Modern GPUs (AMD, Intel and NVidia) have a per process page table so the worse you can do is access memory mapped to the GPU for _your_ process. That is assuming you can guess the virtual address of those, a wrong guess and the GPU context for your process will hang and you loose everything (for the process).

So if IOMMU is disabled you would still have to be able to find a way to re-program your process GPU page table which is not map in anyway to the process (ie only accessible by the kernel driver).

So i say good luck to you sir.