Re: [PATCH v5] random: remove CONFIG_ARCH_RANDOM

Geert Uytterhoeven
2022-08-12T13:58:25Z

Hi Jason,

On Fri, Jul 8, 2022 at 2:44 AM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> When RDRAND was introduced, there was much discussion on whether it
> should be trusted and how the kernel should handle that. Initially, two
> mechanisms cropped up, CONFIG_ARCH_RANDOM, a compile time switch, and
> "nordrand", a boot-time switch.
>
> Later the thinking evolved. With a properly designed RNG, using RDRAND
> values alone won't harm anything, even if the outputs are malicious.
> Rather, the issue is whether those values are being *trusted* to be good
> or not. And so a new set of options were introduced as the real
> ones that people use -- CONFIG_RANDOM_TRUST_CPU and "random.trust_cpu".
> With these options, RDRAND is used, but it's not always credited. So in
> the worst case, it does nothing, and in the best case, maybe it helps.
>
> Along the way, CONFIG_ARCH_RANDOM's meaning got sort of pulled into the
> center and became something certain platforms force-select.
>
> The old options don't really help with much, and it's a bit odd to have
> special handling for these instructions when the kernel can deal fine
> with the existence or untrusted existence or broken existence or
> non-existence of that CPU capability.
>
> Simplify the situation by removing CONFIG_ARCH_RANDOM and using the
> ordinary asm-generic fallback pattern instead, keeping the two options
> that are actually used. For now it leaves "nordrand" for now, as the
> removal of that will take a different route.
>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Alexander Gordeev <agordeev@linux.ibm.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Acked-by: Borislav Petkov <bp@suse.de>
> Acked-by: Heiko Carstens <hca@linux.ibm.com>
> Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Thanks for your patch, which is now commit 9592eef7c16ec5fb ("random:
remove CONFIG_ARCH_RANDOM") upstream.

> --- a/drivers/char/Kconfig
> +++ b/drivers/char/Kconfig
> @@ -431,7 +431,6 @@ config ADI
> config RANDOM_TRUST_CPU
> bool "Initialize RNG using CPU RNG instructions"
> default y
> - depends on ARCH_RANDOM
> help
> Initialize the RNG using random numbers supplied by the CPU's
> RNG instructions (e.g. RDRAND), if supported and available. These

This change means everyone configuring a kernel will be asked this
question, even when configuring for an architecture that does not
support RNG instructions.

Perhaps this question should be hidden behind EXPERT?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds