VM Escape: QEMU Case Study
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For the standard run of the mill VM there is also rarely any need for the multitude of devices. The more stuff you put in the more surface area for exploits. The Qemu install candidate needs to be seriously redesigned and pruned down by package maintainers.
Intel Clear Linux tried running VMs without qemu altogether with Kvmtool but are now using 'Qemu lite'. But there are no packages, instructions or even a readme on what exactly they have done to make 'Qemu lite'. Kvmtool is one of those packages with zero documentation.
Libvirt is needlessly convoluted and its usage complexity has led to an inferior solution like Virtualbox gaining traction on Linux, inspite of kvm in the kernel. For those looking for a simpler way to use kvm I suggest trying something like Aqemu or even just resorting to the command line. I found it far more lightweight and reasonable than libvirt's heavy handed way of running a simple qemu-kvm command line and adding an XML mess to it.
Daniel Berrangé (lead maintainer of libvirt) articulates it much more succintly than I could, allow me to quote him:
"It is a common attitude among people who look at QEMU and see a deceptively simple command-line syntax for launching a VM and don't realize that you'll enter a world of hurt when you go beyond the initial simple command-line syntax.
This[1] was written in 2011, but is pretty valid and probably more besides:
https://www.berrange.com/posts/2011/06/07/what-benefits-does-libvirt-offer-to-developers-targetting-qemukvm/
More on it (from his very detailed response on this thread[+], specifically from the 4th paragraph on):
"Also note that this kind of bug [VENOM] in QEMU device emulation is the poster child example for the benefit of having sVirt (either SELinux or AppArmor backends) enabled on your compute hosts.. With sVirt, QEMU is restricted to only access resources that have been explicitly assigned to it. This makes it very difficult (likely/hopefully impossible[1]) for a compromised QEMU to be used to break out to compromise the host as a whole, likewise protect against compromising other QEMU processes on the same host. The common Linux distros like RHEL, Fedora, Debian, Ubuntu, etc all have sVirt feature available and enabled by default and OpenStack doesn't do anything to prevent it from working. Hopefully no one is actively disabling it themselves leaving themselves open to attack...
[1] I'll never claim anything is 100% foolproof, but it is intended to be impossible to escape sVirt, so any such viable escape routes would themselves be considered security bugs. "
[+] http://lists.openstack.org/pipermail/openstack-operators/201...
We need to highlight the issues with libvirt that leaves a great technology like kvm unused. Security can be used to close down any discussion but the result is many people are using virtualbox and not libvirt.
Seccomp, selinux or Apparmor can be used without libvirt as required so I don't see this as a security issue. On the contrary the needlessly complex XML config files likely put many users off from using it properly or using it as all.
The `virtual-box` equivalent in the KVM stack is `virt-manager` (under the hood, it uses libvirt APIs) -- these are primarily for desktop Virtualization, not for server Virtualisation. To manage a fleet of servers in a data center, people don't fire up a desktop application.
Can some aspects of the experience with `virt-manager` / libvirt stack be improved? Certainly yes. But saying things like it "leaves a great technology like kvm unused" is ridiculous hyperbole.
A clarification about why it is partly a "security issue": The sVirt guest confinement mechanism in libvirt builds on top of basic SELinux / AppArmour confinement. As in, with sVirt: each VM (the QEMU proces that is managed by libvirt) and its associated disk gets a unique SELinux label -- this means, even if a guest (in a fleet of 1000 VMs) is compromised, it is contained to just that specific guest.
Obviously virtualbox users do not need data center level security and should not have to deal with that complexity, and naturally choose not to, and use virtualbox.
You have turned a simple observation about the complexity of libvirt's configuration and the resulting traction of virtualbox on Linux into a discussion on data centers, 1000's of VMs, and security. This seem to be talking around the issue so this discussion is moot.
Are you saying selinux, apparmor and seccomp cannot be used on qemu processes without libvirt?
And you seem to be comparing Virtual Box with libvirt, which is equivalent to comparing apples to aardvarks (okay, that's an exaggeration). But seriously, a fairer comparison would be Virtual Box vs. Virt-Manager on Linux.
