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



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).

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.

--
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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