Re: [PATCH] random: avoid arch_get_random_seed_long() when collecting IRQ randomness

[Marc Zyngier]

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@xxxxxxxxxx>

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@xxxxxxxxxx>

        M.
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