Re: privileged entropy sources in QEMU/KVM guests

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

On Thu, 7 Nov 2019 at 14:44, Laszlo Ersek <lersek@redhat.com> wrote:
>
> On 11/07/19 13:47, Paolo Bonzini wrote:
> > On 07/11/19 12:52, Daniel P. Berrangé wrote:
> >>
> >> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bb5530e4082446aac3a3d69780cd4dbfa4520013
> >>
> >> Is it practical to provide a jitter entropy source for EDK2
> >> too ?
> >
> > The hard part is not collecting jitter (though the firmware might be too
> > deterministic for that), but rather turning it into a random number seed
> > (mixing data from various sources, crediting entropy, etc.).
>
> If there is *any* entropy source that (a) we can trust to be random
> enough and (b) depends only on the CPU, then we shouldn't struggle with
> virtio-rng (or similar devices) at all. RDRAND would be a no-brainer,
> but the "community literature" suggests it should not be trusted in itself.
>
> I've read the commit message linked above, and it appears too good to be
> true.
>
>     The CPU Jitter RNG provides a source of good entropy by collecting
>     CPU executing time jitter. [...] The RNG does not have any
>     dependencies on any other service in the kernel. The RNG only needs
>     a high-resolution time stamp. [...]
>
> http://www.chronox.de/jent.html
>
>     The CPU Jitter Random Number Generator provides a non-physical true
>     random number generator that works equally in kernel and user land.
>     The only prerequisite is the availability of a high-resolution timer
>     that is available in modern CPUs.
>
> http://www.chronox.de/jent/doc/CPU-Jitter-NPTRNG.html
>
>     Today’s operating systems provide non-physical true random number
>     generators which are based on hardware events. With the advent of
>     virtualization and the ever growing need of more high-quality random
>     numbers, these random number generators reach their limits.
>     Additional sources of entropy must be opened up. This document
>     introduces an entropy source based on CPU execution time jitter. The
>     design and implementation of a non-physical true random number
>     generator, the CPU Jitter random number generator, its statistical
>     properties and the maintenance and behavior of entropy is discussed
>     in this document.
>
> If this construct is legit, a core edk2 implementation (available to all
> platforms, and on all edk2 arches) would be a huge win.
>

 "that works equally in kernel and user land"

Firmware running at boot time on a single core without any serious
scheduling or I/O going on is not going to produce any entropy worth
mentioning, I'm afraid. And if it does, it is going to delay the boot
substantially.

> On the other hand, we're having this discussion because the premise of
> TianoCore#1871 is that we shouldn't rely on just the CPU and a high
> resolution timer... I simply cannot decide if this construct is
> trustworthy. (With any solution that was based in the host's /dev/random
> or /dev/urandom, the trustworthiness question would be side-stepped in
> the firmware.)
>