On Mon, Aug 15, 2016 at 3:47 AM, Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > On Fri, Aug 12, 2016 at 11:23:03AM -0700, Kees Cook wrote: >> On Thu, Aug 11, 2016 at 10:44 AM, Catalin Marinas >> <catalin.marinas@xxxxxxx> wrote: >> > The ARMv8 architecture allows execute-only user permissions by clearing >> > the PTE_UXN and PTE_USER bits. However, the kernel running on a CPU >> > implementation without User Access Override (ARMv8.2 onwards) can still >> > access such page, so execute-only page permission does not protect >> > against read(2)/write(2) etc. accesses. Systems requiring such >> > protection must enable features like SECCOMP. >> >> So, UAO CPUs will bypass this protection in userspace if using >> read/write on a memory-mapped file? > > It's the other way around. CPUs prior to ARMv8.2 (when UAO was > introduced) or with the CONFIG_ARM64_UAO disabled can still access > user execute-only memory regions while running in kernel mode via the > copy_*_user, (get|put)_user etc. routines. So a way user can bypass this > protection is by using such address as argument to read/write file > operations. Ah, okay. So exec-only for _userspace_ will always work, but exec-only for _kernel_ will only work on ARMv8.2 with CONFIG_ARM64_UAO? > I don't think mmap() is an issue since such region is already mapped, so > it would require mprotect(). As for the latter, it would most likely be > restricted (probably together with read/write) SECCOMP. > >> I'm just trying to make sure I understand the bypass scenario. And is >> this something that can be fixed? If we add exec-only, I feel like it >> shouldn't have corner case surprises. :) > > I think we need better understanding of the usage scenarios for > exec-only. IIUC (from those who first asked me for this feature), it is > an additional protection on top of ASLR to prevent an untrusted entity > from scanning the memory for ROP/JOP gadgets. An instrumented compiler > would avoid generating the literal pool in the same section as the > executable code, thus allowing the instructions to be mapped as > executable-only. It's not clear to me how such untrusted code ends up > scanning the memory, maybe relying on other pre-existent bugs (buffer > under/overflows). I assume if such code is allowed to do system calls, > all bets are off already. Yeah, the "block gadget scanning" tends to be the largest reason for this. That kind of scanning is usually the result of a wild buffer read of some kind. It's obviously most useful for "unknown" builds, but still has value even for Distro-style kernels since they're updated so regularly that automated attacks must keep an ever-growing mapping of kernels to target. -Kees -- Kees Cook Nexus Security -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>