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

[Theodore Ts'o]

On Fri, Dec 31, 2021 at 12:49:03PM +0100, Jason A. Donenfeld 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>

Reviewed-by: Theodore Ts'o <tytso@xxxxxxx>