If they isolate the memory that each part of the SoC can process, that means large amounts of data (for high definition video) have to be copied from one proces

If they isolate the memory that each part of the SoC can process, that means large amounts of data (for high definition video) have to be copied from one processor to another, so there's direct access. Then if the DSP's firmware has buffer overrun bugs, craft a suitable video and read/write whatever memory area you want to. (Not sure that this is what the bug is, but it's my guess).