Here, I admit -- I'm not a daily Virt-Manager user (I live on the command-line), so I can't tell you where exactly it is lacking compared to Virtual Box.
If you've tried it, & have suggestions, you might want to write to:
https://www.redhat.com/mailman/listinfo/virt-tools-list
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About the libvirt's internal representation (in XML -- yes, I'm not a big fan of it either, and I suspect, nor are the current maintainers; had it been today, they would've likely chosen a more gentler-on-the-eye format like JSON or some such) of guest definition: Most users don't need to touch the XML definition at all. Most tasks that are involved in managing VMs can all be done trivially via Virt-Manager GUI, or the command-line, virsh.
And the regular libvirt configuration files are all in standard Linux configuration file format.
> Are you saying selinux, apparmor and seccomp cannot be used on qemu processes without libvirt?
No, I'm not saying that. My argument (based on daily interactions with users on public IRC & mailing lists) is that, for QEMU-based guests, libvirt makes it easier.
You have taken this discussion to unrelated security and data center issues that have got nothing to do with the topic on hand. You dwell on the security of seccomp, selinux and apparmor that have nothing to do with libvirt specifically.
I am not interested in discussing libvirt, virsh or virt-manager and its various offshoots. And given libvirt is the what virsh and virt-manager use in the context it's disingenuous pedantry to make a distinction.
I am interested in understanding how kvm can be made more accessible to virtualbox users. If that's not something that concerns you and you would rather dwell on defending libvirt then that's not a productive discussion. If libvirt was the solution we would not be having this discussion to start with.
If you look at the list of syscalls whitelisted[1], it's almost a joke -- you could literally do just about anything given those syscalls if you actually had code execution... And every one of those syscalls is a direct interface to the kernel, which becomes the weakest link in a container/LSM environment where your code will be running, presumably. How is seccomp going to stop my exploit when almost everything my exploit could ever need is whitelisted, including hundreds of syscalls that directly talk to the kernel?
I'm glad QEMU is improving their security footprint, but I honestly think its seccomp support is way overblown even though people like to bring it up... When your seccomp policy looks like the one from QEMU -- it just bans a few things they've never used, rather than only allowing things they should use. Taking advantage of it to the level of something like Chrome does (which truly mitigates attack surface with process isolation) would require almost an entire redesign of the whole thing, I think.
https://news.ycombinator.com/item?id=14227605 https://news.ycombinator.com/item?id=14228563
Most QEMU CVEs are related to devices that should never be used in cloud provider scenarios (you'll often find that they are disabled in RHEL for this exact reason). If anything, prompt handling of vulnerabilities in those devices is a sign of taking security seriously...
No doubt in my mind Google has the top-tier working on this, at least now that GCE is public-facing. I was impressed to read they actively monitor/mitigate Rowhammer, something I've not seen mentioned anywhere else (could just be my ignorance).
Xen uses a stripped-down QEMU to boot unpatched guest OSes. However, even Xen doesn't test its qemu-xen components extensively. Writing a new purpose-built emulator (assuming you know what you're doing) is a better idea.
edit: Or use PV guests, and skip all potential QEMU flaws.
From the latest run:
│ run time : 11 days, 23 hrs, 12 min, 49 sec │ cycles done : 0 │
│ last new path : 0 days, 12 hrs, 55 min, 7 sec │ total paths : 364 │
│ last uniq crash : none seen yet │ uniq crashes : 0 │
│ last uniq hang : 0 days, 4 hrs, 4 min, 36 sec │ uniq hangs : 2 │
My understanding was from Qubes choosing Xen and also AWS (they both deal with Xen advisories instead). The Qubes Architecture Specification goes into detail starting on page 11: https://www.qubes-os.org/attachment/wiki/QubesArchitecture/a...
KVM uses the open source qemu emulator for [I/O emulattion]. [...] The I/O emulator is a complex piece of software, and thus it is reasonable to assume that it contains bugs and that it can be exploited by the attacker. In fact both Xen and KVM assume that the I/O emulator can be compromised and they both try to protect the rest of the system from a potentially compromised I/O emulator.
