[PATCH v4] um: seed rng using host OS rng

[Johannes Berg]

On Sun, 2022-07-17 at 12:50 +0200, Jason A. Donenfeld wrote:
> UML generally does not provide access to special CPU instructions like
> RDRAND, and execution tends to be rather deterministic, with no real
> hardware interrupts, making good randomness really very hard, if not
> all together impossible. Not only is this a security eyebrow raiser, but
> it's also quite annoying when trying to do various pieces of UML-based
> automation that takes a long time to boot, if ever.
> 
> Fix this by trivially calling getrandom() in the host and using that
> seed as "bootloader randomness", which initializes the rng immediately
> at UML boot.
> 
> The old behavior can be restored the same way as on any other arch, by
> way of CONFIG_TRUST_BOOTLOADER_RANDOMNESS=n or
> random.trust_bootloader=0. So seen from that perspective, this just
> makes UML act like other archs, which is positive in its own right.
> 
> Additionally, wire up arch_get_random_{int,long}() in the same way, so
> that reseeds can also make use of the host RNG, controllable by
> CONFIG_TRUST_CPU_RANDOMNESS and random.trust_cpu, per usual.
> 
> Cc: stable at vger.kernel.org
> Cc: Johannes Berg <johannes at sipsolutions.net>
> Acked-By: Anton Ivanov <anton.ivanov at cambridgegreys.com>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
> ---
> Johannes - I need to take this through random.git, because it relies on
> some other changes living there. Is that okay with you? -Jason

Sure, go ahead, thanks for doing this work!

> Changes v3->v4:
> - Don't include os.h, per Johannes' suggestion.

Thanks.

Acked-by: Johannes Berg <johannes at sipsolutions.net>


johannes