[Libguestfs] [PATCH for discussion only] lib: libvirt: If root, run qemu subprocess as root.root.

[Daniel P. Berrangé]

On Mon, May 21, 2018 at 06:22:06PM +0100, Richard W.M. Jones wrote:
> libvirt doesn't have a concept of "session qemu" for root:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=890291
> 
> When a libguestfs-using process runs as root, and libvirt runs a qemu
> subprocess, the qemu subprocess is run as a non-root user (typically
> qemu.qemu).  This causes various problems, for example if we try to
> open a file which is readable by root but unreadable by qemu.qemu then
> the operation will fail.
> 
> This can be changed globally via a configuration file, but it can also
> be changed by using a <seclabel/> clause in the XML (although I think
> that's not the only effect):
> 
>   <seclabel type="static" model="dac" relabel="no">
>     <label>0:0</label>
>   </seclabel>
> 
> This patch makes that change.
> 
> I notice that after this change, qemu is indeed running as root.
> However the file being examined still gets relabelled by SELinux (to
> virt_content_t IIRC).  Maybe this relabelling is in fact desirable.

If you turn off relabelling you'll certainly get SELinux denials,
unless the files already have the virt_content_t label or equiv.
So if you wanted to prevent SELinux labelling, you would probably
have to turn off SELinux confinement entirely. 

> Also as you can see from the patch there are cases where we use
> another <seclabel model='selinux'/> element to set labels to a known
> value.  It's not clear if we can include both <seclabel/> elements.
> The patch as shown overrides the selinux seclabel if running as root.

Yes, you can set multiple <seclabel> elements in the same guest - in
fact if you look at a running guest you'll see two present. You just
need to make sure "model" attribute is unique on each.

Regards,
Daniel
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