netdev - Re: [PATCH v2] net/sched: fix use-after-free in taprio_dev

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Message-ID: <20250613205206.fssf4bi4wjgyy53x@skbuf>
Date: Fri, 13 Jun 2025 23:52:06 +0300
From: Vladimir Oltean <vladimir.oltean@....com>
To: Hyunwoo Kim <imv4bel@...il.com>
Cc: Cong Wang <xiyou.wangcong@...il.com>, vinicius.gomes@...el.com,
	jhs@...atatu.com, jiri@...nulli.us, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	horms@...nel.org, netdev@...r.kernel.org, v4bel@...ori.io
Subject: Re: [PATCH v2] net/sched: fix use-after-free in taprio_dev_notifier

On Thu, Jun 12, 2025 at 11:36:12PM -0400, Hyunwoo Kim wrote:
> On Thu, Jun 12, 2025 at 01:23:38PM -0700, Cong Wang wrote:
> > On Thu, Jun 12, 2025 at 07:16:55AM -0400, Hyunwoo Kim wrote:
> > > diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> > > index 14021b812329..bd2b02d1dc63 100644
> > > --- a/net/sched/sch_taprio.c
> > > +++ b/net/sched/sch_taprio.c
> > > @@ -1320,6 +1320,7 @@ static int taprio_dev_notifier(struct notifier_block *nb, unsigned long event,
> > >     if (event != NETDEV_UP && event != NETDEV_CHANGE)
> > >             return NOTIFY_DONE;
> > >
> > > +   rcu_read_lock();
> > >     list_for_each_entry(q, &taprio_list, taprio_list) {
> > >             if (dev != qdisc_dev(q->root))
> > >                     continue;
> > > @@ -1328,16 +1329,17 @@ static int taprio_dev_notifier(struct notifier_block *nb, unsigned long event,
> >
> > There is a taprio_set_picos_per_byte() call here, it calls
> > __ethtool_get_link_ksettings() which could be blocking.
> >
> > For instance, gve_get_link_ksettings() calls
> > gve_adminq_report_link_speed() which is a blocking function.
> >
> > So I am afraid we can't enforce an atomic context here.
> 
> In that case, how about moving the lock as follows so that
> taprio_set_picos_per_byte() isnâ€™t included within it?
> 
> ```
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index 14021b812329..2b14c81a87e5 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1328,13 +1328,15 @@ static int taprio_dev_notifier(struct notifier_block *nb, unsigned long event,
> 
>                 stab = rtnl_dereference(q->root->stab);
> 
> -               oper = rtnl_dereference(q->oper_sched);
> +               rcu_read_lock();
> +               oper = rcu_dereference(q->oper_sched);
>                 if (oper)
>                         taprio_update_queue_max_sdu(q, oper, stab);
> 
> -               admin = rtnl_dereference(q->admin_sched);
> +               admin = rcu_dereference(q->admin_sched);
>                 if (admin)
>                         taprio_update_queue_max_sdu(q, admin, stab);
> +               rcu_read_unlock();
> 
>                 break;
>         }
> ```
> 
> This change still prevents the race condition with advance_sched().

This should work.

And I'm sorry for the bug introduced here, and elsewhere, by assuming
rtnl_dereference() will be fine.
I mostly use taprio with offload, where switch_schedules() runs in
process context with rtnl_lock() held, not the software emulation that
changes the schedules from the advance_sched() hrtimer. Somehow the
different locking requirements for the 2 cases eluded me.