Re: [PATCH] apparmor: fix cred UAF caused by begin_current_label_crit_section()

On Tue, Jul 14, 2026 at 05:38:07PM +0200, Jann Horn wrote:

> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
> index 0646804860ff..ce19fc14060c 100644
> --- a/include/linux/task_work.h
> +++ b/include/linux/task_work.h
> @@ -33,6 +33,7 @@ struct callback_head *task_work_cancel_match(struct task_struct *task,
>  	bool (*match)(struct callback_head *, void *data), void *data);
>  struct callback_head *task_work_cancel_func(struct task_struct *, task_work_func_t);
>  bool task_work_cancel(struct task_struct *task, struct callback_head *cb);
> +bool task_work_has_func(struct task_struct *task, task_work_func_t func);
>  void task_work_run(void);
>  
>  static inline void exit_task_work(struct task_struct *task)
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 0f7519f8e7c9..f83d1528e0bc 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -189,6 +189,20 @@ bool task_work_cancel(struct task_struct *task, struct callback_head *cb)
>  	return ret == cb;
>  }
>  
> +bool task_work_has_func(struct task_struct *task, task_work_func_t func)
> +{
> +	struct callback_head *work;
> +
> +	if (!task_work_pending(task))
> +		return false;
> +	guard(raw_spinlock_irqsave)(&task->pi_lock);
> +	for (work = READ_ONCE(task->task_works); work; work = READ_ONCE(work->next)) {
> +		if (work->func == func)
> +			return true;
> +	}
> +	return false;
> +}
> +
>  /**
>   * task_work_run - execute the works added by task_work_add()
>   *

This thing is quite terrible. And AFAICT the only purpose is to
determine if said task already has said function enqueued. Why not add a
single bit to struct task_struct for this? I'm sure we have a spare bit
somewhere.

> +/* replace the current task's stale label on syscall return */
> +void aa_schedule_stale_label_replacement(void)
> +{
> +	struct callback_head *tw;
> +
> +	if (task_work_has_func(current, aa_replace_stale_label_tw_func))
> +		return;
> +	tw = kmalloc_obj(struct callback_head);
> +	if (!tw)
> +		return;
> +	init_task_work(tw, aa_replace_stale_label_tw_func);
> +	if (task_work_add(current, tw, TWA_RESUME))
> +		kfree(tw);
> +}