kernfs: fix potential null pointer dereference

mainline inclusion
from mainline-5.4-rc1
commit 2fd60da46da76948b8cc561ae8cc902209686253
category: bugfix
bugzilla: 21637
CVE: NA

---------------------------

Get root safely after kn is ensureed to be not null.
Signed-off-by: NPeng Wang <rocking@whu.edu.cn>
Acked-by: NTejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20190708151611.13242-1-rocking@whu.edu.cnSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Nyu kuai <yukuai3@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/kernfs/dir.c

@@ -431,7 +431,6 @@ struct kernfs_node *kernfs_get_active(struct kernfs_node *kn)
  */
 void kernfs_put_active(struct kernfs_node *kn)
 {
-	struct kernfs_root *root = kernfs_root(kn);
 	int v;
 
 	if (unlikely(!kn))
@@ -443,7 +442,7 @@ void kernfs_put_active(struct kernfs_node *kn)
 	if (likely(v != KN_DEACTIVATED_BIAS))
 		return;
 
-	wake_up_all(&root->deactivate_waitq);
+	wake_up_all(&kernfs_root(kn)->deactivate_waitq);
 }
 
 /**