Subject: Re: [PATCH] net: sched: fix use-after-free in taprio_change()

From: Vinicius Costa Gomes <[email-masked]>
Date: Tue, 06 Aug 2024 20:56:41 -0700

Hi,

Dmitry Antipov <[email-masked]> writes:

> In 'taprio_change()', 'admin' pointer may become dangling due to sched
> switch / removal caused by 'advance_sched()', and critical section
> protected by 'q->current_entry_lock' is too small to prevent from such
> a scenario (which causes use-after-free detected by KASAN). Fix this
> by prefer 'rcu_replace_pointer()' over 'rcu_assign_pointer()' to update
> 'admin' immediately before an attempt to schedule freeing.
>
> Fixes: a3d43c0d56f1 ("taprio: Add support adding an admin schedule")
> Reported-by: [email-masked]
> Closes: https://syzkaller.appspot.com/bug?extid=b65e0af58423fc8a73aa
> Signed-off-by: Dmitry Antipov <[email-masked]>
> ---
>  net/sched/sch_taprio.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/sched/sch_taprio.c b/net/sched/sch_taprio.c
> index cc2df9f8c14a..59fad74d5ff9 100644
> --- a/net/sched/sch_taprio.c
> +++ b/net/sched/sch_taprio.c
> @@ -1963,7 +1963,8 @@ static int taprio_change(struct Qdisc *sch, struct nlattr *opt,
>  
>  		taprio_start_sched(sch, start, new_admin);
>  
> -		rcu_assign_pointer(q->admin_sched, new_admin);
> +		admin = rcu_replace_pointer(q->admin_sched, new_admin,
> +					    lockdep_rtnl_is_held());

What I am thinking if, for consistency, the same change could be applied
to the other branch of this comparison (the txtime one). Could be a
separate patch, as the txtime branch is not going to race with
advance_sched().

For this patch:

Acked-by: Vinicius Costa Gomes <[email-masked]>

Cheers,
-- 
Vinicius