[Backport] CVE-2021-30541: Use after free in V8

Cherry-pick of patch originally reviewed on
https://chromium-review.googlesource.com/c/v8/v8/+/3067222:
Fix GC issue in BuildJsonObject

We must ensure that the sweeper is not running or has already swept
mutable_double_buffer. Otherwise the GC can add it to the free list.

(cherry picked from commit 81181a8ad80ac978a6a8732d05f615c645df95d2)

Bug: v8:11837
Bug: chromium:1214842
No-Try: true
No-Presubmit: true
No-Tree-Checks: true
Change-Id: Ifd9cf15f1c94f664fd6489c70bb38b59730cdd78
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#74859}
Commit-Queue: Roger Felipe Zanoni da Silva <rzanoni@google.com>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Reviewed-by: Jana Grill <janagrill@google.com>
Cr-Commit-Position: refs/branch-heads/9.0@{#68}
Cr-Branched-From: bd0108b4c88e0d6f2350cb79b5f363fbd02f3eb7-refs/heads/9.0.257@{#1}
Cr-Branched-From: 349bcc6a075411f1a7ce2d866c3dfeefc2efa39d-refs/heads/master@{#73001}
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

chromium/v8/src/heap/heap.cc

@@ -2117,6 +2117,10 @@ void Heap::CompleteSweepingYoung(GarbageCollector collector) {
   array_buffer_sweeper()->EnsureFinished();
 }
 
+void Heap::EnsureSweepingCompleted() {
+  mark_compact_collector()->EnsureSweepingCompleted();
+}
+
 void Heap::UpdateCurrentEpoch(GarbageCollector collector) {
   if (IsYoungGenerationCollector(collector)) {
     epoch_young_ = next_epoch();

chromium/v8/src/heap/heap.h

@@ -1068,6 +1068,8 @@ class Heap {
   void CompleteSweepingFull();
   void CompleteSweepingYoung(GarbageCollector collector);
 
+  void EnsureSweepingCompleted();
+
   IncrementalMarking* incremental_marking() {
     return incremental_marking_.get();
   }

chromium/v8/src/json/json-parser.cc

@@ -620,6 +620,11 @@ Handle<Object> JsonParser<Char>::BuildJsonObject(
         DCHECK_EQ(mutable_double_address, end);
       }
 #endif
+      // Before setting the length of mutable_double_buffer back to zero, we
+      // must ensure that the sweeper is not running or has already swept the
+      // object's page. Otherwise the GC can add the contents of
+      // mutable_double_buffer to the free list.
+      isolate()->heap()->EnsureSweepingCompleted();
       mutable_double_buffer->set_length(0);
     }
   }