netdev - Re: [PATCH net] net/sched: ets: Always remove class from active list before deleting in ets_qdisc

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Message-ID: <aSna9hYKaG7xvYSn@dcaratti.users.ipa.redhat.com>
Date: Fri, 28 Nov 2025 18:25:10 +0100
From: Davide Caratti <dcaratti@...hat.com>
To: Jamal Hadi Salim <jhs@...atatu.com>
Cc: davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
	pabeni@...hat.com, xiyou.wangcong@...il.com, jiri@...nulli.us,
	netdev@...r.kernel.org, horms@...nel.org,
	zdi-disclosures@...ndmicro.com, w@....eu, security@...nel.org,
	tglx@...utronix.de, victor@...atatu.com
Subject: Re: [PATCH net] net/sched: ets: Always remove class from active list
 before deleting in ets_qdisc_change

On Fri, Nov 28, 2025 at 10:19:19AM -0500, Jamal Hadi Salim wrote:
> zdi-disclosures@...ndmicro.com says:
> 
> The vulnerability is a race condition between `ets_qdisc_dequeue` and
> `ets_qdisc_change`.  It leads to UAF on `struct Qdisc` object.
> Attacker requires the capability to create new user and network namespace
> in order to trigger the bug.
> See my additional commentary at the end of the analysis.

hello, thanks for your patch! 

[...]

> 
> Fixes: de6d25924c2a ("net/sched: sch_ets: don't peek at classes beyond 'nbands'")
> Reported-by: zdi-disclosures@...ndmicro.com
> Tested-by: Victor Nogueira <victor@...atatu.com>
> Signed-off-by: Jamal Hadi Salim <jhs@...atatu.com>
> ---
>  net/sched/sch_ets.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_ets.c b/net/sched/sch_ets.c
> index 82635dd2cfa5..ae46643e596d 100644
> --- a/net/sched/sch_ets.c
> +++ b/net/sched/sch_ets.c
> @@ -652,7 +652,7 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
>  	sch_tree_lock(sch);
>  
>  	for (i = nbands; i < oldbands; i++) {
> -		if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
> +		if (cl_is_active(&q->classes[i]))
>  			list_del_init(&q->classes[i].alist);
>  		qdisc_purge_queue(q->classes[i].qdisc);
>  	}

(nit)

the reported problem is NULL dereference of q->classes[i].qdisc, then
probably the 'Fixes' tag is an hash precedent to de6d25924c2a ("net/sched: sch_ets: don't
peek at classes beyond 'nbands'"). My understanding is: the test on 'q->classes[i].qdisc'
is no more NULL-safe after 103406b38c60 ("net/sched: Always pass notifications when
child class becomes empty"). So we might help our friends  planning backports with something like:

Fixes: de6d25924c2a ("net/sched: sch_ets: don't peek at classes beyond 'nbands'")
Fixes: c062f2a0b04d ("net/sched: sch_ets: don't remove idle classes from the round-robin list")

WDYT?

-- 
davide