuffer(parser, INT_MAX - expectedKeepValue + 1) != NULL)
+    fail("enlarging buffer not failed");
+
+  XML_ParserFree(parser);
+}
+END_TEST
+#endif // defined(XML_CONTEXT_BYTES)
+
 /* Test position information macros */
 START_TEST(test_byte_info_at_end) {
   const char *text = "<doc></doc>";
@@ -11731,6 +11755,9 @@ make_suite(void) {
   tcase_add_test(tc_basic, test_empty_parse);
   tcase_add_test(tc_basic, test_get_buffer_1);
   tcase_add_test(tc_basic, test_get_buffer_2);
+#if defined(XML_CONTEXT_BYTES)
+  tcase_add_test(tc_basic, test_get_buffer_3_overflow);
+#endif
   tcase_add_test(tc_basic, test_byte_info_at_end);
   tcase_add_test(tc_basic, test_byte_info_at_error);
   tcase_add_test(tc_basic, test_byte_info_at_cdata);