Ask Slashdot: Safe Learning Environment For VMs?

First time accepted submitter rarkian writes "I am the teacher in this story. I teach Python and C++ to high school students: grades 9-12. I use CentOS 6 with DRBL to run my computer lab. Some of my students have become Linux experts. Next year I'm planning on allowing students to create and run their own VMs in a segregated LAN. Any advice on which virtualization technology to use and security concerns with allowing students to be root in a VM?"

Comments

Set up VLANs

[Anonymous Coward]

for each of the students and don't allow any interface between them...and certainly no main network/internet access.

Re:Set up VLANs

[TheCarp]

> This is Linux were talking about. What could you possibly teach them without an internet
> connection? The only use I see is teaching shell scripting or something, since other tasks like
> package management, and sane server configuration kinda require an active internet connection.

Well Python, that is what he said he wanted to teach them :)

Beyond that, you are right, I say, the key is not security but recovery. Template boxes so that a new VM can be spun up effortlessly, then let them have at it. Segment the lab off from the rest of the network, maybe let the lab out to the internet, but not to the schools internal network.

However the key is, you can always blow away a machine and reimage it to get class moving, so there is no real danger in letting them play. Hell, I took a class that taught us administration and hacking eachother's machines was explicitly allowed, as long as it stayed in the lab. (we were also required to create a guest account as one of the exercises)

Gives the advanced kids who are bored with the class something to do that lets them learn too; I certanly had fun.

Re:Set up VLANs

[LordLimecat]

Not if they are on private VLANs (still not clear if it is a standard or not). VMWare supports this; the idea is roughly analogous to AP isolation on a wifi AP-- "isolated" nodes can all talk to the gateway (the community / public node), but cannot talk to each other.

And VLANs actually are for security, and can provide far superior security than ACLs, since unless you have a trunk port or layer 3 switches, it is impossible for two devices on different VLANs to communicate, short of a switch misconfiguration. Its probably second to air-gapping in terms of security-- its sort of a logical implementation of "air gapped switches", except that they CAN be joined together if someone gets onto the switch.

Re:Set up VLANs

[ttucker]

VLANs are not for security! Any two things plugged into the same switch, whether virtual or real, can talk to each other if sufficiently motivated.

This is simply not true. You're probably referring to 802.1q tag hopping attacks, which are not particularly difficult to prevent.

Do you really think tag hopping is the only attack on VLAN? Perhaps you should read what Cisco says about the matter, if your job depends on it: http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml . They point out that with an adequately secure switch, and good configuration practices, that the security is adequate. Without all of the correct practices, VLANs provide a very dangerous false sense of security.

Re:Set up VLANs

[MattW]

VLANs are not for security! Any two things plugged into the same switch, whether virtual or real, can talk to each other if sufficiently motivated.

As you pointed out below, VLANs in general are trustworthy when properly configured with a proper switch. I did nothing but netsec work in the late 90s, and everything was airgapped; we'd never have frames from two networks on the same wire. If you wanted to cross security zones, it was at L3 on a firewall and to different wires and switches.

On the other hand, it seemed like back then a new practical way to defeat VLANs was coming out every other week, so this was a wise precaution.

That said, keep in mind that VMware also affords some additional security in terms of VLANs. Physical switches have to connect to virtual switches to interact with the VMware layers (either the hypervisor for control traffic, or with the VMs for VM traffic), and the hypervisor itself will enforce a lot of things. On a VMware vSwitch properly configured:

- VMs can't enter promiscuous mode, change their MAC address, or forge transmits with the wrong L2 address
- QinQ frames are discarded
- The hypervisor itself will determine which virtual nics on a vswitch should receive copies of a frame, depending on which VLAN tag is on a portgroup
- Guests can't send tagged frames if their portgroup is set with a VLAN; you have to specifically configure a trunk on a portgroup to pass VLAN tags in and out of the guest environment

If the network was homogeneously ESX nodes and administratively controlled network equipment, you could likely enforce security between VMs with VLANs even with a dumb hub.

Obviously, airgapping and single-role wires will create better security than VLANs, because there always remains a chance that an undiscovered bug will allow breaching that L2 barrier, but that's true for everything.

