GPU shaders are, by design, arbitrary GPU code execution. You don't have to do anything special or create an attack per-se. Shaders can't lead to execution of CPU code on their own, they are blackboxed and only have access to (limited) GPU memory.

I can imagine the potential for a combined CPU-GPU exploit, but it's not likely to hinge on a shader. I wouldn't say absolutely "impossible" but if I was looking for GPU exploits, I'd poke at the compiler and API first.