When creating an object in Javascript, its |Structure| is created with the constructor's prototype's |VM|.
Here's some snippets of that routine.
```
Structure* InternalFunction::createSubclassStructure(ExecState* exec, JSValue newTarget, Structure* baseClass)
{
...
if (newTarget && newTarget != exec->jsCallee()) {
// newTarget may be an InternalFunction if we were called from Reflect.construct.
JSFunction* targetFunction = jsDynamicCast<JSFunction*>(newTarget);
if (LIKELY(targetFunction)) {
...
return targetFunction->rareData(vm)->createInternalFunctionAllocationStructureFromBase(vm, prototype, baseClass);
...
} else {
...
return vm.prototypeMap.emptyStructureForPrototypeFromBaseStructure(prototype, baseClass);
...
}
}
return baseClass;
}
inline Structure* PrototypeMap::createEmptyStructure(JSObject* prototype, const TypeInfo& typeInfo, const ClassInfo* classInfo, IndexingType indexingType, unsigned inlineCapacity)
{
...
Structure* structure = Structure::create(
prototype->globalObject()->vm(), prototype->globalObject(), prototype, typeInfo, classInfo, indexingType, inlineCapacity);
m_structures.set(key, Weak<Structure>(structure));
...
}
```
As we can see |Structure::create| is called with prototype's |vm| and |globalObject| as arguments. So it could lead to an UXSS condition.
Tested on Safari 10.0.2(12602.3.12.0.1) and Webkit Nightly 10.0.2(12602.3.12.0.1, [r210800](https://crrev.com/210800)).
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