linux-kernel - Re: [PATCH 4.4 119/268] xen/pirq: fix error path cleanup when binding MSIs

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Date:   Wed, 13 Jun 2018 19:48:50 +0100
From:   Ben Hutchings <ben.hutchings@...ethink.co.uk>
To:     Roger Pau Monne <roger.pau@...rix.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Hooman Mirhadi <mirhadih@...zon.com>,
        Amit Shah <aams@...zon.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Sasha Levin <alexander.levin@...rosoft.com>
Subject: Re: [PATCH 4.4 119/268] xen/pirq: fix error path cleanup when
 binding MSIs

On Wed, 2018-02-28 at 09:19 +0000, Roger Pau Monne wrote:
> From: Roger Pau Monne <roger.pau@...rix.com>
> 
> [ Upstream commit 910f8befdf5bccf25287d9f1743e3e546bcb7ce0 ]
> 
> Current cleanup in the error path of xen_bind_pirq_msi_to_irq is
> wrong. First of all there's an off-by-one in the cleanup loop, which
> can lead to unbinding wrong IRQs.
> 
> Secondly IRQs not bound won't be freed, thus leaking IRQ numbers.
> 
> Note that there's no need to differentiate between bound and unbound
> IRQs when freeing them, __unbind_from_irq will deal with both of them
> correctly.

It appears to me that it is safe to call __unbind_from_irq() after
xen_irq_info_common_setup() fails, but *not* if the latter hasn't been
called at all.  In that case the IRQ type will still be set to
IRQT_UNBOUND and this will trigger the BUG_ON() in __unbind_from_irq().

[...]
> --- a/drivers/xen/events/events_base.c
> +++ b/drivers/xen/events/events_base.c
> @@ -764,8 +764,8 @@ out:
>  	mutex_unlock(&irq_mapping_update_lock);
>  	return irq;
>  error_irq:
> -	for (; i >= 0; i--)
> -		__unbind_from_irq(irq + i);
> +	while (nvec--)
> +		__unbind_from_irq(irq + nvec);

If nvec > 1, and xen_irq_info_pirq_setup() fails for i != nvec - 1,
then we reach here without having called xen_irq_info_common_setup()
for all these IRQs.

In that case, I think we will still want to call xen_free_irq() for all
IRQs.  So maybe the fix would be to remove the BUG_ON() in
__unbind_from_irq()?

Ben.

>  	mutex_unlock(&irq_mapping_update_lock);
>  	return ret;
>  }

-- 
Ben Hutchings, Software Developer                         Codethink Ltd
https://www.codethink.co.uk/                 Dale House, 35 Dale Street
                                     Manchester, M1 2HF, United Kingdom