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

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Dec 30, 2021 at 05:50:52PM +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.
> 
> There is one place, though, where such last-ditch moves might be
> quasi-sensible, and that's before the CRNG is actually ready. In that case,
> we're already very much operating from a position of trying to get
> whatever we can, so we might as well throw in the RDRAND call because
> why not.

So I'm not sure we how desperately we *need* the 370% performance
improvement, but realistically speaking, in
crng_init_try_arch_early(), which gets called from rand_initialize(),
we will have already set crng->state[4..15] via RDSEED or RDRAND.

So there's no point in setting crng->state[0] from RDRAND.  So if
we're wanting to speed things up, we should just remove the
crng->state[0] <= RDRAND entirely.

Or if we want to improve the security of get_random_bytes() pre
crng_ready(), then we should try to XOR RDRAND bytes into all returned
buffer from get_random_bytes().  In other words, I'd argue that we
should "go big, or go home".  (And if we do have some real,
security-critical users of get_random_bytes() pre-crng_ready(), maybe
"go big" is the right way to go.  Of course, if those do exist, we're
still screwed for those architectures which don't have an RDRAND or
equivalent --- arm32, RISC-V, I'm looking at you.)

					- Ted