On Fri, Apr 03, 2020 at 01:58:31AM +0100, Al Viro wrote: > 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.? First, CONFIG_CPU_DOMAINS is used on older ARMs, not ARMv7. Second, the kernel image itself is not RO-protected on any ARM32 platform. If we get rid of CONFIG_CPU_DOMAINS, we will use the ARMv7 method of user access, which is to use normal load/stores for the user accessors and every access must check against the address limit, even the __-accessors. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 10.2Mbps down 587kbps up
