x86/cpu: Call verify_cpu() after having entered long mode too

x86/cpu: Call verify_cpu() after having entered long mode too

commit 04633df0c43d710e5f696b06539c100898678235 upstream.

When we get loaded by a 64-bit bootloader, kernel entry point is
startup_64 in head_64.S. We don't trust any and all bootloaders because
some will fiddle with CPU configuration so we go ahead and massage each
CPU into sanity again.

For example, some dell BIOSes have this XD disable feature which set
IA32_MISC_ENABLE[34] and disable NX. This might be some dumb workaround
for other OSes but Linux sure doesn't need it.

A similar thing is present in the Surface 3 firmware - see
https://bugzilla.kernel.org/show_bug.cgi?id=106051 - which sets this bit
only on the BSP:

  # rdmsr -a 0x1a0
  400850089
  850089
  850089
  850089

I know, right?!

There's not even an off switch in there.

So fix all those cases by sanitizing the 64-bit entry point too. For
that, make verify_cpu() callable in 64-bit mode also.

Requested-and-debugged-by: "H. Peter Anvin" <hpa@zytor.com>
Reported-and-tested-by: Bastien Nocera <bugzilla@hadess.net>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1446739076-21303-1-git-send-email-bp@alien8.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>