[openstack-dev] [nova] live-snapshot/cloning of virtual machines

Daniel P. Berrange berrange at redhat.com
Fri Aug 16 10:05:19 UTC 2013

On Wed, Aug 14, 2013 at 04:53:01PM -0700, Vishvananda Ishaya wrote:
> Hi Everyone,
> I have been trying for some time to get the code for the live-snapshot blueprint[1]
> in. Going through the review process for the rpc and interface code[2] was easy. I
> suspect the api-extension code[3] will also be relatively trivial to get in. The
> main concern is with the libvirt driver implementation[4]. I'd like to discuss the
> concerns and see if we can make some progress.
> Short Summary (tl;dr)
> =====================
> I propose we merge live-cloning as an experimental feature for havanna and have the
> api extension disabled by default.
> Overview
> ========
> First of all, let me express the value of live snapshoting. The slowest part of the
> vm provisioning process is generally booting of the OS. The advantage of live-
> snapshotting is that it allows the possibility of bringing up application servers
> while skipping the overhead of vm (and application startup).

For Linux at least I think bootup time is a problem that is being solved by the
distros. It is possible to boot up many modern Linux distros in a couple of seconds
even in physical hardware - VMs can be even faster since they don't have such stupid
BIOS to worry about & have a restricted set of possible hardware. This is on a par
with, or better than, the overheads imposed by Nova itself in the boot up process.

Windows may be a different story, but I've not used it in years so don't know what
its boot performance is like.

> I recognize that this capability comes with some security concerns, so I don't expect
> this feature to go in and be ready to for use in production right away. Similarly,
> containers have a lot of the same benefit, but have had their own security issues
> which are gradually being resolved. My hope is that getting this feature in would
> allow people to start experimenting with live-booting so that we could uncover some
> of these security issues.
> There are two specific concerns that have been raised regarding my patch. The first
> concern is related to my use of libvirt. The second concern is related to the security
> issues above. Let me address them separately.
> 1. Libvirt Issues
> =================
> The only feature I require from the hypervisor is to load memory/processor state for
> a vm from a file. Qemu supports this directly. The only way that libvirt exposes this
> functionality is via its restore command which is specifically for restoring the
> previous state of an existing vm. "Cloning", or restoring the memory state of a
> cloned vm is considered unsafe (which I will address in the second point, below).
> The result of the limited api is that I must include some hacks to make the restore
> command actually allow me to restore the state of the new vm. I recognize that this
> is using an undocumented libvirt api and isn't the ideal solution, but it seemed
> "better" then avoiding libvirt and talking directly to qemu.
> This is obviously not ideal. It is my hope that this 0.1 version of the feature will
> allow us to iteratively improve the live-snapshot/clone proccess and get the security
> to a point where the libvirt maintainers would be willing to accept a patch to directly
> expose an api to load memory from a file.

To characterize this as a libvirt issue is somewhat misleading. The reason why libvirt
does not explicitly allow this, is that from discussions with the upstream QEMU/KVM
developers, the recommendation/advise that this is not a safe operation and should not
be exposed to application developers.

The expectation is that the functionality in QEMU is only targetted for taking point in
time snapshots & allowing rollback of a VM to those snapshots, not creating clones of
active VMs.

> 2. Security Concerns
> ====================
> There are a number of security issues with loading state from another vm. Here is a
> short list of things that need to be done just to make a cloned vm usable:
> a) mac address needs to be recreated
> b) entropy pool needs to be reset
> c) host name must be reset
> d) host keys bust be regenerated
> There are others, and trying to clone a running application as well may expose other
> sensitive data, especially if users are snaphsoting vms and making them public.
> The only issue that I address on the driver side is the mac addresses. This is the
> minimum that needs to be done just to be able to access the vm over the network. This
> is implemented by unplugging all network devices before the snapshot and plugging new
> network devices in on clone. This isn't the most friendly thing to guest applications,
> but it seems like the safest option for the first version of this feature.

This is not really as safe as you portray. When restoring from the snapshot the VM
will initially be running with virtual NIC with a different MAC address from the one
associated with the in memory OS kernel state. Even if you hotunplug the device and
plug in a new one, you still have a period where the virtual hardware exposed to the
guest does not match the memory state of the guest OS. Perhaps you will be lucky and
not hit problems with some OS, but equally you can be unlucky and do bad things to
the OS kernel or application state. Relying on luck in this way does not lead to a
supportable solution IMHO.

There are other unique identifiers exposed in the virtual hardware that will/should
change when you clone VMs too, the host UUID, the storage serial keys and you cannot
easily fix those by just unplugging hardware & replugging it.

> So cloning vms must be done with care. Sensitive data must be removed from the vm
> pre-clone and new data needs to be generated post-clone. Ultimately this should all
> be done via guest-agent of some sort. I have found some volunteers to make the guest
> agent a reality, but it will take a bit of time to get something workable, and it
> will be much more difficult if there isn't a way to test the feature.

Note that even if you think you have removed security data from a VM's filesystem,
it is quite likely that the data will still in fact exist in the unallocated sectors
of VM's block devices and can be fairly easily recovered from them.

The libguestfs project provide tools to perform offline cloning of VM disk images.
Its virt-sysprep knows how to delete alot (but by no means all possible) sensitive
file data for common Linux & Windows OS. It still has to be combined with use of
the virt-sparsify tool though, to ensure the deleted data is actually purged from
the VM disk image as well as the filesystem, by releasing all unused VM disk sectors
back to the host storage (and not all storage supports that).

> Conclusion
> ==========
> There are obviously problems to be solved with the whole idea of live cloning, but
> I think it enables some important new ways of deploying applications. Imagine for
> example a PaaS built on fast-cloning vms instead of containers. This is clearly a
> long term project but if we block it now it may never get the support it needs to
> become a real option.
> Proposal
> ========
> I propose we allow the patch in and we leave the live-snapshot extension disabled
> by default. Deployers can turn on the extension to experiment with the feature.
> This will allow other hypervisors do do an implementation, and the community in
> general to improve the security and usefulness of live-cloned virtual machines.
> I'm very interested in your thoughts and feedback. Thank you to everyone who made
> it this far.

I don't think it is a good idea to add a feature which is considered to
be unsupportable by the developers of the virt platform.

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