[PATCH 3/3] qemu-img: Deprecate use of -b without -F

Peter Krempa pkrempa at redhat.com
Mon Feb 24 17:28:17 UTC 2020


On Mon, Feb 24, 2020 at 10:08:31 -0600, Eric Blake wrote:
> On 2/24/20 5:38 AM, Peter Krempa wrote:
> > On Sat, Feb 22, 2020 at 05:23:41 -0600, Eric Blake wrote:
> > > Creating an image that requires format probing of the backing image is
> > > inherently unsafe (we've had several CVEs over the years based on
> > > probes leaking information to the guest on a subsequent boot).  If our
> > > probing algorithm ever changes, or if other tools like libvirt
> > > determine a different probe result than we do, then subsequent use of
> > > that backing file under a different format will present corrupted data
> > > to the guest.  Start a deprecation clock so that future qemu-img can
> > > refuse to create unsafe backing chains that would rely on probing.
> > > 
> > > However, there is one time where probing is safe: when we first create
> > > an image, no guest has yet used the new image, so as long as we record
> > > what we probed, all future uses of the image will see the same data -
> > 
> > I disagree. If you are creating an overlay on top of an existing image
> > it's not safe to probe the format any more generally. (obviously you'd
> > have to trust the image and express the trust somehow)
> > 
> > The image may have been used in a VM as raw and that means that the VM
> > might have recorded a valid qcow2 header into it. Creating the overlay
> > with probing would legitimize this.
> > 
> > Let's assume we have a malicious image written by the guest but we
> > simulate it by:
> > 
> 
> > b) Now with this patchset:
> > 
> > $ ./qemu-img create -f qcow2 -b /tmp/malicious /tmp/post-patch.qcow2
> > qemu-img: warning: Deprecated use of non-raw backing file without explicit backing format, using detected format of qcow2
> > Formatting '/tmp/post-patch.qcow2', fmt=qcow2 size=2560 backing_file=/tmp/malicious backing_fmt=qcow2 cluster_size=65536 lazy_refcounts=off refcount_bits=16
> 
> > 
> > You now get a warning, but "backing file format" is now recorded in the
> > overlay. Now this is WAY worse than it was before. The overlay now
> > legitimizes the format recorded by the malicious guest which circumvents
> > libvirt's protections. The warning is very easy to miss, and if you run
> > it in scripts you might never get to see it. We can't allow that.
> 
> Good point.  I'll respin this series where v2 never writes the implicit
> format except for a raw image (because probing raw is not only safe to
> record, but also prevents the guest from ever changing that probe, and the
> real risk we are interested in preventing is when a formerly raw image later
> probes as non-raw).
> 
> > 
> > 
> > > so the code now records the probe results as if the user had passed
> > > -F.  When this happens, it is unconditionally safe to record a probe
> > > of 'raw', but any other probe is still worth warning the user in case
> > 
> > While it's safe I don't think it should be encouraged. IMO -F should be
> > made mandatory with -b.
> 
> Making it mandatory will require the completion of the deprecation period.
> For 5.0 and 5.1, the best we can do is the warning, but for 5.2 (assuming v2
> of this series is acceptable), it WILL become a hard error.

Yes, that's fair. I just wanted to point out that the warning and later
error should be reported also if raw is probed. I'm okay with recording
raw into the overlay even now.




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