Re: [Qemu-devel] Image probing: how it can be insecure, and what we could do about it

[Eric Blake]

On 11/06/2014 01:43 PM, Markus Armbruster wrote:

>> Actually, qed requires the backing format to be recorded (it is
>> non-optional) and is therefore immune to probing problems of backing
>> files.  That's one thing it got right.
> 
> If I read the code correctly:
> 
> QED has a feature bit QED_F_BACKING_FORMAT_NO_PROBE.
> 
> It is changed when you set the backing file format.  Setting format to
> "raw" sets the flag, anything else (including nothing) clears the flag.
> The actual non-raw format is not recorded.
> 
> Creating an image counts as setting the backing file format.
> 
> If the flag is set, open uses "raw"for the backing file (no probing).
> 
> If it's unset, open probes, and the probe may yield "raw".

Eww.  Well, looks like a deficiency in the qed spec, and maybe all that
is needed to plug it is:

If the probe yields "raw", refuse to open the backing file (or put
another way, either the probe MUST find a non-raw file, or the user has
a bug that they forgot to set the raw bit so we refuse to open the file
to point out their bug).

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

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