Subject: Re: [PATCH] tun: avoid owner checks on IFF_ATTACH_QUEUE

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 10 Jan 2013 16:41:39 +0200

On Thu, Jan 10, 2013 at 10:27:20PM +0800, Jason Wang wrote:
> On 01/10/2013 10:19 PM, Michael S. Tsirkin wrote:
> > On Thu, Jan 10, 2013 at 10:08:03PM +0800, Jason Wang wrote:
> >> On 01/10/2013 07:31 PM, Michael S. Tsirkin wrote:
> >>> At the moment, we check owner when we enable queue in tun.
> >>> This seems redundant and will break some valid uses
> >>> where fd is passed around: I think TUNSETOWNER is there
> >>> to prevent others from attaching to a persistent device not
> >>> owned by them. Here the fd is already attached,
> >>> enabling/disabling queue is more like read/write.
> >> It also change the number of queues of the tuntap, maybe we should limit
> >> this.
> > Number of active queues? Why does it matter?
> > Max number of queues is already limited by SETIFF.
> 
> Yes the number of active(real) queues in the kernel net device and this
> changing may introduce other events such uevent.

How can it trigger a uevent?

> With this patch, even
> if a owner is set for tap, every user could change the number of real
> queues which I don't think is not expected. Without this patch, we can
> limit a user that just do read and write.

In the end if you want very fine tuned security policy you have to
use an LSM.

Here we are talking about the expected usage without an LSM.
There, enabling/disabling queues is just an optimization:
if an application wants to process data from a single thread
it's better off getting it through a single fd.
Having to channel threading changes through a priveledged
proxy would be very awkward.

> >
> >> Note that if management layer does not call TUNSETOWNER, the check
> >> is just a nop. So if management layer want to limit the behavior, it's
> >> its duty to do this correctly.
> > The point is that management limits tun to allow SETIFF from libvirt
> > only, then passes the fds to qemu.
> 
> Yes, but looks like libvirt does not call TUNSETOWNER before passing it
> to qemu, so we're ok even without this patch. And if libvirt want to do
> this, it can just call TUNSETOWNER to the user of qemu.

No, that would allow qemu to do SETIFF which we don't want.

> >>> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> >>>
> >>> ---
> >>>
> >>> Note: this is unrelated to Stefan's bugfix.
> >>>
> >>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>> index fbd106e..78e3225 100644
> >>> --- a/drivers/net/tun.c
> >>> +++ b/drivers/net/tun.c
> >>> @@ -1789,10 +1792,8 @@ static int tun_set_queue(struct file *file, struct ifreq *ifr)
> >>>  		tun = tfile->detached;
> >>>  		if (!tun)
> >>>  			ret = -EINVAL;
> >>> -		else if (tun_not_capable(tun))
> >>> -			ret = -EPERM;
> >>>  		else
> >>>  			ret = tun_attach(tun, file);
> >>>  	} else if (ifr->ifr_flags & IFF_DETACH_QUEUE) {
> >>>  		tun = rcu_dereference_protected(tfile->tun,
> >>>  						lockdep_rtnl_is_held());
> > --
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