Re:Set up VLANs

[evilviper]

VLANs are not for security! Any two things plugged into the same switch, whether virtual or real, can talk to each other if sufficiently motivated.

Umm, no. Not unless your switch is defective, or massively misconfigured. VLANs are very secure, when done properly. And the same security measures needed to protect VLANs are the same ones you need to protect switching in general (see CAM overflows, arp spoofing, and such).

http://www.cisco.com/en/US/products/hw/switches/ps708/products_white_paper09186a008013159f.shtml

If you leave your trunk/native VLAN at 1, you're in trouble. If you configure user-facing ports as auto-negotiate, or trunk without explicitly specifying allowed VLANs, you're in trouble.

Vagrant

[Pinhedd]

Vagrant is a wrapper for Virtualbox and VMWare Workstation that accelerates the deployment of development environments.

http://www.vagrantup.com/

VM is irrelevant

[onyxruby]

The fact that your using VM's is largely moot and goes back to the line of thought that VM's are somehow not 'real' computers. VM's run the same operating systems, software, have the same bugs, vulnerabilities and everything else as a physical computer. You need to patch them just like any other computer and you need to license them just like a regular computer. The fact that they are VM's really only makes two differences practical differences that matter, fist is that is easy to roll them back and second is that your aren't running on bare metal.

In other words you have a core issue that needs addressed of giving students root access to a computer. In an isolated environment this isn't necessarily a bad thing. Understand that they exploit root and see what they can do with it, however they are there to learn and if you can do so safely and without disruption of what your trying to teach then let them. Your focus needs to be on making it safe for those around them and that means making sure your VLAN and any related Internet access are properly setup. The lab is a lab and as long as you can make sure they aren't getting access to anyone persons computer than let them have at it.

A good rule of thumb is to roll your sessions back prior to the start of every single class. This always gives a fresh machine and the students will quickly learn how to set their VM just the way they want it.

Re:VM is irrelevant

[Archangel Michael]

VMs have one advantage that non-virtualized systems don't have. The ability to put several machines in their own sandboxed network, all managed by a single student who needs to demonstrate cooperating systems. Give every student a template of needed machines and a VM server and you have a small lab on every computer. One that is easily setup, cleared and re-setup for every class, and as needed.

VMs are a perfect solution for advanced computer systems management training.

SELinux on the host

[hpa]

Make sure you have SELinux enabled (and enforcing!) on the VM host, and keep the VMM software updated... there sometimes are security holes in VMM software which can be exploited. SELinux can help contain a breached VMM.

'Create' is the tricky part

[bill_mcgonigle]

Next year I'm planning on allowing students to create and run their own VMs

Running their own VM's is straightforward. Allowing the students to create their own VM's implies that they'll be root on the hypervisor.

Do you intend to run the hypervisor on the client machines of the DRBL system, or run a single hypervisor on the server and deploy the VM's there as DRBL clients?

To satisfy your requirements you probably want to run the hypervisor on the clients so they students can each have their own root on the hypervisor. This would require a hypervisor compatible with DRBL. I don't know how it works, but just from reading the description on the webpage, it sounds like it's geared to PXE booting a host OS.

If you go with Xen, you'll have to probably separately PXE boot Xen and then DRBL boot the Dom0. Which would probably work fine and get you decent performance, but it will expose the students to DRBL (is this what you want?)

If you go with KVM, the performance is a bit slower, but for a student shop that's probably OK, and you'll be able to DRBL-deploy the hypervisor and then let the students create their own non-DRBL (or DRBL) guests. This probably fits your model the best unless you have old hardware that KVM does not support - then you might need to go with the Xen-PXE-Boot model (because it can paravirtualize without hardware assistance).

You could also use VirtualBox, and while it offers a nice GUI, it's probably too simple for teaching your students about virtualization (it just feels like an app).

BTW, it sounds like you're doing great work based on that article. Kudos on your accomplishments and being an inspiration for others in your field.

Re:Virtualbox

[h4rr4r]

And is utter trash for anything that needs to be scalable.

It is fine for a desktop VM system, but it simply does not offer the management interfaces that other solutions have. Basically the options here are VMware and KVM. The first if you want a shiny GUI the latter if you are ok without one. Both will let you script everything they do, which will be very handy when you need to reset 100 VMs for the next batch of students.

