KVM: s390: Sanity check on validity intercept

This patch adds a sanity check for the content of the guest
prefix register content before faulting in the cpu lowcore
that it refers to. The guest might end up in an endless loop
where SIE complains about missing lowcore with incorrect
content of the prefix register without this fix.
Reported-by: Mijo Safradin <mijo@linux.vnet.ibm.com>
Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Signed-off-by: Christian Ehrhardt <ehrhardt@de.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/s390/kvm/intercept.c

@@ -154,17 +154,25 @@ static int handle_stop(struct kvm_vcpu *vcpu)
 static int handle_validity(struct kvm_vcpu *vcpu)
 {
 	int viwhy = vcpu->arch.sie_block->ipb >> 16;
+	int rc;
+
 	vcpu->stat.exit_validity++;
-	if (viwhy == 0x37) {
-		fault_in_pages_writeable((char __user *)
-					 vcpu->kvm->arch.guest_origin +
-					 vcpu->arch.sie_block->prefix,
-					 PAGE_SIZE);
-		return 0;
-	}
-	VCPU_EVENT(vcpu, 2, "unhandled validity intercept code %d",
-		   viwhy);
-	return -ENOTSUPP;
+	if ((viwhy == 0x37) && (vcpu->arch.sie_block->prefix
+		<= vcpu->kvm->arch.guest_memsize - 2*PAGE_SIZE)){
+		rc = fault_in_pages_writeable((char __user *)
+			 vcpu->kvm->arch.guest_origin +
+			 vcpu->arch.sie_block->prefix,
+			 2*PAGE_SIZE);
+		if (rc)
+			/* user will receive sigsegv, exit to user */
+			rc = -ENOTSUPP;
+	} else
+		rc = -ENOTSUPP;
+
+	if (rc)
+		VCPU_EVENT(vcpu, 2, "unhandled validity intercept code %d",
+			   viwhy);
+	return rc;
 }
 
 static int handle_instruction(struct kvm_vcpu *vcpu)