Re: privileged entropy sources in QEMU/KVM guests

[Daniel P . Berrangé]

On Thu, Nov 07, 2019 at 11:10:57AM +0100, Laszlo Ersek wrote:
> Hi,
> 
> related TianoCore BZ:
> 
>   https://bugzilla.tianocore.org/show_bug.cgi?id=1871
> 
> (I'm starting this thread separately because at least some of the topics
> are specific to QEMU, and I didn't want to litter the BZ with a
> discussion that may not be interesting to all participants CC'd on the
> BZ. I am keeping people CC'd on this initial posting; please speak up if
> you'd like to be dropped from the email thread.)
> 
> QEMU provides guests with the virtio-rng device, and the OVMF and
> ArmVirtQemu* edk2 platforms build EFI_RNG_PROTOCOL on top of that
> device. But, that doesn't seem enough for all edk2 use cases.
> 
> Also, virtio-rng (hence EFI_RNG_PROTOCOL too) is optional, and its
> absence may affect some other use cases.

The optional nature of virtio-rng is something that results in the
the same problems for Linux.

If virtio-rng is absent, then Linux now has a general purpose fallback
via the CPU timing jitter entropy source:

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 ?

> (1) For UEFI HTTPS boot, TLS would likely benefit from good quality
> entropy. If the VM config includes virtio-rng (hence the guest firmware
> has EFI_RNG_PROTOCOL), then it should be used as a part of HTTPS boot.
> 
> However, what if virtio-rng (hence EFI_RNG_PROTOCOL) are absent? Should
> UEFI HTTPS boot be disabled completely (or prevented / rejected
> somehow), blaming lack of good entropy? Or should TLS silently fall back
> to "mixing some counters [such as TSC] together and applying a
> deterministic cryptographic transformation"?
> 
> IOW, knowing that the TLS setup may not be based on good quality
> entropy, should we allow related firmware services to "degrade silently"
> (not functionally, but potentially in security), or should we deny the
> services altogether?

If we can guarantee we always present fallback like jitterentropy
then the problem with TLS init goes away IIUC.

Regards,
Daniel
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