sch_htb: make htb_deactivate() idempotent

Alan reported a NULL pointer dereference in htb_next_rb_node()
after we made htb_qlen_notify() idempotent.

It turns out in the following case it introduced some regression:

htb_dequeue_tree():
  |-> fq_codel_dequeue()
    |-> qdisc_tree_reduce_backlog()
      |-> htb_qlen_notify()
        |-> htb_deactivate()
  |-> htb_next_rb_node()
  |-> htb_deactivate()

For htb_next_rb_node(), after calling the 1st htb_deactivate(), the
clprio[prio]->ptr could be already set to  NULL, which means
htb_next_rb_node() is vulnerable here.

For htb_deactivate(), although we checked qlen before calling it, in
case of qlen==0 after qdisc_tree_reduce_backlog(), we may call it again
which triggers the warning inside.

To fix the issues here, we need to:

1) Make htb_deactivate() idempotent, that is, simply return if we
   already call it before.
2) Make htb_next_rb_node() safe against ptr==NULL.

Many thanks to Alan for testing and for the reproducer.

Fixes: 5ba8b83 ("sch_htb: make htb_qlen_notify() idempotent")
Reported-by: Alan J. Wylie <alan@wylie.me.uk>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://patch.msgid.link/20250428232955.1740419-2-xiyou.wangcong@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_htb.c

@@ -348,7 +348,8 @@ static void htb_add_to_wait_tree(struct htb_sched *q,
  */
 static inline void htb_next_rb_node(struct rb_node **n)
 {
-	*n = rb_next(*n);
+	if (*n)
+		*n = rb_next(*n);
 }
 
 /**
@@ -609,8 +610,8 @@ static inline void htb_activate(struct htb_sched *q, struct htb_class *cl)
  */
 static inline void htb_deactivate(struct htb_sched *q, struct htb_class *cl)
 {
-	WARN_ON(!cl->prio_activity);
-
+	if (!cl->prio_activity)
+		return;
 	htb_deactivate_prios(q, cl);
 	cl->prio_activity = 0;
 }
@@ -1485,8 +1486,6 @@ static void htb_qlen_notify(struct Qdisc *sch, unsigned long arg)
 {
 	struct htb_class *cl = (struct htb_class *)arg;
 
-	if (!cl->prio_activity)
-		return;
 	htb_deactivate(qdisc_priv(sch), cl);
 }
 
@@ -1740,8 +1739,7 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg,
 	if (cl->parent)
 		cl->parent->children--;
 
-	if (cl->prio_activity)
-		htb_deactivate(q, cl);
+	htb_deactivate(q, cl);
 
 	if (cl->cmode != HTB_CAN_SEND)
 		htb_safe_rb_erase(&cl->pq_node,
@@ -1949,8 +1947,7 @@ static int htb_change_class(struct Qdisc *sch, u32 classid,
 			/* turn parent into inner node */
 			qdisc_purge_queue(parent->leaf.q);
 			parent_qdisc = parent->leaf.q;
-			if (parent->prio_activity)
-				htb_deactivate(q, parent);
+			htb_deactivate(q, parent);
 
 			/* remove from evt list because of level change */
 			if (parent->cmode != HTB_CAN_SEND) {