linux-kernel - Re: [PATCH 1/2] genirq: introduce update_irq

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Date:   Thu, 8 Aug 2019 21:56:14 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ben Luo <luoben@...ux.alibaba.com>
cc:     alex.williamson@...hat.com, linux-kernel@...r.kernel.org,
        tao.ma@...ux.alibaba.com, gerry@...ux.alibaba.com
Subject: Re: [PATCH 1/2] genirq: introduce update_irq_devid()

On Thu, 8 Aug 2019, Ben Luo wrote:
> +int update_irq_devid(unsigned int irq, void *dev_id, void *new_dev_id)
> +{
> +	struct irq_desc *desc = irq_to_desc(irq);
> +	struct irqaction *action, **action_ptr;
> +	unsigned long flags;
> +
> +	WARN(in_interrupt(),
> +			"Trying to update IRQ %d from IRQ context!\n", irq);

This is broken. The function needs to return on that condition. Actually it
cannot even be called from non-preemptible code.

What's worse is that if the interrupt in question is handled concurrently,
then it will either see the old or the new dev_id and because the interrupt
handler loop runs with desc->lock dropped even more crap can happen because
dev_id can be subject to load and store tearing.

Staring at that, I see that there is the same issue in setup_irq() and
free_irq(). It's actually worse there. I'll have a look.

> +	/*
> +	 * There can be multiple actions per IRQ descriptor, find the right
> +	 * one based on the dev_id:
> +	 */
> +	action_ptr = &desc->action;
> +	for (;;) {
> +		action = *action_ptr;
> +
> +		if (!action) {
> +			WARN(1, "Trying to update already-free IRQ %d\n", irq);

That's wrong in two aspects:

       1) The warn should be outside of the locked region.

       2) Just having the irq number is not useful for debugging either
       	  when the interrupt is shared.

> +			raw_spin_unlock_irqrestore(&desc->lock, flags);
> +			chip_bus_sync_unlock(desc);
> +			return -ENXIO;
> +		}
> +
> +		if (action->dev_id == dev_id) {
> +			action->dev_id = new_dev_id;
> +			break;
> +		}
> +		action_ptr = &action->next;
> +	}
> +
> +	raw_spin_unlock_irqrestore(&desc->lock, flags);
> +	chip_bus_sync_unlock(desc);
> +
> +	/*
> +	 * Make sure it's not being used on another CPU:
> +	 * There is a risk of UAF for old *dev_id, if it is
> +	 * freed in a short time after this func returns
> +	 */
> +	synchronize_irq(irq);
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(update_irq_devid);

EXPORT_SYMBOL_GPL() please.

Thanks,

	tglx