[PATCH] net-sysfs: fix NULL pointer dereference

[Qasim Ijaz <qasdev00@gmail.com>]

Commit <79c61899b5ee> introduces a potential NULL pointer dereference 
in the sysfs_rtnl_lock() function when initialising kn:

	kn = sysfs_break_active_protection(kobj, attr);
	
The commit overlooks the fact that sysfs_break_active_protection can 
return NULL if kernfs_find_and_get() fails to find and get the kernfs_node 
with the given name. 

Later on the code calls sysfs_unbreak_active_protection(kn) 
unconditionally, which could lead to a NULL pointer dereference.

Resolve this bug by introducing a NULL check before using kn
in the sysfs_unbreak_active_protection() call.

Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
Fixes: 79c61899b5ee ("net-sysfs: remove rtnl_trylock from device attributes")
---
 net/core/net-sysfs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 8d9dc048a548..c5085588e536 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -117,7 +117,8 @@ static int sysfs_rtnl_lock(struct kobject *kobj, struct attribute *attr,
 	 * the rtnl lock.
 	 */
 unbreak:
-	sysfs_unbreak_active_protection(kn);
+	if (kn)
+		sysfs_unbreak_active_protection(kn);
 	dev_put(ndev);
 
 	return ret;
-- 
2.39.5