Also pointed out elsewhere on thread: Google skipping QEMU.
Edit: I am digging into it more, but I don't see KVM+QEMU on any top-tier provider (GCE, AWS, Azure [Hyper-V-ish])? My understanding was the only time QEMU was required was to emulate processor architectures, eg. x86 on ARM or vice-versa. QEMU is also used by some reverse-engineering/anti-malware emulation stuff.
Xen also requires a hardware emulator to run HVM guests (including, but not limited to, Windows VMs). I don't know about now, but it definitely used to be QEMU for AWS.
QEMU can do emulation, but with KVM you use the hypervisor to run code at full speed until it has to interact with the emulated hardware.
I didn't dig too far into the AWS vulnerability list to try to find QEMU; XEN shows up right away! Ok: QEMU is last mentioned July 2015, and in none of the mentions is AWS vulnerable.
https://www.google.com/?q=site:https://aws.amazon.com/securi...
Yep, that's because most bugs are found in legacy devices that are never found in production. The big exception was a buffer overflow in the floppy device emulation (the "VENOM" vulnerability).
A lot of AWS security bulletins say "AWS customers' data and instances are not affected by these issues". I read it as "we knew about it a couple weeks in advance and have done a rolling upgrade". :)
The OpenStack aspect is true. Xen lacks support there.
A new Xen guest mode called PVH will remove QEMU when running Linux -- it is basically HVM without QEMU. Windows still requires QEMU.
Without a hardened kernel, LSM can be trivially bypassed and seccomp seems to whitelist everything under the sun. This only leaves us with QEMU code quality to rely on. Since Grsec is not longer available this becomes even more urgent.
Xen relies on stubdoms to isolate QEMU from their TCB which leaves them with bugs in the hypervisor itself as the only avenue of attack. The number of Xen-only bugs vs Linux is way fewer. Please correct me if I'm wrong.
@bonzini I use virtio-9p for shared folders all the time why did you dismiss that as a non-issue: https://news.ycombinator.com/item?id=13755021
If you are a KVM dev please look seriously into using an advanced, intelligent fuzzer like the DARPA Grand Challenge winner Shellphish. It can find security bugs and propose patches for them:
https://github.com/shellphish http://angr.io/
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Security aside I find Libvirt more wanting in UX. The single biggest roadblock is the lack of a virtual appliance implementation that new comers can point and import to from Virt-Manager. I hope this gets resolved down the line.
It would clear things up if you have a table on your site showing which QEMU vulnerabilties affect a specified default configuration of a RHEL/Debian guest out of the box in libvirt. See this for example: https://www.qubes-os.org/security/xsa/
What I want to see:
* Adoption of QEMU-lite as the default mode for Linux guests. There's no point to running Linux in almost any emulated hardware.
* A builtin monitoring solution like Google has that detects excessive DRAM bitflips [1] and cache misses [2] and terminates the guests to foil rowhammer and covert channel attacks.
* A re-design of KSM thats not prone to rowhammer abuse [3]
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[1] https://cloudplatform.googleblog.com/2017/01/7-ways-we-harde...
[2] https://www.usenix.org/system/files/conference/usenixsecurit...
* Rowhammer detection is interesting, but not really related to virtualization. Thanks to KVM's design any such monitoring solution would apply equally to Linux containers. This is not the case for Xen, for example.
* Besides Rowhammer, memory dedup is highly subject to side channel attacks. I think this is a much more important issue, and it already pretty much forces you to disable KSM in multi-tenant applications.
While I'm impressed by the work Microsoft have done to support the Linux kernel interfaces I would imagine the complexity of the effort to implement correct behavior from Windows kernel primitives would lead to more potential security vulnerabilities.
Another comparison might be Linux syscall support within illumos[1] which AFAIK relies on mature Solaris Zones for isolation.
Another possibility would be User-mode Linux (UML), in contrast to containers, it gives each 'virtual machine' its own Linux kernel, where the Linux kernel runs as another Linux program.