On Tue, Aug 16, 2016 at 05:18:24PM +0100, Catalin Marinas wrote:

On Mon, Aug 15, 2016 at 10:45:09AM -0700, Kees Cook wrote:

quoted

On Mon, Aug 15, 2016 at 3:47 AM, Catalin Marinas
[off-list ref] wrote:

quoted

On Fri, Aug 12, 2016 at 11:23:03AM -0700, Kees Cook wrote:

quoted

On Thu, Aug 11, 2016 at 10:44 AM, Catalin Marinas
[off-list ref] wrote:

quoted

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?

Yes (mostly). With UAO, we changed the user access routines in the
kernel to use the LDTR/STTR instructions which always behave
unprivileged even when executed in kernel mode (unless the UAO bit is
set to override this restriction, needed for set_fs(KERNEL_DS)).

Even with UAO, we still have two cases where the kernel cannot perform
unprivileged accesses (LDTR/STTR) since they don't have an exclusives
equivalent (LDXR/STXR). These are in-user futex atomic ops and the SWP
emulation for 32-bit binaries (armv8_deprecated.c). But these require
write permission, so they would always fault even when running in the
kernel. futex_atomic_cmpxchg_inatomic() is able to return the old value
without a write (if it differs from "oldval") but it doesn't look like
such value could leak to user space.

If this was an issue, couldn't we add a dummy LDTR before the LDXR, and
have the fixup handler return -EFAULT?

Either way, this series looks technically fine to me:

Reviewed-by: Will Deacon <redacted>

but it would be good for some security-focussed person (Hi, Kees!) to
comment on whether or not this is useful, given the caveats you've
described. If it is, I can queue it for 4.9.

Will

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>