From 506527a4327dfaac8f02a520d0a823ca37a17690 Mon Sep 17 00:00:00 2001
From: Emmanuele Bassi <ebassi@igalia.com>
Date: Thu, 09 Jul 2026 04:09:31 -0700
Subject: [PATCH] Protect ParseChunk against re-entrancy

The chunk parser in libxml2 is not safe from re-entrancy: the parser
context contains multiple arrays that can be reallocated if a callback
happens to be invoked in the middle of an xmlParseChunk().

Any fix inside libxml2 would require a fair amount of work to make the
whole parser safe, so the easiest solution is to add a guard in the
calling code.

Bug: 501892500
Change-Id: I814c0f2101b8f8a53808bb9184fc119b0f5044d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7902291
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1659452}
---

diff --git a/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc b/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
index ece86d8..48288e2 100644
--- a/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
+++ b/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
@@ -419,7 +419,7 @@
   if (IsStopped() || saw_xsl_transform_)
     return;
 
-  if (parser_paused_) {
+  if (parser_paused_ || in_parse_chunk_) {
     pending_src_.Append(source);
     return;
   }
@@ -970,6 +970,13 @@
   // Protect the libxml context from deletion during a callback
   scoped_refptr<XMLParserContext> context = context_;
 
+  // libxml2's push parser is not re-entrant: xmlParseEndTag2 holds multiple
+  // raw pointers inside ctxt, and a nested xmlParseChunk can xmlRealloc()
+  // those buffers. Crash safely rather than corrupt the heap. (Append()
+  // routes re-entrant data to pending_src_ so this should be unreachable.)
+  CHECK(!in_parse_chunk_);
+  base::AutoReset<bool> reentrancy_guard(&in_parse_chunk_, true);
+
   // libXML throws an error if you try to switch the encoding for an empty
   // string.
   if (parse_string.length()) {
diff --git a/third_party/blink/renderer/core/xml/parser/xml_document_parser.h b/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
index 6e09d417..b88e3d3 100644
--- a/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
+++ b/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
@@ -221,6 +221,10 @@
   bool saw_first_element_;
   bool is_xhtml_document_;
   bool parser_paused_;
+  // Re-entrancy guard for DoWrite()/xmlParseChunk(). libxml2 push-parser
+  // contexts are not re-entrant; calling xmlParseChunk while already inside
+  // a SAX callback corrupts ctxt->pushTab/nsTab.
+  bool in_parse_chunk_ = false;
   bool requesting_script_;
   bool finish_called_;
   bool waiting_for_stylesheets_ = false;