x86: Make the GDT remapping read-only on 64-bit

This patch makes the GDT remapped pages read-only, to prevent accidental
(or intentional) corruption of this key data structure.

This change is done only on 64-bit, because 32-bit needs it to be writable
for TSS switches.

The native_load_tr_desc function was adapted to correctly handle a
read-only GDT. The LTR instruction always writes to the GDT TSS entry.
This generates a page fault if the GDT is read-only. This change checks
if the current GDT is a remap and swap GDTs as needed. This function was
tested by booting multiple machines and checking hibernation works
properly.

KVM SVM and VMX were adapted to use the writeable GDT. On VMX, the
per-cpu variable was removed for functions to fetch the original GDT.
Instead of reloading the previous GDT, VMX will reload the fixmap GDT as
expected. For testing, VMs were started and restored on multiple
configurations.

Signed-off-by: Thomas Garnier <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Ard Biesheuvel <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Joerg Roedel <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Luis R . Rodriguez <[email protected]>
Cc: Matt Fleming <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Radim Krčmář <[email protected]>
Cc: Rafael J . Wysocki <[email protected]>
Cc: Rusty Russell <[email protected]>
Cc: Stanislaw Gruszka <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: Vitaly Kuznetsov <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: zijun_hu <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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