KVM: s390: Do not leak kernel stack data in the KVM_S390_INTERRUPT ioctl

When the userspace program runs the KVM_S390_INTERRUPT ioctl to inject
an interrupt, we convert them from the legacy struct kvm_s390_interrupt
to the new struct kvm_s390_irq via the s390int_to_s390irq() function.
However, this function does not take care of all types of interrupts
that we can inject into the guest later (see do_inject_vcpu()). Since we
do not clear out the s390irq values before calling s390int_to_s390irq(),
there is a chance that we copy random data from the kernel stack which
could be leaked to the userspace later.

Specifically, the problem exists with the KVM_S390_INT_PFAULT_INIT
interrupt: s390int_to_s390irq() does not handle it, and the function
__inject_pfault_init() later copies irq->u.ext which contains the
random kernel stack data. This data can then be leaked either to
the guest memory in __deliver_pfault_init(), or the userspace might
retrieve it directly with the KVM_S390_GET_IRQ_STATE ioctl.

Fix it by handling that interrupt type in s390int_to_s390irq(), too,
and by making sure that the s390irq struct is properly pre-initialized.
And while we're at it, make sure that s390int_to_s390irq() now
directly returns -EINVAL for unknown interrupt types, so that we
immediately get a proper error code in case we add more interrupt
types to do_inject_vcpu() without updating s390int_to_s390irq()
sometime in the future.

Cc: stable@vger.kernel.org
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Link: https://lore.kernel.org/kvm/20190912115438.25761-1-thuth@redhat.comSigned-off-by: Christian Borntraeger <borntraeger@de.ibm.com>

arch/s390/kvm/interrupt.c

@@ -1961,6 +1961,16 @@ int s390int_to_s390irq(struct kvm_s390_interrupt *s390int,
 	case KVM_S390_MCHK:
 		irq->u.mchk.mcic = s390int->parm64;
 		break;
+	case KVM_S390_INT_PFAULT_INIT:
+		irq->u.ext.ext_params = s390int->parm;
+		irq->u.ext.ext_params2 = s390int->parm64;
+		break;
+	case KVM_S390_RESTART:
+	case KVM_S390_INT_CLOCK_COMP:
+	case KVM_S390_INT_CPU_TIMER:
+		break;
+	default:
+		return -EINVAL;
 	}
 	return 0;
 }

arch/s390/kvm/kvm-s390.c

@@ -4335,7 +4335,7 @@ long kvm_arch_vcpu_async_ioctl(struct file *filp,
 	}
 	case KVM_S390_INTERRUPT: {
 		struct kvm_s390_interrupt s390int;
-		struct kvm_s390_irq s390irq;
+		struct kvm_s390_irq s390irq = {};
 
 		if (copy_from_user(&s390int, argp, sizeof(s390int)))
 			return -EFAULT;