At the moment we grant all of /dev/dri, but long term we want only the render nodes accessible. That gives us no modesetting or DRM master capabilities, only rendering.

It is true though that the drivers could very well have leaks in them, but the userspace sandbox is not the place to fix that, it is the drivers themselves. The intent is for the dri driver APIs to give guarantees about client separation, but I'm sure it needs work.