From: "Theodore Ts'o" <tytso@mit.edu>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org
Subject: Re: [PATCH v2] random: avoid superfluous call to RDRAND in CRNG extraction
Date: Fri, 31 Dec 2021 12:13:48 -0500 [thread overview]
Message-ID: <Yc86TIah3w4waDEc@mit.edu> (raw)
In-Reply-To: <20211231114903.60882-1-Jason@zx2c4.com>
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@zx2c4.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
next prev parent reply other threads:[~2021-12-31 17:13 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-30 16:50 [PATCH] random: avoid superfluous call to RDRAND in CRNG extraction Jason A. Donenfeld
2021-12-30 22:13 ` Theodore Ts'o
2021-12-30 22:58 ` Jason A. Donenfeld
2021-12-31 3:35 ` Theodore Ts'o
2021-12-31 11:49 ` [PATCH v2] " Jason A. Donenfeld
2021-12-31 17:13 ` Theodore Ts'o [this message]
2022-01-04 5:03 ` Sandy Harris
2022-01-04 5:55 ` Theodore Ts'o
2022-01-20 15:03 ` Jason A. Donenfeld
2022-01-20 15:07 ` [PATCH] random: use named fields for adjusting chacha state Jason A. Donenfeld
2022-01-20 17:50 ` Theodore Ts'o
2022-01-20 21:53 ` Jason A. Donenfeld
2022-01-05 15:28 ` [PATCH v2] random: avoid superfluous call to RDRAND in CRNG extraction Ard Biesheuvel
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