net/sched: taprio: fix NULL pointer dereference in class dump

When a TAPRIO child qdisc is deleted via RTM_DELQDISC, taprio_graft()
is called with new == NULL and stores NULL into q->qdiscs[cl - 1].
Subsequent RTM_GETTCLASS dump operations walk all classes via
taprio_walk() and call taprio_dump_class(), which calls taprio_leaf()
returning the NULL pointer, then dereferences it to read child->handle,
causing a kernel NULL pointer dereference.

The bug is reachable with namespace-scoped CAP_NET_ADMIN on any kernel
with CONFIG_NET_SCH_TAPRIO enabled. On systems with unprivileged user
namespaces enabled, an unprivileged local user can trigger a kernel
panic by creating a taprio qdisc inside a new network namespace,
grafting an explicit child qdisc, deleting it, and requesting a class
dump. The RTM_GETTCLASS dump itself requires no capability.

 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000007: 0000 [#1] SMP KASAN NOPTI
 KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
 RIP: 0010:taprio_dump_class (net/sched/sch_taprio.c:2478)
 Call Trace:
  <TASK>
  tc_fill_tclass (net/sched/sch_api.c:1966)
  qdisc_class_dump (net/sched/sch_api.c:2326)
  taprio_walk (net/sched/sch_taprio.c:2514)
  tc_dump_tclass_qdisc (net/sched/sch_api.c:2352)
  tc_dump_tclass_root (net/sched/sch_api.c:2370)
  tc_dump_tclass (net/sched/sch_api.c:2431)
  rtnl_dumpit (net/core/rtnetlink.c:6864)
  netlink_dump (net/netlink/af_netlink.c:2325)
  rtnetlink_rcv_msg (net/core/rtnetlink.c:6959)
  netlink_rcv_skb (net/netlink/af_netlink.c:2550)
  </TASK>

Fix this by substituting &noop_qdisc when new is NULL in
taprio_graft(), a common pattern used by other qdiscs (e.g.,
multiq_graft()) to ensure the q->qdiscs[] slots are never NULL.
This makes control-plane dump paths safe without requiring individual
NULL checks.

Since the data-plane paths (taprio_enqueue and taprio_dequeue_from_txq)
previously had explicit NULL guards that would drop/skip the packet
cleanly, update those checks to test for &noop_qdisc instead. Without
this, packets would reach taprio_enqueue_one() which increments the root
qdisc's qlen and backlog before calling the child's enqueue; noop_qdisc
drops the packet but those counters are never rolled back, permanently
inflating the root qdisc's statistics.

After this change *old can be a valid qdisc, NULL, or &noop_qdisc.
Only call qdisc_put(*old) in the first case to avoid decreasing
noop_qdisc's refcount, which was never increased.

Fixes: 665338b2a7 ("net/sched: taprio: dump class stats for the actual q->qdiscs[]")
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Tested-by: Weiming Shi <bestswngs@gmail.com>
Link: https://patch.msgid.link/20260422161958.2517539-3-bestswngs@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/sch_taprio.c

@@ -634,7 +634,7 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 	queue = skb_get_queue_mapping(skb);
 
 	child = q->qdiscs[queue];
-	if (unlikely(!child))
+	if (unlikely(child == &noop_qdisc))
 		return qdisc_drop(skb, sch, to_free);
 
 	if (taprio_skb_exceeds_queue_max_sdu(sch, skb)) {
@@ -717,7 +717,7 @@ static struct sk_buff *taprio_dequeue_from_txq(struct Qdisc *sch, int txq,
 	int len;
 	u8 tc;
 
-	if (unlikely(!child))
+	if (unlikely(child == &noop_qdisc))
 		return NULL;
 
 	if (TXTIME_ASSIST_IS_ENABLED(q->flags))
@@ -2184,6 +2184,9 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
 	if (!dev_queue)
 		return -EINVAL;
 
+	if (!new)
+		new = &noop_qdisc;
+
 	if (dev->flags & IFF_UP)
 		dev_deactivate(dev, false);
 
@@ -2197,14 +2200,14 @@ static int taprio_graft(struct Qdisc *sch, unsigned long cl,
 	*old = q->qdiscs[cl - 1];
 	if (FULL_OFFLOAD_IS_ENABLED(q->flags)) {
 		WARN_ON_ONCE(dev_graft_qdisc(dev_queue, new) != *old);
-		if (new)
+		if (new != &noop_qdisc)
 			qdisc_refcount_inc(new);
-		if (*old)
+		if (*old && *old != &noop_qdisc)
 			qdisc_put(*old);
 	}
 
 	q->qdiscs[cl - 1] = new;
-	if (new)
+	if (new != &noop_qdisc)
 		new->flags |= TCQ_F_ONETXQUEUE | TCQ_F_NOPARENT;
 
 	if (dev->flags & IFF_UP)