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

Blue Swirl blauwirbel at gmail.com
Wed Jul 20 20:01:37 UTC 2011

On Wed, Jul 20, 2011 at 9:17 PM, Eric Blake <eblake at redhat.com> wrote:
> On 07/20/2011 12:00 PM, Blue Swirl wrote:
>>>> Let's have files A, B, C etc. with backing files AA etc. How would
>>>> libvirt know that when QEMU wants to write to file CA, this is because
>>>> it's needed to access C, or is it just trickery by a devious guest to
>>>> corrupt storage?
>>> The fix for CVE-2010-2238 already deals with this: if primary image C
>>> refers
>>> to backing file CA of raw format, but does not state what file format CA
>>> contains, then a malicious guest can modify the contents of CA to appear
>>> to
>>> be yet another qcow2 image.  At which point, if libvirt follows the
>>> backing
>>> file specified in CA, then yes, the malicious guest really can cause
>>> libvirt
>>> to expose arbitrary file CB for manipulation by the guest.  But that
>>> security hole was already plugged - by default, libvirt refuses to probe
>>> backing files parsed from qcow2 headers for file format, but instead
>>> requires the outer qcow2 header to also include the a file format
>>> designation for the backing file.  At which point, you then have a safe
>>> chain: if C refers to CA, then libvirt knows that both C and CA are
>>> essential to the storage presented by giving qemu the file name C, and
>>> the
>>> guest will already be modifying CA, but there is no storage corruption
>>> involved.
>> But what if CA is accessed even if C is not? For example, QEMU opens C
>> (to determine CA and write new information about the path), closes it
>> and then requests CA?
> Why is qemu trying to access CA?
> Either because CA was mentioned as a backing file for C (in which case
> libvirt already knows about it, because either libvirt handed C to qemu at
> startup time after already parsing C's headers to learn that CA is a backing
> file, or because libvirt called the snapshot_blkdev command with the intent
> of having qemu populate CA with C as its backing file), or because qemu has
> a bug (in which case, libvirt should refuse the access to CA).

So no new backing files can be introduced by QEMU after it has started
without libvirt knowing it?

> Libvirt is already perfectly capable of tracking all files that qemu might
> need to access, and whether it is qemu or libvirt that does the open() of
> those files, we can still have libvirt validate whether each request for a
> file makes sense given the context of all previous files in use from the
> time the qemu command line was invoked and across all monitor commands in
> the meantime.
> On non-NFS solutions, where every file can have a SELinux label, then the
> security is then present by merely having libvirt relabel all such files to
> a unique label for that particular qemu process, and SELinux merely enforces
> that qemu cannot open() anything but what libvirt has already labeled.  And
> since libvirt already knows which files to label in the non-NFS scenario, it
> already knows which fds to pass in the NFS scenario, at which point the
> ability to prevent qemu from open()ing an NFS file is a security
> enhancement.
> Your question about qemu wanting to use CA is thus answered independently of
> whether the fd management solution is solved by libvirt handing an fd for CA
> to qemu prior to any monitor command where qemu will then need to use CA, or
> whether qemu is taught to asynchronously ask libvirt to open an fd for CA on
> qemu's behalf.  The answer is that libvirt already tracks whether qemu
> should access CA, and just needs a way to enforce that knowledge.  The
> enforcement already exists for non-NFS via SELinux labels, and the proposal
> to add fd handling will expand that enforcement to also cover NFS.

OK. I think fds would be useful internally in a privilege separation
mode in plain QEMU too.

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