tuntap: fix a possible race between queue selection and changing queues

Complier may generate codes that re-read the tun->numqueues during
tun_select_queue(). This may be a race if vlan->numqueues were changed in the
same time and 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 tun->numqueues were only read once.

Bug were introduced by commit c8d68e6be1c3b242f1c598595830890b65cea64a
(tuntap: multiqueue support).

Reported-by: Michael S. Tsirkin <mst@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
---
 drivers/net/tun.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

On Wed, Jun 05, 2013 at 04:44:57PM +0800, Jason Wang wrote:
> Complier may generate codes that re-read the tun->numqueues during
> tun_select_queue(). This may be a race if vlan->numqueues were changed in the
> same time and 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 tun->numqueues were only read once.
> 
> Bug were introduced by commit c8d68e6be1c3b242f1c598595830890b65cea64a
> (tuntap: multiqueue support).
> 
> Reported-by: Michael S. Tsirkin <mst@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

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

This is a theoretical problem, right?
So no need for stable.

> ---
>  drivers/net/tun.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index f042b03..adfcde7 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -352,7 +352,7 @@ static u16 tun_select_queue(struct net_device *dev, struct sk_buff *skb)
>  	u32 numqueues = 0;
>  
>  	rcu_read_lock();
> -	numqueues = tun->numqueues;
> +	numqueues = ACCESS_ONCE(tun->numqueues);
>  
>  	txq = skb_get_rxhash(skb);
>  	if (txq) {
> -- 
> 1.7.1
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From: "Michael S. Tsirkin" <mst@redhat.com>
Date: Wed, 5 Jun 2013 13:33:32 +0300

> On Wed, Jun 05, 2013 at 04:44:57PM +0800, Jason Wang wrote:
>> Complier may generate codes that re-read the tun->numqueues during
>> tun_select_queue(). This may be a race if vlan->numqueues were changed in the
>> same time and 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 tun->numqueues were only read once.
>> 
>> Bug were introduced by commit c8d68e6be1c3b242f1c598595830890b65cea64a
>> (tuntap: multiqueue support).
>> 
>> Reported-by: Michael S. Tsirkin <mst@redhat.com>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
> 
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> 
> This is a theoretical problem, right?
> So no need for stable.

Applied, thanks.
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