netdev - Re: [PATCH v1 net] net: sched: sch_qfq: Fix use-after-free in qfq_reset

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Message-ID: <aRJJaCVk181ErQWJ@pop-os.localdomain>
Date: Mon, 10 Nov 2025 12:22:00 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Paolo Valente <paolo.valente@...more.it>,
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
	syzbot+ec7176504e5bcc33ca4e@...kaller.appspotmail.com
Subject: Re: [PATCH v1 net] net: sched: sch_qfq: Fix use-after-free in
 qfq_reset_qdisc().

On Thu, Nov 06, 2025 at 07:10:49AM +0000, Kuniyuki Iwashima wrote:
>  static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  			    struct nlattr **tca, unsigned long *arg,
>  			    struct netlink_ext_ack *extack)
> @@ -511,6 +541,10 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  		new_agg = kzalloc(sizeof(*new_agg), GFP_KERNEL);
>  		if (new_agg == NULL) {
>  			err = -ENOBUFS;
> +
> +			if (existing)
> +				goto delete_class;
> +
>  			gen_kill_estimator(&cl->rate_est);
>  			goto destroy_class;
>  		}
> @@ -528,40 +562,14 @@ static int qfq_change_class(struct Qdisc *sch, u32 classid, u32 parentid,
>  	*arg = (unsigned long)cl;
>  	return 0;
>  
> -destroy_class:
> -	qdisc_put(cl->qdisc);
> -	kfree(cl);
> +delete_class:
> +	qfq_delete_class(sch, (unsigned long)cl, extack);
>  	return err;

Is it better to just call qfq_delete_class() directly? Two reasons:
1) It is only used by this code path
2) It reads odd to place a 'return' above 'destroy_class' label below.

And, what about the error patch of gen_new_estimator()? 'existing' could
be true for that case too, which I assume requires the same error
handling?

Thanks!