Re: [PATCH] genirq: fix reference leaks on irq affinity notifiers

[Ben Hutchings <ben@decadent.org.uk>]

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On Fri, 2020-03-13 at 20:33 +0000, Edward Cree wrote:
> The handling of notify->work did not properly maintain notify->kref in two
>  cases:
> 1) where the work was already scheduled, another irq_set_affinity_locked()
>    would get the ref and (no-op-ly) schedule the work.  Thus when
>    irq_affinity_notify() ran, it would drop the original ref but not the
>    additional one.
> 2) when cancelling the (old) work in irq_set_affinity_notifier(), if there
>    was outstanding work a ref had been got for it but was never put.

This makes sense, but...

> Fix both by checking the return values of the work handling functions
>  (schedule_work() for (1) and cancel_work_sync() for (2)) and put the
>  extra ref if the return value indicates preexisting work.
> 
> Fixes: cd7eab44e994 ("genirq: Add IRQ affinity notifiers")
> Fixes: 59c39840f5ab ("genirq: Prevent use-after-free and work list corruption")
> Signed-off-by: Edward Cree <ecree@solarflare.com>

...since the pending work item holds a reference to the notification
state, it's still not clear to me why or whether "genirq: Prevent use-
after-free and work list corruption" was needed.

If it's reasonable to cancel_work_sync() when removing a notifier, I
think we can remove the kref and call the release function directly.

Ben.

> ---
>  kernel/irq/manage.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 7eee98c38f25..b3aa1db895e6 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -323,7 +323,10 @@ int irq_set_affinity_locked(struct irq_data *data, const struct cpumask *mask,
>  
>  	if (desc->affinity_notify) {
>  		kref_get(&desc->affinity_notify->kref);
> -		schedule_work(&desc->affinity_notify->work);
> +		if (!schedule_work(&desc->affinity_notify->work))
> +			/* Work was already scheduled, drop our extra ref */
> +			kref_put(&desc->affinity_notify->kref,
> +				 desc->affinity_notify->release);
>  	}
>  	irqd_set(data, IRQD_AFFINITY_SET);
>  
> @@ -423,7 +426,9 @@ irq_set_affinity_notifier(unsigned int irq, struct irq_affinity_notify *notify)
>  	raw_spin_unlock_irqrestore(&desc->lock, flags);
>  
>  	if (old_notify) {
> -		cancel_work_sync(&old_notify->work);
> +		if (cancel_work_sync(&old_notify->work))
> +			/* Pending work had a ref, put that one too */
> +			kref_put(&old_notify->kref, old_notify->release);
>  		kref_put(&old_notify->kref, old_notify->release);
>  	}
>  
-- 
Ben Hutchings
Humour is the best antidote to reality.


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