vhost_vdpa: Return -EFAULT if copy_from_user() fails

The copy_to/from_user() functions return the number of bytes which we
weren't able to copy but the ioctl should return -EFAULT if they fail.

Fixes: a127c5b ("vhost-vdpa: fix backend feature ioctls")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201023120853.GI282278@mwanda
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Jason Wang <jasowang@redhat.com>

drivers/vhost/vdpa.c

@@ -432,12 +432,11 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 	void __user *argp = (void __user *)arg;
 	u64 __user *featurep = argp;
 	u64 features;
-	long r;
+	long r = 0;
 
 	if (cmd == VHOST_SET_BACKEND_FEATURES) {
-		r = copy_from_user(&features, featurep, sizeof(features));
-		if (r)
-			return r;
+		if (copy_from_user(&features, featurep, sizeof(features)))
+			return -EFAULT;
 		if (features & ~VHOST_VDPA_BACKEND_FEATURES)
 			return -EOPNOTSUPP;
 		vhost_set_backend_features(&v->vdev, features);
@@ -480,7 +479,8 @@ static long vhost_vdpa_unlocked_ioctl(struct file *filep,
 		break;
 	case VHOST_GET_BACKEND_FEATURES:
 		features = VHOST_VDPA_BACKEND_FEATURES;
-		r = copy_to_user(featurep, &features, sizeof(features));
+		if (copy_to_user(featurep, &features, sizeof(features)))
+			r = -EFAULT;
 		break;
 	case VHOST_VDPA_GET_IOVA_RANGE:
 		r = vhost_vdpa_get_iova_range(v, argp);