Re: [libvirt] Problem configuring selective dropping of root

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(a)redhat.com&gt; 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(a)redhat.com&gt; 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