CHERRY-PICK: CHROMIUM: msr: whitelist the i915 thermal control for wrmsr

CHERRY-PICK: CHROMIUM: msr: whitelist the i915 thermal control for wrmsr

Deny all userspace MSR writes except those explicitly whitelisted for
i915 thermal controls. Without this, processes with CAP_SYS_RAWIO can
run arbitrary kernel code via MSR writing.

BUG=chromium-os:38756
TEST=link build, wrmsr works only on i915 thermal registers

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/42910
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Change-Id: Iaba154c76d48414633a4df8d07fe94b2a5e81a90
(cherry picked from ToT commit 3b16706f52c471365ed9a391c4803fd7cfcb0c0d)
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/43573
Reviewed-by: Sameer Nanda <snanda@chromium.org>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>