libvirt users - Dec 2013 - Re: virsh attach makes qcow2 format disk to raw format

Hi all

I have a problem when I use `virsh attach` to attach a qcow2 disk to vm without
argument --subdirver=qcow2
and this makes the qcow2 disk become a raw format disk, and the data in this
disk is missing in guest os
and also I have used guestmout to confirm this, the same result.

Is there any way to let me find back the data in this disk ?

the version of libvirt and kvm is :

Compiled against library: libvir 0.9.8
Using library: libvir 0.9.8
Using API: QEMU 0.9.8
Running hypervisor: QEMU 1.0.0






Best Regards
Li Yuezhou

On 12/09/2013 09:03 PM, lyz_pro@163.com wrote:
> Hi all
> 
> I have a problem when I use `virsh attach` to attach a qcow2 disk to vm
without argument --subdirver=qcow2
> and this makes the qcow2 disk become a raw format disk, and the data in
this disk is missing in guest os
> and also I have used guestmout to confirm this, the same result.
The format of a file is important.  By default, libvirt refuses to probe
a file for its format, because any raw file can contain contents that
look like any other format, making libvirt and/or qemu misbehave if the
guest abused the raw contents to intentionally resemble a different
format (this is a known CVE in the past).  You can flip an option in
/etc/libvirt/qemu.conf to request libvirt to probe image formats, at
which point you can then omit the format but the guest will still get
the data from the qcow2 format instead of abusing the file as raw; but
this is a security risk if you do not trust any guest that has ever run
on the image file.
> 
> Is there any way to let me find back the data in this disk ?
If the guest wrote to the disk, then I hope you had a good backup.  If
the guest merely read from the disk but didn't write any data, then
unplugging the disk and replugging with the correct qcow2 format will
fix things.

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

Hi Eric


Thanks for you answer, help me a lot


At 2013-12-13 06:25:44,"Eric Blake" <eblake@redhat.com>
wrote:
>On 12/09/2013 09:03 PM, lyz_pro@163.com wrote:
>> Hi all
>> 
>> I have a problem when I use `virsh attach` to attach a qcow2 disk to vm
without argument --subdirver=qcow2
>> and this makes the qcow2 disk become a raw format disk, and the data in
this disk is missing in guest os
>> and also I have used guestmout to confirm this, the same result.
>
>The format of a file is important.  By default, libvirt refuses to probe
>a file for its format, because any raw file can contain contents that
>look like any other format, making libvirt and/or qemu misbehave if the
>guest abused the raw contents to intentionally resemble a different
>format (this is a known CVE in the past).  You can flip an option in
>/etc/libvirt/qemu.conf to request libvirt to probe image formats, at
>which point you can then omit the format but the guest will still get
>the data from the qcow2 format instead of abusing the file as raw; but
>this is a security risk if you do not trust any guest that has ever run
>on the image file.
>
>> 
>> Is there any way to let me find back the data in this disk ?
>
>If the guest wrote to the disk, then I hope you had a good backup.  If
>the guest merely read from the disk but didn't write any data, then
>unplugging the disk and replugging with the correct qcow2 format will
>fix things.
>
>-- 
>Eric Blake   eblake redhat com    +1-919-301-3266
>Libvirt virtualization library http://libvirt.org
>