sch_htb: fix refcount leak in htb_parent_to_leaf_offload

The commit ae81feb7 ("sch_htb: fix null pointer dereference
on a null new_q") fixes a NULL pointer dereference bug, but it
is not correct.

Because htb_graft_helper properly handles the case when new_q
is NULL, and after the previous patch by skipping this call
which creates an inconsistency : dev_queue->qdisc will still
point to the old qdisc, but cl->parent->leaf.q will point to
the new one (which will be noop_qdisc, because new_q was NULL).
The code is based on an assumption that these two pointers are
the same, so it can lead to refcount leaks.

The correct fix is to add a NULL pointer check to protect
qdisc_refcount_inc inside htb_parent_to_leaf_offload.

Fixes: ae81feb7 ("sch_htb: fix null pointer dereference on a null new_q")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Suggested-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_htb.c

@@ -1488,6 +1488,7 @@ static void htb_parent_to_leaf_offload(struct Qdisc *sch,
 	struct Qdisc *old_q;
 
 	/* One ref for cl->leaf.q, the other for dev_queue->qdisc. */
+	if (new_q)
 		qdisc_refcount_inc(new_q);
 	old_q = htb_graft_helper(dev_queue, new_q);
 	WARN_ON(!(old_q->flags & TCQ_F_BUILTIN));
@@ -1675,12 +1676,11 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg,
 					  cl->parent->common.classid,
 					  NULL);
 		if (q->offload) {
-			if (new_q) {
+			if (new_q)
 				htb_set_lockdep_class_child(new_q);
 			htb_parent_to_leaf_offload(sch, dev_queue, new_q);
 		}
 	}
-	}
 
 	sch_tree_lock(sch);