Re: [PATCH v2] random: avoid superfluous call to RDRAND in CRNG extraction

[Ard Biesheuvel]

On Fri, 31 Dec 2021 at 12:50, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
>
> RDRAND is not fast. RDRAND is actually quite slow. We've known this for
> a while, which is why functions like get_random_u{32,64} were converted
> to use batching of our ChaCha-based CRNG instead.
>
> Yet CRNG extraction still includes a call to RDRAND, in the hot path of
> every call to get_random_bytes(), /dev/urandom, and getrandom(2).
>
> This call to RDRAND here seems quite superfluous. CRNG is already
> extracting things based on a 256-bit key, based on good entropy, which
> is then reseeded periodically, updated, backtrack-mutated, and so
> forth. The CRNG extraction construction is something that we're already
> relying on to be secure and solid. If it's not, that's a serious
> problem, and it's unlikely that mixing in a measly 32 bits from RDRAND
> is going to alleviate things.
>
> And in the case where the CRNG doesn't have enough entropy yet, we're
> already initializing the ChaCha key row with RDRAND in
> crng_init_try_arch_early().
>
> Removing the call to RDRAND improves performance on an i7-11850H by
> 370%. In other words, the vast majority of the work done by
> extract_crng() prior to this commit was devoted to fetching 32 bits of
> RDRAND.
>
> Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
> ---
>  drivers/char/random.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 4de0feb69781..17ec60948795 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1023,7 +1023,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
>  static void _extract_crng(struct crng_state *crng,
>                           __u8 out[CHACHA_BLOCK_SIZE])
>  {
> -       unsigned long v, flags, init_time;
> +       unsigned long flags, init_time;
>
>         if (crng_ready()) {
>                 init_time = READ_ONCE(crng->init_time);
> @@ -1033,8 +1033,6 @@ static void _extract_crng(struct crng_state *crng,
>                                     &input_pool : NULL);
>         }
>         spin_lock_irqsave(&crng->lock, flags);
> -       if (arch_get_random_long(&v))
> -               crng->state[14] ^= v;
>         chacha20_block(&crng->state[0], out);
>         if (crng->state[12] == 0)
>                 crng->state[13]++;

Given that arch_get_random_long() may be backed by other things than
special instructions on some architectures/platforms, avoiding it if
we can on any path that may be a hot path is good, so

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>