File _service:tar_scm:backport-CVE-2024-28757-001.patch of Package expat

From 1d50b80cf31de87750103656f6eb693746854aa8 Mon Sep 17 00:00:00 2001
From: Sebastian Pipping <sebastian@pipping.org>
Date: Mon, 4 Mar 2024 23:49:06 +0100
Subject: [PATCH] lib/xmlparse.c: Detect billion laughs attack with isolated
 external parser

When parsing DTD content with code like ..

XML_Parser parser = XML_ParserCreate(NULL);
  XML_Parser ext_parser = XML_ExternalEntityParserCreate(parser, NULL, NULL);
  enum XML_Status status = XML_Parse(ext_parser, doc, (int)strlen(doc), XML_TRUE);

.. there are 0 bytes accounted as direct input and all input from  accounted
as indirect input.  Now function accountingGetCurrentAmplification cannot calculate
the current amplification ratio as "(direct + indirect) / direct", and it did refuse
to divide by 0 as one would expect, but it returned 1.0 for this case to indicate
no amplification over direct input.  As a result, billion laughs attacks from
DTD-only input were not detected with this isolated way of using an external parser.

The new approach is to assume direct input of length not 0 but 22 -- derived from
ghost input "<!ENTITY a SYSTEM 'b'>", the shortest possible way to include an external
DTD --, and do the usual "(direct + indirect) / direct" math with "direct := 22".

GitHub issue #839 has more details on this issue and its origin in ClusterFuzz
finding 66812.
---
 expat/lib/xmlparse.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/lib/xmlparse.c b/expat/lib/xmlparse.c
index b884d82b..d44baa68 100644
--- a/lib/xmlparse.c
+++ b/lib/xmlparse.c
@@ -7787,6 +7787,8 @@ copyString(const XML_Char *s, const XML_Memory_Handling_Suite *memsuite) {
 
 static float
 accountingGetCurrentAmplification(XML_Parser rootParser) {
+  //                                          1.........1.........12 => 22
+  const size_t lenOfShortestInclude = sizeof("<!ENTITY a SYSTEM 'b'>") - 1;
   const XmlBigCount countBytesOutput
       = rootParser->m_accounting.countBytesDirect
         + rootParser->m_accounting.countBytesIndirect;
@@ -7794,7 +7796,9 @@ accountingGetCurrentAmplification(XML_Parser rootParser) {
       = rootParser->m_accounting.countBytesDirect
             ? (countBytesOutput
                / (float)(rootParser->m_accounting.countBytesDirect))
-            : 1.0f;
+            : ((lenOfShortestInclude
+                + rootParser->m_accounting.countBytesIndirect)
+               / (float)lenOfShortestInclude);
   assert(! rootParser->m_parentParser);
   return amplificationFactor;
 }
-- 
2.33.0