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Re: [libvirt] [PATCH 0/12] Improve security driver handling & QEMU DAC management



On Tue, Jan 26, 2010 at 05:00:53PM +0200, Dan Kenigsberg wrote:
> On Tue, Jan 26, 2010 at 12:56:00PM +0000, Daniel P. Berrange wrote:
> > On Tue, Jan 26, 2010 at 02:28:45PM +0200, Dan Kenigsberg wrote:
> > > On Tue, Jan 26, 2010 at 10:23:01AM +0000, Daniel P. Berrange wrote:
> > > > On Tue, Jan 26, 2010 at 12:06:58PM +0200, Dan Kenigsberg wrote:
> > > > > On Wed, Jan 20, 2010 at 03:14:57PM +0000, Daniel P. Berrange wrote:
> > > > > > This patch series does some work on te security drivers, and the QEMU code
> > > > > > for managing DAC permissions on files. 
> > > > > > 
> > > > > > The core goal is to turn the QEMU driver DAC file management code into a
> > > > > > security driver. Instead of QEMU calling into the SELinux/AppArmour drivers
> > > > > > directly, a stacked driver module is introduced. This delegates all operations
> > > > > > to first the QEMU DAC driver, and then the main SELinux/AppArmour driver. 
> > > > > > The end result is that all the permissions management code is removed from 
> > > > > > the QEMU driver, and we're left with just simple security driver calls.
> > > > > > 
> > > > > > In the process of this a number of flaws in the current hotplug  code were
> > > > > > found, and code was generally tidied up with a view to making it easier to
> > > > > > manage.
> > > > > > 
> > > > > > Finally, we add the ability to turn off the QEMU  DAC file managment code,
> > > > > > and also deal gracefully with failures to change ownership (eg on NFS with
> > > > > > root squash, or readonly FS).
> > > > > 
> > > > > hmmm, there's another problem which this patch set does not address:
> > > > > error : virStorageFileGetMetadata:415 : cannot open file '/deep/into/my/root/squashing/export': Permission denied
> > > > > 
> > > > > With dynamic_ownership = 0, libvirt down not mess with chown, but it now
> > > > > assumes that it can read disk images.
> > > > 
> > > > That is a not unreasonable assumption. The model we want to support is one
> > > > where 
> > > > 
> > > >  - App uses virStoragePool APIs to setup the NFS mount point on the host
> > > >  - App uses virStorageVol APIs to create the new volume with correct
> > > >    ownership at time of creation
> > > >  - libvirt starts the guest, without needing chown() due to step 2
> > > >    getting ownership correct at time of creation.
> > > > 
> > > > If libvirt can't read the directories leading upto the image, then it 
> > > > would never have been able to create the volume in the first place. The
> > > > patchs we have done have aimed at avoiding root squash problems which
> > > > prevent chmod/chown of files between root & non-root. We still assume
> > > > that we can read files / browse directories.
> > > 
> > > Understood. But if I am handling storage myself, I have a regression
> > > when I switch to libvirt from the sordid art of playing with qemu
> > > directly. I don't want to make my repository world-readable, and I don't
> > > want (yet) to use libvirt storage API.
> > > 
> > > Do I have a third option?
> > 
> > virStorageFileGetMetadata() is used to determine if the disk configured
> > for a guest has a backing file (eg qcow linked images). This is called
> > from the DAC driver to setup ownership, and from the SELinux driver to
> > setup labelling. You say you've already disabled the DAC driver ownership
> > so it shouldn't be hitting it there. Thus I assume it must the SELinux
> > driver that's causing the error. 
> > 
> > > Note that before this patchset, I could apply a patch disabling chown's
> > > and I managed to start domains over root squashing nfs.
> > 
> > I'm surprised if that's true, because I don't believe my patches changed
> > this behaviour of the SELinux driver - it was always calling into the
> > virStorageFileGetMetadata() method to determine backing store, so should
> > always have hit the issue you describe
> 
> Ok I was probably too hasty to blame this patchset, after all, I tested
> it earlier on a different machine with a different os and a different
> storage export.
> 
> However my question still stands: what can I do to make this work when
> the storage is not readable by root?

Have you tried disabling the SELinux driver ?  That ought to at least 
stop this code being called. We might also need to make the function
virStorageFileGetMetadata() treat failure to open the file as non-fatal,
though I'm kind of loathe todo that. I fear all the places where we're
putting in hacks to ignore errors on NFS, are going to make it harder
to see / diagnose real important errors on non-NFS filesystems

Daniel
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