libvirt users - Aug 2013 - Re: Stop the relabeling of CD images

Hi,

I'm installing the operating system for my virtual machines from CD
images and I would like for libvirtd to stop relabeling the
corresponding files.  Since the installation media is no big secret, I
have labeled the files with system_u:object_r:public_content_t:s0, but
libvirtd keeps changing them to system_u:object_r:svirt_image_t:s0.  It
also changes the ownership to qemu:qemu.  This means that I can not make
the files immutable (chattr +i).

The XML dump of the machine looks like this :

    <disk type='file' device='cdrom'>
        <driver name='qemu' type='raw'/>
        <source
file='/mnt/extra/Software/Linux/Fedora/Fedora-Live-Desktop-x86_64-19/Fedora-Live-Desktop-x86_64-19-1.iso'>
            <seclabel relabel='no'/>
        </source>
        <target dev='hdc' bus='ide'/>
        <readonly/>
        <shareable/>
        <address type='drive' controller='0' bus='1'
target='0' unit='0'/>
    </disk>

I am using libvirt-daemon-1.1.1-2.fc19.x86_64.  You might also find
interesting bug #568935 [1].

So how do I stop libvirtd from changing the permissions of the CD
images?

Thank you,
Cristian Ciupitu


[1]: https://bugzilla.redhat.com/show_bug.cgi?id=568935

On 08/19/2013 01:51 PM, Cristian Ciupitu wrote:
> Hi,
> 
> I'm installing the operating system for my virtual machines from CD
> images and I would like for libvirtd to stop relabeling the
> corresponding files.  Since the installation media is no big secret, I
> have labeled the files with system_u:object_r:public_content_t:s0, but
> libvirtd keeps changing them to system_u:object_r:svirt_image_t:s0.  It
> also changes the ownership to qemu:qemu.  This means that I can not make
> the files immutable (chattr +i).
Caveat - this is not something I have tried myself, so try it out, and
feel free to post back if it works or doesn't work for your case.
> 
> The XML dump of the machine looks like this :
> 
>     <disk type='file' device='cdrom'>
>         <driver name='qemu' type='raw'/>
>         <source
file='/mnt/extra/Software/Linux/Fedora/Fedora-Live-Desktop-x86_64-19/Fedora-Live-Desktop-x86_64-19-1.iso'>
>             <seclabel relabel='no'/>
Hmm, the documentation at http://libvirt.org/formatdomain.html#seclabel
is a bit stale; the last paragraph mentions that you can apply
per-<disk> seclabel overrides, but fails to mention that
model='...' is
valid in that XML.

Since you already know what label you want, it might be worth trying to
force that particular label instead of requesting no relabel, as in:

<source file=...>
  <seclabel model='selinux' relabel='yes'>
    <label>system_u:object_r:public_content_t:s0</label>
  </seclabel>
</source>

so that might get libvirt to stop doing the SELinux relabel dance on
your .iso file.  Then again, I'm not sure if that will prevent libvirt
from trying to "un-label" the device when your guest shuts down.

Then there's the question of the chown, which is caused by the DAC
driver rather than the SELinux driver.  So maybe this would do it:

<source file=...>
  <seclabel model='selinux' relabel='no'/>
  <seclabel model='dac' relabel='no'/>
</source>

I wouldn't be surprised if we need to patch our docs to be more clear
about this.  I also know that there has been talk of a patch for
teaching libvirt how to restore labels to their original state, instead
of the current problem of "restoring" labels to a single default
setting
(even where that does not match the original setting).

I'm also not sure why you think to resort to chattr +i, but if using
that causes libvirt heartburn, maybe we have a bug to fix to be more
tolerant of failed label attempts due to chattr.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

----- Original Message -----
> From: Eric Blake <eblake@redhat.com>
> To: Cristian Ciupitu <cristian.ciupitu@yahoo.com>
> Cc: libvirt-users <libvirt-users@redhat.com>
> Sent: Monday, August 19, 2013 11:24 PM
> Subject: Re: [libvirt-users] Stop the relabeling of CD images
> So maybe this would do it:
>
> <source file=...>
>   <seclabel model='selinux' relabel='no'/>
>   <seclabel model='dac' relabel='no'/>
> </source>
I've just tried it and the SELinux label is not changed anymore, but the
ownership is still changed to qemu:qemu.
> I'm also not sure why you think to resort to chattr +i, but if using
> that causes libvirt heartburn, maybe we have a bug to fix to be more
> tolerant of failed label attempts due to chattr.
I resorted to `chattr +i` because I got tired of libvirtd messing with
my files even if it wasn't required.  The official versions of libvirtd
from Fedora 18 or 19 used to complain about not being able to change the
files, but the current bleeding edge version hasn't complained (with the
XML config from above).

To sum it up, SELinux - solved, DAC - not (yet).

Thank you,
Cristian Ciupitu