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Date: Wed, 12 Dec 2012 11:10:22 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Paul Moore <pmoore@...hat.com> Cc: netdev@...r.kernel.org, linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov, jasowang@...hat.com Subject: Re: [RFC PATCH v2 3/3] tun: fix LSM/SELinux labeling of tun/tap devices On Mon, Dec 10, 2012 at 05:43:49PM -0500, Paul Moore wrote: > On Monday, December 10, 2012 07:50:35 PM Michael S. Tsirkin wrote: > > On Mon, Dec 10, 2012 at 12:33:49PM -0500, Paul Moore wrote: > > > On Monday, December 10, 2012 07:26:56 PM Michael S. Tsirkin wrote: > > > > On Mon, Dec 10, 2012 at 12:04:35PM -0500, Paul Moore wrote: > > > > > On Friday, December 07, 2012 02:25:16 PM Michael S. Tsirkin wrote: > > > > > > On Thu, Dec 06, 2012 at 04:09:51PM -0500, Paul Moore wrote: > > > > > > > On Thursday, December 06, 2012 10:57:16 PM Michael S. Tsirkin > wrote: > > > > > > > > On Thu, Dec 06, 2012 at 11:56:45AM -0500, Paul Moore wrote: > > > > > > > > > The SETQUEUE/tun_socket:create_queue permissions do not yet > > > > > > > > > exist > > > > > > > > > in any released SELinux policy as we are just now adding them > > > > > > > > > with > > > > > > > > > this patchset. With current policies loaded into a kernel with > > > > > > > > > this patchset applied the SETQUEUE/tun_socket:create_queue > > > > > > > > > permission would be treated according to the policy's unknown > > > > > > > > > permission setting. > > > > > > > > > > > > > > > > OK I think we need to rethink what we are doing here: what you > > > > > > > > sent > > > > > > > > addresses the problem as stated but I think we mis-stated it. > > > > > > > > Let > > > > > > > > me try to restate the problem: it is not just selinux problem. > > > > > > > > Let's > > > > > > > > assume qemu wants to use tun, I (libvirt) don't want to run it > > > > > > > > as > > > > > > > > root. > > > > > > > > > > > > > > > > 1. TUNSETIFF: I can open tun, attach an fd and pass it to qemu. > > > > > > > > Now, qemu does not invoke TUNSETIFF so it can run without > > > > > > > > kernel priveledges. > > > > > > > > > > > > > > Correct me if I'm wrong, but I believe libvirt does this while > > > > > > > running > > > > > > > as root. Assuming that is the case, why not simply > > > > > > > setuid()/setgid() > > > > > > > to the same credentials as the QEMU instance before creating the > > > > > > > TUN > > > > > > > device? You can always (re)configure the device afterwards while > > > > > > > running as root/CAP_NET_ADMIN. > > > > > > > > > > > > We want isolation between qemu instances. > > > > > > > > > > Understood, I agree. > > > > > > > > > > Achieving separation via SELinux is easily done, with libvirt/sVirt > > > > > already doing this for us automatically in most cases; the only thing > > > > > we > > > > > will want to do is make sure the SELinux policy is aware of the new > > > > > permission. > > > > > > > > > > Achieving separation via DAC should also be easily done, simply run > > > > > each > > > > > QEMU instance with a separate UID and/or GID. > > > > > > > > > > > Giving qemu right to open tun and SETIFF would give it rights > > > > > > to access any tun device. > > > > > > > > > > I'm quickly looked at tun_chr_open() again and I don't see any special > > > > > rights/privileges required, the same for tun_chr_ioctl() and > > > > > __tun_chr_ioctl(). Looking at tun_set_queue() I see we call > > > > > tun_not_capable() which does a simple DAC check; it must have the same > > > > > UID/GID or have CAP_NET_ADMIN. > > > > > > > > > > I'm having a hard time seeing the problem you are describing; help me > > > > > understand. > > > > > > > > The issue is guest controls the number of queues in use. > > > > So qemu would be required to be allowed to call tun_set_queue. > > > > If we allow this we have a problem as one qemu will be > > > > able to access any tun. > > > > > > QEMU can call tun_set_queue() as long as it satisfies tun_not_capable(), > > > which from a practical point of view means that the TUN device was > > > created with the same UID/GID as the QEMU instance. If you want TUN > > > device separation between QEMU instances using DAC you need to run each > > > QEMU instance with a different UID/GID (which you should be doing anyway > > > if you want DAC enforced general separation). > > > > > > I believe I've stated this point several times now and I don't feel you've > > > addressed it properly. > > > > Look at how it works at the moment: > > a priveledged libvirt server calls tun_set_iff > > and passes the fd to qemu which is not priveledged. > > > > The result is isolation between qemu instances without > > need to create uid per qemu instance. > > Okay, good. That is my understanding. > > > How do we create multiple queues? It makes sense to > > follow this model and pass in fds for individual queues. > > Okay. > > > However they need to be disabled initially > > so libvirt can not do tun_set_queue for us. > > Unrelated question: why do the queues need to be disabled initially? Is this > to prevent traffic from being queued up? Some other reason? I'm just curious > as to the reason ... Yes. Basically because old guests only use a single queue. If a guest comes along and declares multiqueue support we can queue up traffic on new queues but if we do this with a legacy guest it will not be able to consume it. > > can't utilize multiqueue. > > I still don't understand why in the multiqueue case libvirt doesn't just > change it's effective UID/GID when creating the TUN device, or just use the > TUNSETOWNER/TUNSETGROUP commands. This would solve the problem you describe > above and - at least to me - seems like a better solution conceptually. > > Help me understand why you believe that will not work. > > Do you not want to give ownership of the TUN device to QEMU? That would be > the only reason I can think of, but all of your comments that I can recall > have been about isolation between QEMU instances and not access control > between a QEMU instance and its assigned TUN device. I think I might have confused things more than clarified them. Let me comment on specific lines in patch that worry me that will make it clear I hope. > > My solution is an unpriveledged variant > > of tun_set_queue that only enables/disables > > a queue without attach/detach. > > -- > paul moore > security and virtualization @ redhat -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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