KVM: nSVM: always intercept VMLOAD/VMSAVE when nested (CVE-2021-3656)

If L1 disables VMLOAD/VMSAVE intercepts, and doesn't enable
Virtual VMLOAD/VMSAVE (currently not supported for the nested hypervisor),
then VMLOAD/VMSAVE must operate on the L1 physical memory, which is only
possible by making L0 intercept these instructions.

Failure to do so allowed the nested guest to run VMLOAD/VMSAVE unintercepted,
and thus read/write portions of the host physical memory.

Fixes: 89c8a4984f ("KVM: SVM: Enable Virtual VMLOAD VMSAVE feature")

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm/nested.c

@@ -158,6 +158,9 @@ void recalc_intercepts(struct vcpu_svm *svm)
 	/* If SMI is not intercepted, ignore guest SMI intercept as well  */
 	if (!intercept_smi)
 		vmcb_clr_intercept(c, INTERCEPT_SMI);
+
+	vmcb_set_intercept(c, INTERCEPT_VMLOAD);
+	vmcb_set_intercept(c, INTERCEPT_VMSAVE);
 }
 
 static void copy_vmcb_control_area(struct vmcb_control_area *dst,