linux-kernel - Re: [PATCH 3.8-rc] tuntap: refuse to re-attach to different tun

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Date:	Thu, 10 Jan 2013 13:07:37 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Stefan Hajnoczi <stefanha@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3.8-rc] tuntap: refuse to re-attach to different
 tun_struct

On Thu, Jan 10, 2013 at 06:43:20PM +0800, Jason Wang wrote:
> On 01/10/2013 06:23 PM, Michael S. Tsirkin wrote:
> > On Thu, Jan 10, 2013 at 08:59:48AM +0100, Stefan Hajnoczi wrote:
> >> Multiqueue tun devices support detaching a tun_file from its tun_struct
> >> and re-attaching at a later point in time.  This allows users to disable
> >> a specific queue temporarily.
> >>
> >> ioctl(TUNSETIFF) allows the user to specify the network interface to
> >> attach by name.  This means the user can attempt to attach to interface
> >> "B" after detaching from interface "A".
> >>
> >> The driver is not designed to support this so check we are re-attaching
> >> to the right tun_struct.  Failure to do so may lead to oops.
> >>
> >> Signed-off-by: Stefan Hajnoczi <stefanha@...hat.com>
> >> ---
> >> This fix is for 3.8-rc.
> >>
> >>  drivers/net/tun.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >>
> >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >> index fbd106e..cf6da6e 100644
> >> --- a/drivers/net/tun.c
> >> +++ b/drivers/net/tun.c
> >> @@ -491,6 +491,8 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
> >>  	err = -EINVAL;
> >>  	if (rcu_dereference_protected(tfile->tun, lockdep_rtnl_is_held()))
> >>  		goto out;
> >> +	if (tfile->detached && tun != tfile->detached)
> >> +		goto out;
> >>  
> >>  	err = -EBUSY;
> >>  	if (!(tun->flags & TUN_TAP_MQ) && tun->numqueues == 1)
> >> -- 
> >> 1.8.0.2
> >
> > I agree this is a bug but even with this patch, we still allow:
> >
> > SETIFF
> > SETQUEUE (DISABLED)
> > SETIFF
> >
> > Originally the rule always was that repeated setiff calls fail with
> > EINVAL. We broke that when we set tun to NULL.  It's probably worth
> > preserving that, even if queue is disabled.  Applying something like the below
> > instead will address this concern, won't it?
> >
> > Note: works with regular userspace but I didn't test
> > multiqueue userspace. What do you think.
> 
> Or just fail when tun->detached is not NULL at the beginning of
> tun_set_iff()?

Yes that'll also work I think. If you prefer this pls send patch.

> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> >
> > ---
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index fbd106e..5ec8b08 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -483,7 +483,7 @@ static void tun_detach_all(struct net_device *dev)
> >  	BUG_ON(tun->numdisabled != 0);
> >  }
> >  
> > -static int tun_attach(struct tun_struct *tun, struct file *file)
> > +static int tun_attach(struct tun_struct *tun, struct file *file, bool setiff)
> >  {
> >  	struct tun_file *tfile = file->private_data;
> >  	int err;
> > @@ -492,6 +492,9 @@ static int tun_attach(struct tun_struct *tun, struct file *file)
> >  	if (rcu_dereference_protected(tfile->tun, lockdep_rtnl_is_held()))
> >  		goto out;
> >  
> > +	if (setiff && tfile->detached)
> > +		goto out;
> > +
> >  	err = -EBUSY;
> >  	if (!(tun->flags & TUN_TAP_MQ) && tun->numqueues == 1)
> >  		goto out;
> > @@ -1561,7 +1564,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
> >  		if (err < 0)
> >  			return err;
> >  
> > -		err = tun_attach(tun, file);
> > +		err = tun_attach(tun, file, true);
> >  		if (err < 0)
> >  			return err;
> >  
> > @@ -1627,7 +1630,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
> >  		dev->features = dev->hw_features;
> >  
> >  		INIT_LIST_HEAD(&tun->disabled);
> > -		err = tun_attach(tun, file);
> > +		err = tun_attach(tun, file, true);
> >  		if (err < 0)
> >  			goto err_free_dev;
> >  
> > @@ -1792,7 +1795,7 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
> >  		else if (tun_not_capable(tun))
> >  			ret = -EPERM;
> >  		else
> > -			ret = tun_attach(tun, file);
> > +			ret = tun_attach(tun, file, false);
> >  	} else if (ifr->ifr_flags & IFF_DETACH_QUEUE) {
> >  		tun = rcu_dereference_protected(tfile->tun,
> >  						lockdep_rtnl_is_held());
> > --
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