Most modern phones don't really have a physically separate baseband processor and application processor, they run a real-time microvisor (usually an L4 based system, and frequently OKL4 - see
http://wiki.ok-labs.com/#OKL4Microvisor4.0). The microvisor runs multiple virtual 'cells' (which are ARM operating systems that think they are running as supervisor), and ensures that the hard realtime requirements of the radio driver are met even if the kernel in the application cell is stuck in a loop.
Because the application processor is actually running in a virtual cell and not on bare metal, it doesn't have full access to the hardware and can't interfere with the radio cell - but the radio cell might still have access to everything.