Messages in this thread | | | Date | Thu, 10 Jan 2013 23:27:21 +0800 | From | Jason Wang <> | Subject | Re: [PATCH] tun: avoid owner checks on IFF_ATTACH_QUEUE |
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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 <mst@redhat.com> >>>>>>> >>>>>>> --- >>>>>>> >>>>>>> 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
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