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

From: Jason Wang <[email-masked]>
Date: Thu, 10 Jan 2013 23:27:21 +0800

On 01/10/2013 11:10 PM, Michael S. Tsirkin wrote:
> On Thu, Jan 10, 2013 at 10:47:49PM +0800, Jason Wang wrote:
>> 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.
> Look SETOWNER is a tool intended mostly for persistent taps,
> where you give a specific user the rights to attach to
> specific taps but not others.

True.
> The uevent issue is preventing a DOS by a uevent flood?
> Then it applies to persistent and non persistent as one.
> So if one cares about this one should use an LSM
> or we can add a separate capability to limit this if we
> care enough.

Ok.
>>>> 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.
> This only happens on startup. Threading changes can happen
> at any time.

Yes. So no objection from my side. Thanks for the explanation.
>>>>>> 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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