KVM: x86: Handle errors when RIP is set during far jumps

KVM: x86: Handle errors when RIP is set during far jumps

commit d1442d85cc30ea75f7d399474ca738e0bc96f715 upstream.

Far jmp/call/ret may fault while loading a new RIP.  Currently KVM does not
handle this case, and may result in failed vm-entry once the assignment is
done.  The tricky part of doing so is that loading the new CS affects the
VMCS/VMCB state, so if we fail during loading the new RIP, we are left in
unconsistent state.  Therefore, this patch saves on 64-bit the old CS
descriptor and restores it if loading RIP failed.

This fixes CVE-2014-3647.

Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - __load_segment_descriptor() does not take an in_task_switch parameter]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>