ubi: Fix possible null-ptr-deref in ubi_free_volume()

It willl cause null-ptr-deref in the following case:

uif_init()
  ubi_add_volume()
    cdev_add() -> if it fails, call kill_volumes()
    device_register()

kill_volumes() -> if ubi_add_volume() fails call this function
  ubi_free_volume()
    cdev_del()
    device_unregister() -> trying to delete a not added device,
			   it causes null-ptr-deref

So in ubi_free_volume(), it delete devices whether they are added
or not, it will causes null-ptr-deref.

Handle the error case whlie calling ubi_add_volume() to fix this
problem. If add volume fails, set the corresponding vol to null,
so it can not be accessed in kill_volumes() and release the
resource in ubi_add_volume() error path.

Fixes: 801c135c ("UBI: Unsorted Block Images")
Suggested-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/build.c

@@ -470,6 +470,7 @@ static int uif_init(struct ubi_device *ubi)
 			err = ubi_add_volume(ubi, ubi->volumes[i]);
 			if (err) {
 				ubi_err(ubi, "cannot add volume %d", i);
+				ubi->volumes[i] = NULL;
 				goto out_volumes;
 			}
 		}

drivers/mtd/ubi/vmt.c

@@ -582,6 +582,7 @@ int ubi_add_volume(struct ubi_device *ubi, struct ubi_volume *vol)
 	if (err) {
 		ubi_err(ubi, "cannot add character device for volume %d, error %d",
 			vol_id, err);
+		vol_release(&vol->dev);
 		return err;
 	}
 
@@ -592,15 +593,14 @@ int ubi_add_volume(struct ubi_device *ubi, struct ubi_volume *vol)
 	vol->dev.groups = volume_dev_groups;
 	dev_set_name(&vol->dev, "%s_%d", ubi->ubi_name, vol->vol_id);
 	err = device_register(&vol->dev);
-	if (err)
-		goto out_cdev;
+	if (err) {
+		cdev_del(&vol->cdev);
+		put_device(&vol->dev);
+		return err;
+	}
 
 	self_check_volumes(ubi);
 	return err;
-
-out_cdev:
-	cdev_del(&vol->cdev);
-	return err;
 }
 
 /**