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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 27 Nov 2020 13:08:37 +0100
Message-ID: <CAMj1kXFeYB+UdWkn30dwzb61LUXVztWU4kKG+ncYLy0+S4AEeQ@mail.gmail.com>
Subject: Re: [PATCH] random: avoid arch_get_random_seed_long() when collecting
 IRQ randomness
To:     Marc Zyngier <maz@kernel.org>, "Theodore Y. Ts'o" <tytso@mit.edu>,
        Eric Biggers <ebiggers@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Andre Przywara <andre.przywara@arm.com>
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On Fri, 20 Nov 2020 at 16:27, Marc Zyngier <maz@kernel.org> wrote:
>
> On 2020-11-05 15:29, Ard Biesheuvel wrote:
> > When reseeding the CRNG periodically, arch_get_random_seed_long() is
> > called to obtain entropy from an architecture specific source if one
> > is implemented. In most cases, these are special instructions, but in
> > some cases, such as on ARM, we may want to back this using firmware
> > calls, which are considerably more expensive.
> >
> > Another call to arch_get_random_seed_long() exists in the CRNG driver,
> > in add_interrupt_randomness(), which collects entropy by capturing
> > inter-interrupt timing and relying on interrupt jitter to provide
> > random bits. This is done by keeping a per-CPU state, and mixing in
> > the IRQ number, the cycle counter and the return address every time an
> > interrupt is taken, and mixing this per-CPU state into the entropy pool
> > every 64 invocations, or at least once per second. The entropy that is
> > gathered this way is credited as 1 bit of entropy. Every time this
> > happens, arch_get_random_seed_long() is invoked, and the result is
> > mixed in as well, and also credited with 1 bit of entropy.
> >
> > This means that arch_get_random_seed_long() is called at least once
> > per second on every CPU, which seems excessive, and doesn't really
> > scale, especially in a virtualization scenario where CPUs may be
> > oversubscribed: in cases where arch_get_random_seed_long() is backed
> > by an instruction that actually goes back to a shared hardware entropy
> > source (such as RNDRRS on ARM), we will end up hitting it hundreds of
> > times per second.
> >
> > So let's drop the call to arch_get_random_seed_long() from
> > add_interrupt_randomness(), and instead, rely on crng_reseed() to call
> > the arch hook to get random seed material from the platform.
> >
> > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>
> Looks sensible. Having this on the interrupt path looks quite
> heavy handed, and my understanding of the above is that it has
> an adverse effect on the entropy pool.
>
> Acked-by: Marc Zyngier <maz@kernel.org>
>

Thanks Marc.

Ted, any thoughts?