linux-kernel - Re: [PATCH v1 5/7] random: do not xor RDRAND when writing into /dev/random

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Date:   Tue, 8 Feb 2022 15:11:54 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>
Subject: Re: [PATCH v1 5/7] random: do not xor RDRAND when writing into
 /dev/random

On Tue, Feb 08, 2022 at 04:53:33PM +0100, Jason A. Donenfeld wrote:
> Continuing the reasoning of "random: ensure early RDSEED goes through
> mixer on init", we don't want RDRAND interacting with anything without
> going through the mixer function, as a backdoored CPU could presumably
> cancel out data during an xor, which it'd have a harder time doing when
> being forced through a cryptographic hash function. There's actually no
> need at all to be calling RDRAND in write_pool(), because before we
> extract from the pool, we always do so with 32 bytes of RDSEED hashed in
> at that stage. Xoring at this stage is needless and introduces a minor
> liability.
> 
> Cc: Theodore Ts'o <tytso@....edu>
> Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  drivers/char/random.c | 14 ++------------
>  1 file changed, 2 insertions(+), 12 deletions(-)

Looks good,

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

- Eric