[libvirt] Problem configuring selective dropping of root

Wed Jul 10 13:55:04 UTC 2019

On Wed, Jul 10, 2019 at 03:44:54PM +0200, Stephan von Krawczynski wrote:
>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?
>

Our Makefile specifies what to do on installation:

  $(MKDIR_P) -m 0751 "$(DESTDIR)$(localstatedir)/lib/libvirt/qemu"

so I guess this is a packaging issue.  No idea where/how the arch packaging
works, sorry.

>Thank you Martin, Pavel and Daniel for tracking that down.
>
>-- 
>Regards,
>Stephan
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