[Backport] CVE-2020-6449: Use after free in audio.

Manual backport of patch originally reviewed on:
https://chromium-review.googlesource.com/c/chromium/src/+/2098260
https://chromium-review.googlesource.com/c/chromium/src/+/2104992
Make finished_source_handlers_ hold scoped_refptrs

Previously, finished_source_handlers_ held raw pointers to
AudioHandlers and assumed that active_source_handlers_ also had a
copy.  But when the context goes away, active_source_handlers_ would
be cleared, but not finished_source_handlers_, leaving pointers to
deleted objects.

So do two things:
1. Change finished_source_handlers_ to hold scoped_refptrs to manage
   lifetime of the objects
2. Clear finished_source_handler_ in ClearHandlersToBeDeleted()

Either of these fix the repro case, but let's do both.  Don't want to
leaving dangling objects.

Manually tested the repro case which no longer reproduces.

Bug: 1059686
Change-Id: I11e999e6d7243351771d9530ceb924bd635578fd
Reviewed-by: Michal Klocek <michal.klocek@qt.io>

2 files changed, 4 insertions, 3 deletions

diff --git a/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.cc b/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.cc
index fca70458c5a..b836b4d23ca 100644
--- a/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.cc
+++ b/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.cc
@@ -77,7 +77,7 @@ void DeferredTaskHandler::BreakConnections() {
   // connection.
   wtf_size_t size = finished_source_handlers_.size();
   if (size > 0) {
-    for (auto* finished : finished_source_handlers_) {
+    for (auto finished : finished_source_handlers_) {
       // Break connection first and then remove from the list because that can
       // cause the handler to be deleted.
       finished->BreakConnectionWithLock();
@@ -358,6 +358,7 @@ void DeferredTaskHandler::ClearHandlersToBeDeleted() {
   deletable_orphan_handlers_.clear();
   automatic_pull_handlers_.clear();
   rendering_automatic_pull_handlers_.clear();
+  finished_source_handlers_.clear();
   active_source_handlers_.clear();
 }
 
diff --git a/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.h b/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.h
index 0ede5f5b5da..5d7416e481e 100644
--- a/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.h
+++ b/chromium/third_party/blink/renderer/modules/webaudio/deferred_task_handler.h
@@ -188,7 +188,7 @@ class MODULES_EXPORT DeferredTaskHandler final
     return &active_source_handlers_;
   }
 
-  Vector<AudioHandler*>* GetFinishedSourceHandlers() {
+  Vector<scoped_refptr<AudioHandler>>* GetFinishedSourceHandlers() {
     return &finished_source_handlers_;
   }
 
@@ -257,7 +257,7 @@ class MODULES_EXPORT DeferredTaskHandler final
   // connection and elements here are removed from |active_source_handlers_|.
   //
   // This must be accessed only from the audio thread.
-  Vector<AudioHandler*> finished_source_handlers_;
+  Vector<scoped_refptr<AudioHandler>> finished_source_handlers_;
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;