Re: [PATCH] genirq: Synchronize only with single thread on free_irq()

[Lukas Wunner <lukas@wunner.de>]

On Thu, Jun 21, 2018 at 10:21:44PM +0200, Thomas Gleixner wrote:
> On Thu, 24 May 2018, Lukas Wunner wrote:
> >  static int irq_wait_for_interrupt(struct irqaction *action)
> >  {
> > -	set_current_state(TASK_INTERRUPTIBLE);
> > +	for (;;) {
> > +		set_current_state(TASK_INTERRUPTIBLE);
> >  
> > -	while (!kthread_should_stop()) {
> > +		if (kthread_should_stop()) {
> > +			/* may need to run one last time. */
> > +			if (test_and_clear_bit(IRQTF_RUNTHREAD,
> > +					       &action->thread_flags)) {
> > +				__set_current_state(TASK_RUNNING);
> > +				return 0;
> > +			}
> > +			__set_current_state(TASK_RUNNING);
> > +			return -1;
> > +		}
> >  
> >  		if (test_and_clear_bit(IRQTF_RUNTHREAD,
> >  				       &action->thread_flags)) {
> > @@ -766,10 +776,7 @@ static int irq_wait_for_interrupt(struct irqaction *action)
> >  			return 0;
> >  		}
> >  		schedule();
> > -		set_current_state(TASK_INTERRUPTIBLE);
> >  	}
> > -	__set_current_state(TASK_RUNNING);
> > -	return -1;
> >  }
> >  
> >  /*
> > @@ -990,7 +997,7 @@ static int irq_thread(void *data)
> >  	/*
> >  	 * This is the regular exit path. __free_irq() is stopping the
> >  	 * thread via kthread_stop() after calling
> > -	 * synchronize_irq(). So neither IRQTF_RUNTHREAD nor the
> > +	 * synchronize_hardirq(). So neither IRQTF_RUNTHREAD nor the
> >  	 * oneshot mask bit can be set. We cannot verify that as we
> >  	 * cannot touch the oneshot mask at this point anymore as
> >  	 * __setup_irq() might have given out currents thread_mask
> > @@ -1595,7 +1602,7 @@ static struct irqaction *__free_irq(struct irq_desc *desc, void *dev_id)
> >  	unregister_handler_proc(irq, action);
> >  
> >  	/* Make sure it's not being used on another CPU: */
> > -	synchronize_irq(irq);
> > +	synchronize_hardirq(irq);
> 
> So after that, action can be freed and when the thread above tries to
> access it. Nice Use After Free. That needs more thought.

No, after that, kthread_stop() is called which blocks until the IRQ
thread has completed.  Only then is the action freed.

Thanks,

Lukas