Randomness in virtual machines

I always felt that entropy available to the operating system must be affected by running said operating system in a virtual environment – after all, unpredictable phenomena used to feed the entropy pool are commonly based on hardware and in a VM most hardware either is simulated or has the hypervisor mediate access to it. While looking for something tangentially related to the subject, I have recently stumbled upon a paper commissioned by the German Federal Office for Information Security which covers this subject, with particular emphasis on entropy sources used by the standard Linux random-number generator (i.e. what feeds /dev/random and /dev/urandom), in extreme detail:

https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/Publikationen/Studien/ZufallinVMS/Randomness-in-VMs.pdf?__blob=publicationFile&v=3

Personally I have found this paper very interesting but since not everyone can stomach a 142-page technical document, here are some of the main points made by its authors regarding entropy.

  1. As a reminder – the RNG in recent versions of Linux uses five sources of random noise, three of which do not require dedicated hardware (emulated or otherwise): Human-Interface Devices, rotational block devices, and interrupts.
  2. Running in a virtual machine does not affect entropy sources implemented purely in hardware or purely in software. However, it does affect hybrid sources – which in case of Linux essentially means all of the default ones.
  3. Surprisingly enough, virtualisation seems to have no negative effect on the quality of delivered entropy. This is at least in part due to additional CPU execution-time jitter introduced by the hypervisor compensating for increased predictability of certain emulated devices.
  4. On the other hand, virtualisation can strongly affect the quantity of produced entropy. More on this below.
  5. Low quantity of available entropy means among other things that it takes virtual machines visibly longer to bring /dev/urandom to usable state. This is a problem if /dev/urandom is used by services started at boot time because they can be initialised using low-quality random data.

Why exactly is the quantity of entropy low in virtual machines? The problem is that in a lot of configurations, only the last of the three standard noise sources will be active. On the one hand, even physical servers tend to be fairly inactive on the HID front. On the other, the block-device source does nothing unless directly backed by a rotational device – which has been becoming less and less likely, especially when we talk about large cloud providers who, chances are, hold your persistent storage on distributed networked file systems which are miles away from actual hard drives. This leaves interrupts as the only available noise source. Now take a machine configured this way and have it run a VPN endpoint, a HTTPS server, a DNSSEC-enabled DNS server… You get the idea.

But wait, it gets worse. Chances are many devices in your VM, especially ones like network cards which are expected to be particularly active and therefore the main source of interrupts, are in fact paravirtualised rather than fully emulated. Will such devices still generate enough interrupts? That depends on the underlying hypervisor, or to be precise on the paravirtualisation framework it uses. The BSI paper discusses this for KVM/QEMU, Oracle VirtualBox, Microsoft Hyper-V, and VMWare ESXi:

  • the former two use the the VirtIO framework, which is integrated with the Linux interrupt-handling code;
  • VMWare drivers trigger the interrupt handler similarly to physical devices;
  • Hyper-V drivers use a dedicated communication channel called VMBus which does not invoke the LRNG interrupt handler. This means it is entirely feasible for a Linux Hyper-V guest to have all noise sources disabled, and reenabling the interrupt one comes at a cost of performance loss caused by the use of emulated rather than paravirtualised devices.

 

All in all, the paper in question has surprised me with the news of unchanged quality of entropy in VMs and confirmed my suspicions regarding its quantity. It also briefly mentions (which is how I have ended up finding it) the way I typically work around this problem, i.e. by employing VirtIO-RNG – a paravirtualised hardware RNG in the KVM/VirtualBox guests which which interfaces with a source of entropy (typically /dev/random, although other options are possible too) on the host. Combine that with haveged on the host, sprinkle a bit of rate limiting on top (I typically set it to 1 kB/s per guest, even though in theory haveged is capable of acquiring entropy at a rate several orders of magnitude higher) and chances are you will never have to worry about lack of entropy in your virtual machines again.

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