CHROMIUM: msr: whitelist the i915 thermal control for wrmsr

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

Change-Id: Iff5b9a466dbddd5d94e9246ff99b63a21c975406
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/42910
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>