Re:Safety?

[Archangel Michael]

Which is why they need to setup their own VLAN to isolate the VMs to the classroom. VM traffic is isolated to non-routing VLANS. They call this setup a "sandbox", and it is generally a good practice for classroom work.

As for which VM technology to use ... VMWare, or ZEN or even Microsoft's version are usable. VMWare is sort of free, Xen definitely is. I'm not familiar with pricing on Microsoft's versions but schools tend to get steep discounts for server licenses. Look at OpenStack for management, I hear it is decent when it works.

VLANs, RH Virtualization Security manual, virt-man

[raymorris]

Thanks for going the extra mile with your students.

As AC said, a separate LAN or VLAN, or multiple separate LANs/VLANs handles most of what's posted below. For example, a rogue DHCP server would only be visible on that VLAN.

Red Hat has a Virtualization Security section in their manual:
https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Administration_Guide/chap-Virtualization-Security_for_virtualization.html

CentOS/RHEL includes comprehensive support for KVM with virt-manager. While VirtualBox et al are fine for running one or two virtual machines on your desktop, for many VMs, with new ones created and removed each semester, the enterprise level support of KVM built into the distro is more appropriate. That support includes creating VLANs within the same management interface, for example, and integrates with the built in storage stack administration tools. Again, VirtualBox may be simpler to set up for one or to two machines, so I'm not saying it's not good - it's just not the best tool in this particular scenario. In this type of scenario, the KVM / virt-manager / virsh stack that RH baked in is probably a better match to the needs.

Re:VirtualBOX

[Archangel Michael]

Forkbomb is only successful if you don't have limits on your VM environment. You have put limits on your environment, right?

APT-Cacher, Squid

[SgtChaireBourne]

A good rule of thumb is to roll your sessions back prior to the start of every single class. This always gives a fresh machine and the students will quickly learn how to set their VM just the way they want it.

They can start each class with a fresh snapshot. In effect they would be restoring from backups. The configuration files from some other networked storage or their thumb drives and the applications themselves from the repositories. I've done something similar, but on bare metal, and after about half a dozen times they don't notice -- it had become such second nature to install and restore applications. Heck you might even have them practice installing the whole system from scratch. If you go that route, they can become quite proficient with installation and resource allocation. PXE booting a netinstall image helps there.

However, once you start to load packages from the net things can really slow down unless you prepare. The best way is to have a cache like APT-Cacher or Squid on your LAN or host system and have them configure their systems to use it for APT. For the cache to be most effective, you have to pre-load it before each class. That's easy and can be done while doing other things. It only takes time not attention. But once you have the cache loaded, installation will fly and can be done in 15 - 20 minutes. After that they weren't shy about installing on their own computers at home or helping their friends.

It depends on what you're trying to protect

[dankney]

So far, I see lots of advice about VM breakouts and network isolation. If this were a production datacenter where uptime was a criteria, this is all well and good. I suspect that this isn't what you need to hear, however.

I see three things you could be attempting to protect:

1) The larger school network.
2) The VM host infrastructure.
3) The VMs themselves.

1) A student on a VM is no more dangerous to the network than one who can connect to the school wireless with a laptop or smartphone. If the lab uplinks to the same network as the broader access, your risk profile is unchanged.

2) Make sure the VMs can't route to the host and keep it patched. If a student managed to break out of a VM in a patched hosting environment, do some forensics and find the bug then sell it. It's probably worth more than you make in a year. Seriously, if they can do this, they deserve to win. You might as well worry about protecting against nation-state sponsored attacks.

3) Make sure that the class work is backed up (a git server, perhaps) and then don't worry about it. Seriously, just throw the VMs away after each class (or every night, etc) and start with a clean one the next time they log in. Don't spend time trying to outsmart a classroom full of bored highschoolers. Instead, make it so it doesn't matter when they break something.

Re:Safety?

[Joce640k]

Unless they take down the network, e.g. running a rogue DHCP server. Or they use it to hack other systems on the network, e.g. password-sniffing the other student's credentials when they log in from their VMs.

So... nothing they couldn't do much easier/more safely by just pulling the network cable out of the physical machine and connecting it to their netbook?