s390: vfio-ap: tighten the NIB validity check

The NIB is architecturally invalid if the address designates a
storage location that is not installed or if it is zero.
Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Janosch Frank <frankja@linux.ibm.com>
Fixes: ec89b55e ("s390: ap: implement PAPQ AQIC interception in kernel")
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

drivers/s390/crypto/vfio_ap_ops.c

@@ -352,6 +352,8 @@ static int vfio_ap_validate_nib(struct kvm_vcpu *vcpu, dma_addr_t *nib)
 {
 	*nib = vcpu->run->s.regs.gprs[2];
 
+	if (!*nib)
+		return -EINVAL;
 	if (kvm_is_error_hva(gfn_to_hva(vcpu->kvm, *nib >> PAGE_SHIFT)))
 		return -EINVAL;