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. 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. -- Catalin -- 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>