Add a static key which controls the invocation of the CPU buffer clear
mechanism on exit to user space and add the call into
prepare_exit_to_usermode() and do_nmi() right before actually returning.

Add documentation which kernel to user space transition this covers and
explain why some corner cases are not mitigated.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

CVE-2018-12126
CVE-2018-12127
CVE-2018-12130

(backported from commit b5ce1d36407a2324d2ab63c30ca1f71f24e9861e)
[juergh:
 - Adjusted context.]
 - Included linux/static_key.h for DEFINE_STATIC_KEY_FALSE.]
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>