Subject: Re: [PATCH net v2] net/sched: taprio: fix NULL pointer dereference in class dump

From: Weiming Shi <[email-masked]>
Date: Tue, 14 Apr 2026 16:28:38 +0800

On 26-04-14 10:16, Paolo Abeni wrote:
> On 4/10/26 5:39 PM, Weiming Shi wrote:
> > 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:2475)
> >  Call Trace:
> >   <TASK>
> >   tc_fill_tclass (net/sched/sch_api.c:1966)
> >   qdisc_class_dump (net/sched/sch_api.c:2329)
> >   taprio_walk (net/sched/sch_taprio.c:2510)
> >   tc_dump_tclass_qdisc (net/sched/sch_api.c:2353)
> >   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:6827)
> >   netlink_dump (net/netlink/af_netlink.c:2325)
> >   rtnetlink_rcv_msg (net/core/rtnetlink.c:6927)
> >   netlink_rcv_skb (net/netlink/af_netlink.c:2550)
> >   </TASK>
> > 
> > Fix this by substituting &noop_qdisc when new is NULL in
> > taprio_graft(), following the same pattern used by multiq_graft() and
> > prio_graft(). This ensures q->qdiscs[] slots are never NULL, making
> > control-plane dump paths safe without requiring individual NULL checks.
> > 
> > Also update the data-plane NULL guards in taprio_enqueue() and
> > taprio_dequeue_from_txq() to check for &noop_qdisc, so that packets
> > are still dropped cleanly without inflating qlen/backlog counters.
> > 
> > Fixes: 665338b2a7a0 ("net/sched: taprio: dump class stats for the actual q->qdiscs[]")
> > Reported-by: Xiang Mei <[email-masked]>
> > Signed-off-by: Weiming Shi <[email-masked]>
> > ---
> > v2:
> >   - Update NULL checks in taprio_enqueue() and taprio_dequeue_from_txq()
> >     to test for &noop_qdisc instead of NULL, preventing qlen/backlog
> >     counter inflation when noop_qdisc drops packets (Sashiko)
> > ---
> >  net/sched/sch_taprio.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> > index f721c03514f60..XXXXXXXXX 100644
> > --- a/net/sched/sch_taprio.c
> > +++ b/net/sched/sch_taprio.c
> > @@ -634,7 +634,7 @@ static int taprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
> > 
> >  	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 prio;
> >  	int len;
> >  	u8 tc;
> > 
> > -	if (unlikely(!child))
> > +	if (unlikely(child == &noop_qdisc))
> >  		return NULL;
> > 
> >  	if (TXTIME_ASSIST_IS_ENABLED(q->flags))
> > @@ -2183,6 +2183,9 @@ 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);
> > 
> > @@ -2196,14 +2199,14 @@ 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)
> >  			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)
> > --
> > 2.43.0
> 
> Does not apply cleanly to net and looks seriously mangled. I suspect the
> above chunks should be part of the actual patch ?!?
> 
> Please, whatever tool you are using to help crafting the patch, double
> check the result manually before the actual submission.
> 
> /P
> 
Hi,

Sorry about the broken v2.  I'll double-check everything carefully and
resend as v3.

Thanks,
Weiming