On Thu, Apr 02, 2020 at 11:35:57AM -0700, Kees Cook wrote: > Yup, I think it's a weakness of the ARM implementation and I'd like to > not extend it further. AFAIK we should never nest, but I would not be > surprised at all if we did. > > If we were looking at a design goal for all architectures, I'd like > to be doing what the public PaX patchset did for their memory access > switching, which is to alarm if calling into "enable" found the access > already enabled, etc. Such a condition would show an unexpected nesting > (like we've seen with similar constructs with set_fs() not getting reset > during an exception handler, etc etc). FWIW, maybe I'm misreading the ARM uaccess logics, but... it smells like KERNEL_DS is somewhat more dangerous there than on e.g. x86. Look: with CONFIG_CPU_DOMAINS, set_fs(KERNEL_DS) tells MMU to ignore per-page permission bits in DOMAIN_KERNEL (i.e. for kernel address ranges), allowing them even if they would normally be denied. We need that for actual uaccess loads/stores, since those use insns that pretend to be done in user mode and we want them to access the kernel pages. But that affects the normal loads/stores as well; unless I'm misreading that code, it will ignore (supervisor) r/o on a page. And that's not just for the code inside the uaccess blocks; *everything* done under KERNEL_DS is subject to that. Why do we do that (modify_domain(), that is) inside set_fs() and not in uaccess_enable() et.al.?
