[libvirt] [Qemu-devel] live snapshot wiki updated

Stefan Hajnoczi stefanha at gmail.com
Fri Jul 22 09:11:17 UTC 2011

On Fri, Jul 22, 2011 at 8:22 AM, Kevin Wolf <kwolf at redhat.com> wrote:
> Am 21.07.2011 17:01, schrieb Stefan Hajnoczi:
>> On Thu, Jul 21, 2011 at 3:02 PM, Eric Blake <eblake at redhat.com> wrote:
>>> Thank you for persisting - you've found another hole that needs to be
>>> plugged.  It sounds like you are proposing that after a qemu process dies,
>>> that libvirt re-reads the qcow2 metadata headers, and validates that the
>>> backing file information has not changed in a manner unexpected by libvirt.
>>>  If it has, then the qemu process that just died was compromised to the
>>> point that restarting a new qemu process from the old image is now a
>>> security risk.  So this is _yet another_ security aspect that needs to be
>>> coded into libvirt as part of hardening sVirt.
>> The backing file information changes when image streaming completes.
>> Before: fedora.img <- my_vm.qed
>> After: my_vm.qed (fedora.img is no longer referenced)
>> The image streaming operation copies data out of fedora.img and
>> populates my_vm.qed.  When image streaming completes, the backing file
>> is no longer needed and my_vm.qed is updated to drop the backing file.
>> I think we need to design carefully to prevent QEMU and libvirt making
>> incorrect assumptions about who does what.  I really wish that all
>> this image file business was outside QEMU and libvirt - that we had a
>> separate storage management service which handled the details.  QEMU
>> would only do block device operations (no image format manipulation),
>> and libvirt would only delegate to the storage management service.
> And how do you implement that in a way that works on all platforms, and
> without root privileges? I can't see this happen unless it stays
> completely optional.

The cross-platform way would be an iSCSI target that understands image
formats.  But iSCSI requires copying when doing I/O and we can't pass
through virtio-blk.

I'm not sure I see the root privilege issue.


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