Re: [PATCH v1 1/7] random: use RDSEED instead of RDRAND in entropy extraction

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Feb 08, 2022 at 04:53:29PM +0100, Jason A. Donenfeld wrote:
> When /dev/random was directly connected with entropy extraction, without
> any expansion stage, extract_buf() was called for every 10 bytes of data
> read from /dev/random. For that reason, RDRAND was used rather than
> RDSEED. At the same time, crng_reseed() was still only called every 5
> minutes, so there RDSEED made sense.
> 
> Those olden days were also a time when the entropy collector did not use
> a cryptographic hash function, which meant most bets were off in terms
> of real preimage resistance. For that reason too it didn't matter
> _that_ much whether RDSEED was mixed in before or after entropy
> extraction; both choices were sort of bad.
> 
> But now we have a cryptographic hash function at work, and with that we
> get real preimage resistance. We also now only call extract_entropy()
> every 5 minutes, rather than every 10 bytes. This allows us to do two
> important things.
> 
> First, we can switch to using RDSEED in extract_entropy(), as Dominik
> suggested. Second, we can ensure that RDSEED input always goes into the
> cryptographic hash function with other things before being used
> directly. This eliminates a category of attacks in which the CPU knows
> the current state of the crng and knows that we're going to xor RDSEED
> into it, and so it computes a malicious RDSEED. By going through our
> hash function, it would require the CPU to compute a preimage on the
> fly, which isn't going to happen.
> 
> Cc: Theodore Ts'o <tytso@mit.edu>
> Suggested-by: Dominik Brodowski <linux@dominikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/char/random.c | 22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)

Looks good,

Reviewed-by: Eric Biggers <ebiggers@google.com>

- Eric