UPSTREAM: arm64: Introduce a way to disable the 32bit vdso

commit 97884ca8c2925d14c32188e865069f21378b4b4f upstream.

[this is a redesign rather than a backport, further hacked to
 cope with Android half natve, half backport implementation]

We have a class of errata (grouped under the ARM64_WORKAROUND_1418040
banner) that force the trapping of counter access from 32bit EL0.

We would normally disable the whole vdso for such defect, except that
it would disable it for 64bit userspace as well, which is a shame.

Instead, add a new vdso_clock_mode, which signals that the vdso
isn't usable for compat tasks.  This gets checked in the new
vdso_clocksource_ok() helper, now provided for the 32bit vdso.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200706163802.1836732-2-maz@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Marc Zyngier <mzyngier@google.com>
Change-Id: I95686faacc27674b89478d3f3a545ff2e88b027e
(cherry picked from commit 97884ca8c2925d14c32188e865069f21378b4b4f)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>