ubi: Revert "ubi: wl: Close down wear-leveling before nand is suspended"

Commit 5580cda ("ubi: wl: Close down wear-leveling before nand is
suspended") added a reboot notification in UBI layer to shutdown the
wear-leveling subsystem, which imported an UAF problem[1]. Besides that,
the method also brings other potential UAF problems, for example:
       reboot             kworker
 ubi_wl_reboot_notifier
  ubi_wl_close
   ubi_fastmap_close
    kfree(ubi->fm)
                     update_fastmap_work_fn
		      ubi_update_fastmap
		       old_fm = ubi->fm
		       if (old_fm && old_fm->e[i]) // UAF!

Actually, the problem fixed by commit 5580cda ("ubi: wl: Close down
wear-leveling before nand is suspended") has been solved by commit
8cba323 ("mtd: rawnand: protect access to rawnand devices while in
suspend"), which was discussed in [2]. So we can revert the commit
5580cda ("ubi: wl: Close down wear-leveling before nand is
suspended") directly.

[1] https://lore.kernel.org/linux-mtd/20241208175211.9406-2-dennis.lamerice@gmail.com/
[2] https://lore.kernel.org/all/9bf76f5d-12a4-46ff-90d4-4a7f0f47c381@axis.com/

Fixes: 5580cda ("ubi: wl: Close down wear-leveling before nand is suspended")
Reported-by: Dennis Lam <dennis.lamerice@gmail.com>
Closes: https://lore.kernel.org/linux-mtd/20241208175211.9406-2-dennis.lamerice@gmail.com/
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Acked-by: Mårten Lindahl <marten.lindahl@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/ubi.h

@@ -549,7 +549,6 @@ struct ubi_debug_info {
  * @peb_buf: a buffer of PEB size used for different purposes
  * @buf_mutex: protects @peb_buf
  * @ckvol_mutex: serializes static volume checking when opening
- * @wl_reboot_notifier: close all wear-leveling work before reboot
  *
  * @dbg: debugging information for this UBI device
  */
@@ -652,7 +651,6 @@ struct ubi_device {
 	void *peb_buf;
 	struct mutex buf_mutex;
 	struct mutex ckvol_mutex;
-	struct notifier_block wl_reboot_notifier;
 
 	struct ubi_debug_info dbg;
 };

drivers/mtd/ubi/wl.c

@@ -89,7 +89,6 @@
 #include <linux/crc32.h>
 #include <linux/freezer.h>
 #include <linux/kthread.h>
-#include <linux/reboot.h>
 #include "ubi.h"
 #include "wl.h"
 
@@ -128,8 +127,6 @@ static int self_check_in_wl_tree(const struct ubi_device *ubi,
 				 struct ubi_wl_entry *e, struct rb_root *root);
 static int self_check_in_pq(const struct ubi_device *ubi,
 			    struct ubi_wl_entry *e);
-static int ubi_wl_reboot_notifier(struct notifier_block *n,
-				  unsigned long state, void *cmd);
 
 /**
  * wl_tree_add - add a wear-leveling entry to a WL RB-tree.
@@ -1953,13 +1950,6 @@ int ubi_wl_init(struct ubi_device *ubi, struct ubi_attach_info *ai)
 	if (!ubi->ro_mode && !ubi->fm_disabled)
 		ubi_ensure_anchor_pebs(ubi);
 #endif
-
-	if (!ubi->wl_reboot_notifier.notifier_call) {
-		ubi->wl_reboot_notifier.notifier_call = ubi_wl_reboot_notifier;
-		ubi->wl_reboot_notifier.priority = 1; /* Higher than MTD */
-		register_reboot_notifier(&ubi->wl_reboot_notifier);
-	}
-
 	return 0;
 
 out_free:
@@ -2005,17 +1995,6 @@ void ubi_wl_close(struct ubi_device *ubi)
 	kfree(ubi->lookuptbl);
 }
 
-static int ubi_wl_reboot_notifier(struct notifier_block *n,
-				  unsigned long state, void *cmd)
-{
-	struct ubi_device *ubi;
-
-	ubi = container_of(n, struct ubi_device, wl_reboot_notifier);
-	ubi_wl_close(ubi);
-
-	return NOTIFY_DONE;
-}
-
 /**
  * self_check_ec - make sure that the erase counter of a PEB is correct.
  * @ubi: UBI device description object