libvirt users - Aug 2018 - Efficacy of jitterentropy RNG on qemu-kvm Guests

Hello. I'm a distro maintainer and was wondering about the efficacy of
entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
authors of haveged [0] pointed out if the hardware cycles counter is
emulated and deterministic, and thus predictible. He therefore does not
recommend using HAVEGE on those systems. Is this the case with KVM's
counters?

PS. I will be setting VM CPU settings to host-passthrough.

Bonus: Also if anyone knows the answer to this question about Xen please
let me know because its the other main platform we support and they
don't have the luxury of virtio-rng in PVH mode.

Thanks.

[0]
https://github.com/BetterCrypto/Applied-Crypto-Hardening/commit/cf7cef7a870c1b77089b1bd6209ded6525b5a4e0#commitcomment-23006392

On Fri, Aug 10, 2018 at 08:33:00PM +0000, procmem wrote:
>Hello. I'm a distro maintainer and was wondering about the efficacy of
>entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
>authors of haveged [0] pointed out if the hardware cycles counter is
>emulated and deterministic, and thus predictible. He therefore does not
>recommend using HAVEGE on those systems. Is this the case with KVM's
>counters?
>
I don't know the answer.  But try asking on qemu-discuss@nongnu.org or maybe
KVM-related ML.

Have a nice day,
Martin
>PS. I will be setting VM CPU settings to host-passthrough.
>
>Bonus: Also if anyone knows the answer to this question about Xen please
>let me know because its the other main platform we support and they
>don't have the luxury of virtio-rng in PVH mode.
>
>Thanks.
>
>[0]
>https://github.com/BetterCrypto/Applied-Crypto-Hardening/commit/cf7cef7a870c1b77089b1bd6209ded6525b5a4e0#commitcomment-23006392
>
>_______________________________________________
>libvirt-users mailing list
>libvirt-users@redhat.com
>https://www.redhat.com/mailman/listinfo/libvirt-users

On Fri, Aug 10, 2018 at 08:33:00PM +0000, procmem wrote:
> Hello. I'm a distro maintainer and was wondering about the efficacy of
> entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
> authors of haveged [0] pointed out if the hardware cycles counter is
> emulated and deterministic, and thus predictible. He therefore does not
> recommend using HAVEGE on those systems. Is this the case with KVM's
> counters?
> 
> PS. I will be setting VM CPU settings to host-passthrough.
Hardware from circa 2011 onwards has RDRAND support, and with host-passthrough
this will be available to the guest.  The rngd daemon, running in the guest,
can use this as a source to feed the kernel entropy.

In addition QEMU has support for virtio-rng which can pull entropy from
/dev/urandom on the host, and feed it into the guest, where again rngd can
give it to the kernel.

So why do you need to consider haveged / jitterentropyd at all with QEMU ?
It should suffice to just enable virtio-rng in the host and run rngd in
all guests. If the host has RDRAND, that's an extra bonus.

haveged / jitterentropyd should only be needed on other non-QEMU hypervisors
which don't support something equiv to virtio-rng, and are on hardware that
is too old for RDRAND.

Regards,
Daniel
-- 
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Martin Kletzander:
> On Fri, Aug 10, 2018 at 08:33:00PM +0000, procmem wrote:
>> Hello. I'm a distro maintainer and was wondering about the efficacy
of
>> entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
>> authors of haveged [0] pointed out if the hardware cycles counter is
>> emulated and deterministic, and thus predictible. He therefore does not
>> recommend using HAVEGE on those systems. Is this the case with
KVM's
>> counters?
>>
> 
> I don't know the answer.  But try asking on qemu-discuss@nongnu.org or
> maybe
> KVM-related ML.
> 
> Have a nice day,
> Martin
> 
Thanks. Yes I had already sent there. Waiting patiently for an answer :)

Daniel P. Berrangé:
> On Fri, Aug 10, 2018 at 08:33:00PM +0000, procmem wrote:
>> Hello. I'm a distro maintainer and was wondering about the efficacy
of
>> entropy daemons like haveged and jitterentropyd in qemu-kvm. One of the
>> authors of haveged [0] pointed out if the hardware cycles counter is
>> emulated and deterministic, and thus predictible. He therefore does not
>> recommend using HAVEGE on those systems. Is this the case with
KVM's
>> counters?
>>
>> PS. I will be setting VM CPU settings to host-passthrough.
> 
> Hardware from circa 2011 onwards has RDRAND support, and with
host-passthrough
> this will be available to the guest.  The rngd daemon, running in the
guest,
> can use this as a source to feed the kernel entropy.
> 
> In addition QEMU has support for virtio-rng which can pull entropy from
> /dev/urandom on the host, and feed it into the guest, where again rngd can
> give it to the kernel.
> 
> So why do you need to consider haveged / jitterentropyd at all with QEMU ?
> It should suffice to just enable virtio-rng in the host and run rngd in
> all guests. If the host has RDRAND, that's an extra bonus.
> 
> haveged / jitterentropyd should only be needed on other non-QEMU
hypervisors
> which don't support something equiv to virtio-rng, and are on hardware
that
> is too old for RDRAND.
> 
> Regards,
> Daniel
> 
Makes sense. Thanks for your answer.