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

[Ard Biesheuvel]

(+ Eric)

On Thu, 5 Nov 2020 at 16:29, Ard Biesheuvel <ardb at kernel.org> 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 at kernel.org>
> ---
>  drivers/char/random.c | 15 +--------------
>  1 file changed, 1 insertion(+), 14 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 2a41b21623ae..a9c393c1466d 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1261,8 +1261,6 @@ void add_interrupt_randomness(int irq, int irq_flags)
>         cycles_t                cycles = random_get_entropy();
>         __u32                   c_high, j_high;
>         __u64                   ip;
> -       unsigned long           seed;
> -       int                     credit = 0;
>
>         if (cycles == 0)
>                 cycles = get_reg(fast_pool, regs);
> @@ -1298,23 +1296,12 @@ void add_interrupt_randomness(int irq, int irq_flags)
>
>         fast_pool->last = now;
>         __mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
> -
> -       /*
> -        * If we have architectural seed generator, produce a seed and
> -        * add it to the pool.  For the sake of paranoia don't let the
> -        * architectural seed generator dominate the input from the
> -        * interrupt noise.
> -        */
> -       if (arch_get_random_seed_long(&seed)) {
> -               __mix_pool_bytes(r, &seed, sizeof(seed));
> -               credit = 1;
> -       }
>         spin_unlock(&r->lock);
>
>         fast_pool->count = 0;
>
>         /* award one bit for the contents of the fast pool */
> -       credit_entropy_bits(r, credit + 1);
> +       credit_entropy_bits(r, 1);
>  }
>  EXPORT_SYMBOL_GPL(add_interrupt_randomness);
>
> --
> 2.17.1
>