Allocate your resources according to their available time. If a given resource is working on something that "not of your business" just plan for it.
If your project is already in flight and resources once committed full time are partially being pulled out of your project, you should raise a risk (if impact hasn't eventuated) or an issue (if impact has eventuated) and formally articulate it with your senior management (registers, status report, email). Your forecast should also reflect the schedule impacts of resources not being available as planned.
By doing this, if your senior management doesn't take action they are accountable for the consequences, and you did your PM homework. No emotions, just PM techniques applied.
Open a change request to address the impacts to your plan and submit it to your senior management approval. In the change request details, clearly articulate the resource problems and ask your senior management to approve it. This way you will track to a realistic plan and deliver to your new baseline.
Stand in front of the CEO and you may loose your job. Do your PM homework and perhaps they will bring you to the "inner circle" ;)