[libvirt] Problem configuring selective dropping of root

[Stephan von Krawczynski]

On Tue, 9 Jul 2019 14:26:08 +0200
Pavel Hrdina <phrdina at redhat.com> wrote:

> On Tue, Jul 09, 2019 at 02:03:15PM +0200, Stephan von Krawczynski wrote:
> > On Tue, 9 Jul 2019 09:40:23 +0100
> > Daniel P. Berrangé <berrange at redhat.com> wrote:
> >   
> > > On Mon, Jul 08, 2019 at 09:47:24PM +0200, Stephan von Krawczynski
> > > wrote:  
> > > > Hello list,
> > > > 
> > > > I came across a fundamental flaw in the libvirt user configuration
> > > > lately and try to find a solution now. Here is the problem:
> > > > I run several qemu instances on arch linux all configured via libvirt.
> > > > The default config as user nobody:kvm was fine up to the day I tried
> > > > to use a host filesystem via 9p. If you want to gain all user rights
> > > > on the guest inside that fs you have to run qemu as root. So far so
> > > > good. But if you have several qemus running and only one needs to be
> > > > root, what to do? You can try to give a -runas by using <qemu:args>.
> > > > But that does not work, qemu instantly crashes. I think this is
> > > > because to have _one_ root qemu, you have to configure libvirt to use
> > > > root user. This means all rights to fs and so on are set to root and
> > > > this is what lets qemu probably go crazy if dropping root by -runas.
> > > > The whole thing would be a lot easier and more transparent if the user
> > > > in libvirt wouldn't be a global config, but instead be part of the
> > > > domain xml. This way every qemu started could use a different user and
> > > > have different rights. In my case all but one could be nobody:kvm, and
> > > > one root:root. This should not be to complicated based on whats
> > > > already there, is it?    
> > > 
> > > Libvirt needs to know about the user/group QEMU is running at in order to
> > > ensure it gets given access to the various files it needs to use. If you
> > > look at the XML of the running guest you should see a <seclabel>
> > > describing the user/group it is running as currently.
> > > 
> > > If no <seclabel> is in the offline config, libvirt adds the default
> > > seclabel, but if you want a different user/group, you can add the
> > > <seclabel> yourself.
> > > 
> > > Regards,
> > > Daniel  
> > 
> > Hello Daniel,
> > 
> > well, tried that (as good as the docs are) by adding:
> > 
> > <seclabel type='dynamic' model='dac'>
> > 	<label>nobody:kvm</label>
> > </seclabel>
> > 
> > This edit worked in virsh without giving errors.
> > Starting the domain and then looking into the xml showed:
> > 
> >   <seclabel type='dynamic' model='dac' relabel='yes'/>
> > 
> > Consequently qemu runs still as root. My user:group setting simply
> > vanished.
> > 
> > I think at least some better docs are needed with a striking example of
> > how to change user and group ...
> > I may be biased, but how to set user and group is probably the most basic
> > example of how to use seclabel - and I cannot find one.  
> 
> I agree that the documentation is not the best one.
> 
> You need to use type='static' relabel='yes':
> 
> <seclabel type='static' model='dac' relabel='yes'>
>   <label>nobody:kvm</label>
> </seclabel>
> 
> To achieve that.
> 
> In addition if you would like to have only one VM as root:root you
> should keep the default config as nobody:kvm and use the root:root for
> that specific VM.
> 
> Pavel

Hello Pavel,

thank you for taking up the thread, but unfortunately your suggestion does not
work:

virsh # start collabora
Fehler: Domain collabora konnte nicht gestartet werden
Fehler: Interner Fehler: process exited while connecting to monitor:
2019-07-09T12:34:00.735392Z qemu-system-x86_64: -object
secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-17-collabora/master-key.aes:
Unable to read /var/lib/libvirt/qemu/domain-17-collabora/master-key.aes:
Failed to open file
“/var/lib/libvirt/qemu/domain-17-collabora/master-key.aes”: Permission denied

Obviously this is because "type='static'" means that libvirt does not care
about setting the user rights for qemu, which then leads to this.
I did think "relabel='yes'" should do that, but does not - or I have a deep
misunderstanding concerning the seclabel parameters ...
Thanks for any help to solve this, if there is no bug involved.

Dumpxml shows this btw:

  <seclabel type='static' model='dac' relabel='yes'>
    <label>nobody:kvm</label>
  </seclabel>

which at least is what was configured.
-- 
Regards,
Stephan