ubi: Fix UAF wear-leveling entry in eraseblk_count_seq_show()

Wear-leveling entry could be freed in error path, which may be accessed
again in eraseblk_count_seq_show(), for example:

__erase_worker                eraseblk_count_seq_show
                                wl = ubi->lookuptbl[*block_number]
				if (wl)
  wl_entry_destroy
    ubi->lookuptbl[e->pnum] = NULL
    kmem_cache_free(ubi_wl_entry_slab, e)
		                   erase_count = wl->ec  // UAF!

Wear-leveling entry updating/accessing in ubi->lookuptbl should be
protected by ubi->wl_lock, fix it by adding ubi->wl_lock to serialize
wl entry accessing between wl_entry_destroy() and
eraseblk_count_seq_show().

Fetch a reproducer in [Link].

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216305
Fixes: 7bccd12d ("ubi: Add debugfs file for tracking PEB state")
Fixes: 801c135c ("UBI: Unsorted Block Images")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/wl.c

@@ -890,8 +890,11 @@ static int wear_leveling_worker(struct ubi_device *ubi, struct ubi_work *wrk,
 
 	err = do_sync_erase(ubi, e1, vol_id, lnum, 0);
 	if (err) {
-		if (e2)
+		if (e2) {
+			spin_lock(&ubi->wl_lock);
 			wl_entry_destroy(ubi, e2);
+			spin_unlock(&ubi->wl_lock);
+		}
 		goto out_ro;
 	}
 
@@ -1130,14 +1133,18 @@ static int __erase_worker(struct ubi_device *ubi, struct ubi_work *wl_wrk)
 		/* Re-schedule the LEB for erasure */
 		err1 = schedule_erase(ubi, e, vol_id, lnum, 0, false);
 		if (err1) {
+			spin_lock(&ubi->wl_lock);
 			wl_entry_destroy(ubi, e);
+			spin_unlock(&ubi->wl_lock);
 			err = err1;
 			goto out_ro;
 		}
 		return err;
 	}
 
+	spin_lock(&ubi->wl_lock);
 	wl_entry_destroy(ubi, e);
+	spin_unlock(&ubi->wl_lock);
 	if (err != -EIO)
 		/*
 		 * If this is not %-EIO, we have no idea what to do. Scheduling