mount: handle NULL values in mnt_ns_release()

commit 6c7ca6a upstream.

When calling in listmount() mnt_ns_release() may be passed a NULL
pointer. Handle that case gracefully.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: brauner

fs/namespace.c

@@ -153,7 +153,7 @@ static void mnt_ns_release(struct mnt_namespace *ns)
 	lockdep_assert_not_held(&mnt_ns_tree_lock);
 
 	/* keep alive for {list,stat}mount() */
-	if (refcount_dec_and_test(&ns->passive)) {
+	if (ns && refcount_dec_and_test(&ns->passive)) {
 		put_user_ns(ns->user_ns);
 		kfree(ns);
 	}