The type of kernel is orthogonal to this particular design flaw from what I know. It's about how page tables are set up for user mode processes. I may be wrong but pretty much every Unix or Unix-like kernel (including Plan 9) follows the page table model Windows and Linux use: the ring 0 PTEs are present in the page table, even in user mode, and we count on the architecture to prevent ring 3 access to ring 0 memory. This is certainly the case on all the ones I've worked on.