linux-kernel - Re: [PATCH v2] random: avoid mis-detecting a slow counter as a cycle counter

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Message-ID: <YmHraZcGnY3stnp9@zx2c4.com>
Date:   Fri, 22 Apr 2022 01:40:25 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Theodore Ts'o <tytso@....edu>, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org
Subject: Re: [PATCH v2] random: avoid mis-detecting a slow counter as a cycle
 counter

Hi Eric,

Thanks. This looks better.

On Thu, Apr 21, 2022 at 04:31:52PM -0700, Eric Biggers wrote:
> Therefore, increase the number of counter comparisons from 1 to 3, to
> greatly reduce the rate of false positive cycle counter detections.
> +	for (i = 0; i < 3; i++) {
> +		unsigned long entropy = random_get_entropy();
 
Wondering: why do you do 3 comparisons rather than 2? What does 3 get
you that 2 doesn't already? I thought the only real requirement was that
in the event where (a)!=(b), (b) is read as meaningfully close as
possible to when the counter changes.

Jason