Re: [PATCH v1 net] net: sched: sch_qfq: Fix use-after-free in qfq_reset_qdisc().

[Cong Wang <xiyou.wangcong@gmail.com>]

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!