There's no special mask for Smart Objects that I'm aware of. You can add the same pixel mask or vector mask as you'd add to a regular pixel layer.
There is, you can mask out layer effects.
If it's something you need to mask on the layer before you converted to a smart object, you can double click on the smart object icon in the layers panel and edit the source file.
Then press save to update the smart object.
1 person found this helpful
I get it! You're talking about Smart Filter mask. It appears when you add the first filter to a Smart Object
If you've removed that mask, another can be added from the right-click menu of the words "Smart Filters" under the layer in the Layers Panel.
I solved it