commit a27507ca upstream.

Check the repeat_nmi .. end_repeat_nmi special case first.  The next
patch will rework the RSP check and, as a side effect, the RSP check
will no longer detect repeat_nmi .. end_repeat_nmi, so we'll need
this ordering of the checks.

Note: this is more subtle than it appears.  The check for repeat_nmi
.. end_repeat_nmi jumps straight out of the NMI code instead of
adjusting the "iret" frame to force a repeat.  This is necessary,
because the code between repeat_nmi and end_repeat_nmi sets "NMI
executing" and then writes to the "iret" frame itself.  If a nested
NMI comes in and modifies the "iret" frame while repeat_nmi is also
modifying it, we'll end up with garbage.  The old code got this
right, as does the new code, but the new code is a bit more
explicit.

If we were to move the check right after the "NMI executing" check,
then we'd get it wrong and have random crashes.

This is a prerequisite for the fix for CVE-2015-3291.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
[bwh: Backported to 4.0: adjust filename, spacing]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>