From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] tun: avoid owner checks on IFF_ATTACH_QUEUE Date: Thu, 10 Jan 2013 17:10:37 +0200 Message-ID: <20130110151037.GD30731@redhat.com> References: <20130110113108.GF13451@redhat.com> <50EECB43.8020405@redhat.com> <20130110141939.GA30557@redhat.com> <50EECFC8.9020402@redhat.com> <20130110144138.GA30731@redhat.com> <50EED495.4040405@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Stefan Hajnoczi , "David S. Miller" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Paul Moore To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <50EED495.4040405@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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. 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. > > > >> 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. > >>>> 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 > >>>>> > >>>>> --- > >>>>> > >>>>> 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()); > >>> -- > >>> To unsubscribe from this list: send the line "unsubscribe netdev" in > >>> the body of a message to majordomo@vger.kernel.org > >>> More majordomo info at http://vger.kernel.org/majordomo-info.html