[libvirt] Problem configuring selective dropping of root

[Stephan von Krawczynski]

On Wed, 10 Jul 2019 09:56:35 +0200
Pavel Hrdina <phrdina at redhat.com> wrote:

> On Wed, Jul 10, 2019 at 12:01:18AM +0200, Stephan von Krawczynski wrote:
> > On Tue, 9 Jul 2019 14:26:08 +0200
> > Pavel Hrdina <phrdina at redhat.com> wrote:
> >   
> > > [...]
> > > 
> > > 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  
> > 
> > Let me answer this part in another post.
> > Generally I agree with you. But there is one question: if I configure
> > libvirt to use nobody:kvm as user, how is it possible to start a qemu with
> > root privileges? I thought it not to be possible that it runs a root
> > process while its config says it should be nobody ...?  
> 
> That configuration is in /etc/libvirt/qemu.conf which configures things
> related to QEMU process and the user:group configuration tells how the
> QEMU process will be started.  The system libvirtd daemon runs always as
> root:root in order to have permissions to execute QEMU process under any
> user and to configure a lot of other things when starting a VM.
> 
> > I thought it can only _drop_ privileges from root to nobody, because its
> > primary user is root.
> > Or is it in fact always running as root, and only "fake-dropping" to the
> > configured user (maybe a spawned child), while still being able to spawn
> > other root processes?  
> 
> I'm not sure what do you mean by "fake-dropping", libvirt forks itself
> in order to create a new process where the QEMU binary is executed and
> the permissions are configured for that newly created process.
> 
> All of this is true only for the system libvirt, that means if you use
> qemu:///system connection, for the session libvirt everything runs as
> your user and there is no session libvirt for root user.
> 
> The XML and configuration that I've suggested should work as I've tried
> it before sending the mail.
> 
> Pavel

Thank you for clearing things up a bit for me.
I am using arch linux (see OP) and the libvirt version gives:

virsh # version
Kompiliert gegen die Bibliothek: libvirt 5.4.0
Verwende Bibliothek: libvirt 5.4.0

Verwende API: QEMU 5.4.0
Laufender Hypervisor: QEMU 4.0.0

Using your seclabel with root:root in libvirt/qemu.conf throws:

virsh # start collabora
Fehler: Domain collabora konnte nicht gestartet werden
Fehler: Interner Fehler: process exited while connecting to monitor:
2019-07-10T09:49:52.519211Z qemu-system-x86_64: -object
secret,id=masterKey0,format=raw,file=/var/lib/libvirt/qemu/domain-18-collabora/master-key.aes:
Unable to read /var/lib/libvirt/qemu/domain-18-collabora/master-key.aes:
Failed to open file
“/var/lib/libvirt/qemu/domain-18-collabora/master-key.aes”: Permission denied

Unfortunately I cannot verify what file permissions the requested file has, as
it is vanished as soon as the crash happens.
I bet though that it has root:root and the correctly set qemu user nobody:kvm
has no read rights. So obviously relabel does not work.
As it works on your side question is what's different? You are sure that you
did not try the other way round and seclabel to root:root for a setup with
standard user nobody:kvm. This would explain why you do not get this error...
I generally try not to patch around in libvirt or qemu or the hosts' arch
system. Which makes this probably at least a bug in the distro...

-- 
Regards,
Stephan