ubi: fastmap: Free unused fastmap anchor peb during detach

When CONFIG_MTD_UBI_FASTMAP is enabled, fm_anchor will be assigned
a free PEB during ubi_wl_init() or ubi_update_fastmap(). However
if fastmap is not used or disabled on the MTD device, ubi_wl_entry
related with the PEB will not be freed during detach.

So Fix it by freeing the unused fastmap anchor during detach.

Fixes: f9c34bb5 ("ubi: Fix producing anchor PEBs")
Reported-by: syzbot+f317896aae32eb281a58@syzkaller.appspotmail.com
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/fastmap-wl.c

@@ -39,6 +39,13 @@ static struct ubi_wl_entry *find_anchor_wl_entry(struct rb_root *root)
 	return victim;
 }
 
+static inline void return_unused_peb(struct ubi_device *ubi,
+				     struct ubi_wl_entry *e)
+{
+	wl_tree_add(e, &ubi->free);
+	ubi->free_count++;
+}
+
 /**
  * return_unused_pool_pebs - returns unused PEB to the free tree.
  * @ubi: UBI device description object
@@ -52,8 +59,7 @@ static void return_unused_pool_pebs(struct ubi_device *ubi,
 
 	for (i = pool->used; i < pool->size; i++) {
 		e = ubi->lookuptbl[pool->pebs[i]];
-		wl_tree_add(e, &ubi->free);
-		ubi->free_count++;
+		return_unused_peb(ubi, e);
 	}
 }
 
@@ -361,6 +367,11 @@ static void ubi_fastmap_close(struct ubi_device *ubi)
 	return_unused_pool_pebs(ubi, &ubi->fm_pool);
 	return_unused_pool_pebs(ubi, &ubi->fm_wl_pool);
 
+	if (ubi->fm_anchor) {
+		return_unused_peb(ubi, ubi->fm_anchor);
+		ubi->fm_anchor = NULL;
+	}
+
 	if (ubi->fm) {
 		for (i = 0; i < ubi->fm->used_blocks; i++)
 			kfree(ubi->fm->e[i]);