linux-kernel - Re: [PATCH v2 5/9] random: do not xor RDRAND when writing into /dev/random

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Message-ID: <YgN7OKSJRNZNxuGm@owl.dominikbrodowski.net>
Date:   Wed, 9 Feb 2022 09:28:40 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        tytso@....edu, ebiggers@...nel.org,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v2 5/9] random: do not xor RDRAND when writing into
 /dev/random

Am Wed, Feb 09, 2022 at 02:19:15AM +0100 schrieb Jason A. Donenfeld:
> 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.

Looks good generally, just one unrelated change slipped in:

>  		bytes = min(count, sizeof(buf));
> -		if (copy_from_user(&buf, p, bytes))
> +		if (copy_from_user(buf, p, bytes))
>  			return -EFAULT;

Otherwise:

	Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>

Thanks,
	Dominik