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

From: Jason Wang <[email-masked]>
Date: Thu, 10 Jan 2013 22:47:49 +0800

On 01/10/2013 10:41 PM, Michael S. Tsirkin wrote:
> 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?

netif_set_real_num_{tx|rx}_queues() will update the queue kobjects which
may trigger an 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.

Yes, but we have something similar like bridge-helper in qemu which
create devices through a privileged proxy.
>>>> 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.

True, I was wrong.
>
>>>>> 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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