linux-kernel - Re: [net-next rfc V3 1/9] macvtap: fix a possible race between queue selection and changing queues

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Date:	Wed, 5 Jun 2013 13:35:28 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, sergei.shtylyov@...entembedded.com
Subject: Re: [net-next rfc V3 1/9] macvtap: fix a possible race between queue
 selection and changing queues

On Wed, Jun 05, 2013 at 02:36:24PM +0800, Jason Wang wrote:
> Complier may generate codes that re-read the vlan->numvtaps during
> macvtap_get_queue(). This may lead a race if vlan->numvtaps were changed in the
> same time and which can lead unexpected result (e.g. very huge value).
> 
> We need prevent the compiler from generating such codes by adding an
> ACCESS_ONCE() to make sure vlan->numvtaps were only read once.
> 
> Signed-off-by: Jason Wang <jasowang@...hat.com>

I would add that this is a theoretical issue, so no need
for stable.

Acked-by: Michael S. Tsirkin <mst@...hat.com>


> ---
>  drivers/net/macvtap.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/macvtap.c b/drivers/net/macvtap.c
> index 68efb91..5e485e3 100644
> --- a/drivers/net/macvtap.c
> +++ b/drivers/net/macvtap.c
> @@ -172,7 +172,7 @@ static struct macvtap_queue *macvtap_get_queue(struct net_device *dev,
>  {
>  	struct macvlan_dev *vlan = netdev_priv(dev);
>  	struct macvtap_queue *tap = NULL;
> -	int numvtaps = vlan->numvtaps;
> +	int numvtaps = ACCESS_ONCE(vlan->numvtaps);
>  	__u32 rxq;
>  
>  	if (!numvtaps)
> -- 
> 1.7.1
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