Bug 172572 – JSObject::getPropertySlot does not appear to access the prototype in a safe way

• Status: NEW
• Component: JavaScriptCore

[Saam Barati]

It just accesses Structure's storedPrototype, which may not call the method table method.

See:
```
// It may seem crazy to inline a function this large but it makes a big difference
// since this is function very hot in variable lookup
ALWAYS_INLINE bool JSObject::getPropertySlot(ExecState* exec, PropertyName propertyName, PropertySlot& slot)
{
    VM& vm = exec->vm();
    auto& structureIDTable = vm.heap.structureIDTable();
    JSObject* object = this;
    while (true) {
        if (UNLIKELY(TypeInfo::overridesGetOwnPropertySlot(object->inlineTypeFlags()))) {
            // If propertyName is an index then we may have missed it (as this loop is using
            // getOwnNonIndexPropertySlot), so we cannot safely call the overridden getOwnPropertySlot
            // (lest we return a property from a prototype that is shadowed). Check now for an index,
            // if so we need to start afresh from this object.
            if (std::optional<uint32_t> index = parseIndex(propertyName))
                return getPropertySlot(exec, index.value(), slot);
            // Safe to continue searching from current position; call getNonIndexPropertySlot to avoid
            // parsing the int again.
            return object->getNonIndexPropertySlot(exec, propertyName, slot);
        }
        ASSERT(object->type() != ProxyObjectType);
        Structure* structure = structureIDTable.get(object->structureID());
        if (object->getOwnNonIndexPropertySlot(vm, structure, propertyName, slot))
            return true;
        JSValue prototype = structure->storedPrototype();
        if (!prototype.isObject())
            break;
        object = asObject(prototype);
    }

    if (std::optional<uint32_t> index = parseIndex(propertyName))
        return getPropertySlot(exec, index.value(), slot);
    return false;
}
```

[GSkachkov]

Is this issue connected to this https://bugs.webkit.org/show_bug.cgi?id=171915?