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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
To:     vladimir.murzin@arm.com, linux-kernel@vger.kernel.org
Cc:     linux-crypto@vger.kernel.org, ebiggers@google.com, tytso@mit.edu,
        torvalds@linux-foundation.org,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH] random: cap jitter samples per bit to factor of HZ
Date:   Wed, 13 Jul 2022 17:11:15 +0200
Message-Id: <20220713151115.1014188-1-Jason@zx2c4.com>
In-Reply-To: <CAHmME9rrQVm72P6cLL4dUnSw+9nnXszDbQXRd3epRaQgKTy8BQ@mail.gmail.com>
References: <CAHmME9rrQVm72P6cLL4dUnSw+9nnXszDbQXRd3epRaQgKTy8BQ@mail.gmail.com>
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Currently the jitter mechanism will require two timer ticks per
iteration, and it requires N iterations per bit. This N is determined
with a small measurement, and if it's too big, it won't waste time with
jitter entropy because it'd take too long or not have sufficient entropy
anyway.

With the current max N of 32, there are large timeouts on systems with a
small CONFIG_HZ. Rather than set that maximum to 32, instead choose a
factor of CONFIG_HZ. In this case, 1/30 seems to yield sane values for
different configurations of CONFIG_HZ.

Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Fixes: 78c768e619fb ("random: vary jitter iterations based on cycle counter speed")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
Vladimir - Can you let me know if this appears to fix the issue you're
seeing? -Jason

 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index e3dd1dd3dd22..a1af90bacc9f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1174,7 +1174,7 @@ static void __cold entropy_timer(struct timer_list *timer)
  */
 static void __cold try_to_generate_entropy(void)
 {
-	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 };
+	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
-- 
2.35.1