[libvirt] Problem configuring selective dropping of root

[Stephan von Krawczynski]

On Wed, 10 Jul 2019 14:48:14 +0200
Martin Kletzander <mkletzan at redhat.com> wrote:

> On Tue, Jul 09, 2019 at 02:45:18PM +0200, Stephan von Krawczynski wrote:
> >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.  
> 
> No, 'static' means you tell libvirt what the label should be, 'dynamic' means
> libvirt will generate it automatically.
> 
> >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.
> >  
> 
> Relabel definitely does that and unless there is a bug it uses the seclabel for
> the domain.  What could be happening is that one of the parent directories is
> missing a browse permission for others (the last 'x' in rwxrwxrwx).  Could you
> run the following commands and reply with the output?
> 
> ls -ld /var/lib/
> ls -ld /var/lib/libvirt/
> ls -ld /var/lib/libvirt/qemu/
> 
> Also what distribution are you using?  I remember there were some differences in
> packaging related to the directory permissions.
> 
> Martin
> 
> >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

Hello Martin,

thanks for picking up. Here is the output you requested:

[root at vserver-a01 /]# ls -ld /var/lib/
drwxr-xr-x 21 root root 4096 10. Jul 00:00 /var/lib/
[root at vserver-a01 /]# ls -ld /var/lib/libvirt/
drwxr-xr-x 11 root root 4096  4. Jul 10:08 /var/lib/libvirt/
[root at vserver-a01 /]# ls -ld /var/lib/libvirt/qemu/
drwxrwx--- 11 root root 4096 10. Jul 15:36 /var/lib/libvirt/qemu/

It seems your guess is right. After

[root at vserver-a01 /]# chmod a+x /var/lib/libvirt/qemu/
[root at vserver-a01 /]# ls -ld /var/lib/libvirt/qemu/
drwxrwx--x 11 root root 4096 10. Jul 15:41 /var/lib/libvirt/qemu/

it started:

virsh # start collabora
Domain collabora gestartet

So that's a bug in the Arch Linux packaging?
Who to tell?

Thank you Martin, Pavel and Daniel for tracking that down.

-- 
Regards,
Stephan