mtd: Fix gluebi NULL pointer dereference caused by ftl notifier

If both ftl.ko and gluebi.ko are loaded, the notifier of ftl
triggers NULL pointer dereference when trying to access
‘gluebi->desc’ in gluebi_read().

ubi_gluebi_init
  ubi_register_volume_notifier
    ubi_enumerate_volumes
      ubi_notify_all
        gluebi_notify    nb->notifier_call()
          gluebi_create
            mtd_device_register
              mtd_device_parse_register
                add_mtd_device
                  blktrans_notify_add   not->add()
                    ftl_add_mtd         tr->add_mtd()
                      scan_header
                        mtd_read
                          mtd_read_oob
                            mtd_read_oob_std
                              gluebi_read   mtd->read()
                                gluebi->desc - NULL

Detailed reproduction information available at the Link [1],

In the normal case, obtain gluebi->desc in the gluebi_get_device(),
and access gluebi->desc in the gluebi_read(). However,
gluebi_get_device() is not executed in advance in the
ftl_add_mtd() process, which leads to NULL pointer dereference.

The solution for the gluebi module is to run jffs2 on the UBI
volume without considering working with ftl or mtdblock [2].
Therefore, this problem can be avoided by preventing gluebi from
creating the mtdblock device after creating mtd partition of the
type MTD_UBIVOLUME.

Fixes: 2ba3d76 ("UBI: make gluebi a separate module")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217992 [1]
Link: https://lore.kernel.org/lkml/441107100.23734.1697904580252.JavaMail.zimbra@nod.at/ [2]
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20231220024619.2138625-1-wangzhaolong1@huawei.com

drivers/mtd/mtd_blkdevs.c

@@ -463,7 +463,7 @@ static void blktrans_notify_add(struct mtd_info *mtd)
 {
 	struct mtd_blktrans_ops *tr;
 
-	if (mtd->type == MTD_ABSENT)
+	if (mtd->type == MTD_ABSENT || mtd->type == MTD_UBIVOLUME)
 		return;
 
 	list_for_each_entry(tr, &blktrans_majors, list)
@@ -503,7 +503,7 @@ int register_mtd_blktrans(struct mtd_blktrans_ops *tr)
 	mutex_lock(&mtd_table_mutex);
 	list_add(&tr->list, &blktrans_majors);
 	mtd_for_each_device(mtd)
-		if (mtd->type != MTD_ABSENT)
+		if (mtd->type != MTD_ABSENT && mtd->type != MTD_UBIVOLUME)
 			tr->add_mtd(tr, mtd);
 	mutex_unlock(&mtd_table_mutex);
 	return 0;