SELinux labels change in libvirt

[Daniel P. Berrangé]

On Tue, Jul 14, 2020 at 04:02:17PM +0300, Ram Lavi wrote:
> On Tue, Jul 14, 2020 at 3:33 PM Daniel P. Berrangé <berrange at redhat.com>
> wrote:
> 
> > On Tue, Jul 14, 2020 at 03:21:17PM +0300, Ram Lavi wrote:
> > > Hello all,
> > >
> > > tl;dr, can you point me to the point in the libvirt repo where it's
> > trying
> > > to change a tap-device's SELinux label?
> > >
> > > I am trying to create a tap device with libvirt on a
> > > super-privileged container, and then use it on another,
> > > unprivileged container with libvirt.
> > > User wise, I know I need the super-privileged container to open the tap
> > > device with the user of the unprivileged one - that I already did and
> > it's
> > > not the issue.
> > > But I have a problem when I open the tap device in the
> > > non-privileged container: the tap device currently has the spc_t label
> > > since the tun_socket inherited the selinux context from the
> > > super-privileged container who creates it. then libvirt is trying to
> > change
> > > the SELinux labels, and since it's not privileged then it fails.
> > > But I didn't find where and how libvirt is trying to change the tap
> > > device's label.
> > >
> > > Can you point me to that specific code on libvirt?
> >
> > If the SELinux policy that libvirtd is running under prevents it from
> > re-labelling, then TAP devices label failure is just going to be one
> > out of 100's of labelling failures.
> >
> IIUC normally libvirtd would use devices created by itself, so there
> shouldn't be relabeling failures, right?

Whether or not there are relabeling failures is determined by what
SELinux policy libvirtd is running under.

> >
> > Either the SELinux policy needs to be changed to allow libvirtd to
> > relabel stuff in the normal manner, or you will have to turn off
> > SELinux  support in libvirtd. in /etc/libvirt/qemu.conf via the
> > param security_driver = "none".  If you turn off SELinux in
> > libvirt, then you no longer have separation of QEMU processes
> > which may be a security flaw depending on your deplyoment
> > scenario.
> >
> turning SELinux in libvirtd off or allowing libvirt to relabel are tempting
> options but it is not an option I'm afraid due to security concerns.
> Our plan (or at least this particular effort) is to try to relabel the
> tun-socket after created in the super-privileged container to be the same
> one as the one used in the unprivileged one, then it won't  have an issue
> consuming it.
> 
> btw the change I (or rather Migule's my teammate) made is in this PR, where
> I want to add a tap device in virt-handler (i.e. the super privileged
> container) to be further uses in virt-launcher (i.e. the non-privileged
> container): https://github.com/kubevirt/kubevirt/pull/3290

In normal host OS deployment,  libvirtd runs under virtd_t, and when
it spawns QEMU, it will relabel files to svirt_image_t:s0:$MCS, and
spawn QEMU as svirt_t:s0:$MCS.

My understanding is what in kubevirt, things work differently. Docker
(or podman), launch the container as  container_t:s0:$MCS.  libvirtd
*and* QEMU thus both run as container_t:s0:$MCS. ie All the labelling
is setup when the container is launched and libvirtd should not do
anything.

So I'm really not sure why you have libvirtd configured to do relabelling
at all ?  I'd be expecting it to have security_driver=none in the qemu.conf
file so that libvirtd doesn't do anything.

If libvirtd is doing relabelling, I'm not sure how it works for anything,
let alone tap devices. 

Regards,
Daniel
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