linux-kernel - Re: [PATCH v2 4/5] arm64: Add support for SMCCC TRNG entropy source

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Message-ID: <CAMj1kXEsaZAGT0jnPNyj_K1Q4W7E1+WP=tQf3bTPQi=KOCUv4g@mail.gmail.com>
Date:   Thu, 5 Nov 2020 15:04:57 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Mark Brown <broonie@...nel.org>,
        Andre Przywara <andre.przywara@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm <kvmarm@...ts.cs.columbia.edu>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [PATCH v2 4/5] arm64: Add support for SMCCC TRNG entropy source

On Thu, 5 Nov 2020 at 15:03, Mark Rutland <mark.rutland@....com> wrote:
>
> On Thu, Nov 05, 2020 at 01:41:42PM +0000, Mark Brown wrote:
> > On Thu, Nov 05, 2020 at 12:56:55PM +0000, Andre Przywara wrote:
> >
> > >  static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
> > >  {
> > > +   struct arm_smccc_res res;
> > >     unsigned long val;
> > > -   bool ok = arch_get_random_seed_long(&val);
> > >
> > > -   *v = val;
> > > -   return ok;
> > > +   if (cpus_have_const_cap(ARM64_HAS_RNG)) {
> > > +           if (arch_get_random_seed_long(&val)) {
> > > +                   *v = val;
> > > +                   return true;
> > > +           }
> > > +           return false;
> > > +   }
> >
> > It isn't obvious to me why we don't fall through to trying the SMCCC
> > TRNG here if for some reason the v8.5-RNG didn't give us something.
> > Definitely an obscure possibility but still...
>
> I think it's better to assume that if we have a HW RNG and it's not
> giving us entropy, it's not worthwhile trapping to the host, which might
> encounter the exact same issue.
>
> I'd rather we have one RNG source that we trust works, and use that
> exclusively.
>
> That said, I'm not sure it's great to plumb this under the
> arch_get_random*() interfaces, e.g. given this measn that
> add_interrupt_randomness() will end up trapping to the host all the time
> when it calls arch_get_random_seed_long().
>

As it turns out, add_interrupt_randomness() isn't actually used on ARM.

> Is there an existing interface for "slow" runtime entropy that we can
> plumb this into instead?
>
> Thanks,
> Mark.