vfio-ccw: protect the I/O region

Introduce a mutex to disallow concurrent reads or writes to the
I/O region. This makes sure that the data the kernel or user
space see is always consistent.

The same mutex will be used to protect the async region as well.
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

drivers/s390/cio/vfio_ccw_drv.c

@@ -84,7 +84,9 @@ static void vfio_ccw_sch_io_todo(struct work_struct *work)
 		if (is_final)
 			cp_free(&private->cp);
 	}
+	mutex_lock(&private->io_mutex);
 	memcpy(private->io_region->irb_area, irb, sizeof(*irb));
+	mutex_unlock(&private->io_mutex);
 
 	if (private->io_trigger)
 		eventfd_signal(private->io_trigger, 1);
@@ -129,6 +131,7 @@ static int vfio_ccw_sch_probe(struct subchannel *sch)
 
 	private->sch = sch;
 	dev_set_drvdata(&sch->dev, private);
+	mutex_init(&private->io_mutex);
 
 	spin_lock_irq(sch->lock);
 	private->state = VFIO_CCW_STATE_NOT_OPER;

drivers/s390/cio/vfio_ccw_ops.c

@@ -169,16 +169,20 @@ static ssize_t vfio_ccw_mdev_read(struct mdev_device *mdev,
 {
 	struct vfio_ccw_private *private;
 	struct ccw_io_region *region;
+	int ret;
 
 	if (*ppos + count > sizeof(*region))
 		return -EINVAL;
 
 	private = dev_get_drvdata(mdev_parent_dev(mdev));
+	mutex_lock(&private->io_mutex);
 	region = private->io_region;
 	if (copy_to_user(buf, (void *)region + *ppos, count))
-		return -EFAULT;
-
-	return count;
+		ret = -EFAULT;
+	else
+		ret = count;
+	mutex_unlock(&private->io_mutex);
+	return ret;
 }
 
 static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
@@ -188,23 +192,29 @@ static ssize_t vfio_ccw_mdev_write(struct mdev_device *mdev,
 {
 	struct vfio_ccw_private *private;
 	struct ccw_io_region *region;
+	int ret;
 
 	if (*ppos + count > sizeof(*region))
 		return -EINVAL;
 
 	private = dev_get_drvdata(mdev_parent_dev(mdev));
+	if (!mutex_trylock(&private->io_mutex))
+		return -EAGAIN;
 
 	region = private->io_region;
-	if (copy_from_user((void *)region + *ppos, buf, count))
-		return -EFAULT;
+	if (copy_from_user((void *)region + *ppos, buf, count)) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
 
 	vfio_ccw_fsm_event(private, VFIO_CCW_EVENT_IO_REQ);
-	if (region->ret_code != 0) {
+	if (region->ret_code != 0)
 		private->state = VFIO_CCW_STATE_IDLE;
-		return region->ret_code;
-	}
+	ret = (region->ret_code != 0) ? region->ret_code : count;
 
-	return count;
+out_unlock:
+	mutex_unlock(&private->io_mutex);
+	return ret;
 }
 
 static int vfio_ccw_mdev_get_device_info(struct vfio_device_info *info)

drivers/s390/cio/vfio_ccw_private.h

@@ -28,6 +28,7 @@
  * @mdev: pointer to the mediated device
  * @nb: notifier for vfio events
  * @io_region: MMIO region to input/output I/O arguments/results
+ * @io_mutex: protect against concurrent update of I/O regions
  * @cp: channel program for the current I/O operation
  * @irb: irb info received from interrupt
  * @scsw: scsw info
@@ -42,6 +43,7 @@ struct vfio_ccw_private {
 	struct mdev_device	*mdev;
 	struct notifier_block	nb;
 	struct ccw_io_region	*io_region;
+	struct mutex		io_mutex;
 
 	struct channel_program	cp;
 	struct irb		irb;