linux-kernel - [PATCH 3/3] crypto: exynos - Reseed PRNG after generating 2^16 random bytes

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Message-id: <20171205123558.31087-4-l.stelmach@samsung.com>
Date:   Tue, 05 Dec 2017 13:35:58 +0100
From:   Łukasz Stelmach <l.stelmach@...sung.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>, robh+dt@...nel.org,
        Stephan Mueller <smueller@...onox.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Kukjin Kim <kgene@...nel.org>, linux-crypto@...r.kernel.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Łukasz Stelmach <l.stelmach@...sung.com>,
        m.szyprowski@...sung.com, b.zolnierkie@...sung.com
Subject: [PATCH 3/3] crypto: exynos - Reseed PRNG after generating 2^16
 random bytes

Reseed PRNG after reading 65 kB of randomness. Although this may reduce
performance, in most casese the loss is not noticable.

Signed-off-by: Łukasz Stelmach <l.stelmach@...sung.com>
---
 drivers/crypto/exynos-rng.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
index 002e9d2a83cc..0bf07a655813 100644
--- a/drivers/crypto/exynos-rng.c
+++ b/drivers/crypto/exynos-rng.c
@@ -54,12 +54,15 @@ enum exynos_prng_type {
 };
 
 /*
- * Driver re-seeds itself with generated random numbers to increase
- * the randomness.
+ * Driver re-seeds itself with generated random numbers to hinder
+ * backtracking of the original seed.
  *
  * Time for next re-seed in ms.
  */
-#define EXYNOS_RNG_RESEED_TIME		100
+#define EXYNOS_RNG_RESEED_TIME		1000
+#define EXYNOS_RNG_RESEED_BYTES		65536
+
+
 /*
  * In polling mode, do not wait infinitely for the engine to finish the work.
  */
@@ -81,6 +84,8 @@ struct exynos_rng_dev {
 	unsigned int			seed_save_len;
 	/* Time of last seeding in jiffies */
 	unsigned long			last_seeding;
+	/* Bytes generated since last seeding */
+	unsigned long			bytes_seeding;
 };
 
 static struct exynos_rng_dev *exynos_rng_dev;
@@ -125,6 +130,7 @@ static int exynos_rng_set_seed(struct exynos_rng_dev *rng,
 	}
 
 	rng->last_seeding = jiffies;
+	rng->bytes_seeding = 0;
 
 	return 0;
 }
@@ -166,6 +172,8 @@ static int exynos_rng_get_random(struct exynos_rng_dev *rng,
 	memcpy_fromio(dst, rng->mem + EXYNOS_RNG_OUT_BASE, *read);
 
 	return 0;
+
+
 }
 
 /* Re-seed itself from time to time */
@@ -177,7 +185,8 @@ static void exynos_rng_reseed(struct exynos_rng_dev *rng)
 	unsigned int read = 0;
 	u8 seed[EXYNOS_RNG_SEED_SIZE];
 
-	if (time_before(now, next_seeding))
+	if (time_before(now, next_seeding) &&
+	    rng->bytes_seeding < EXYNOS_RNG_RESEED_BYTES)
 		return;
 
 	if (exynos_rng_get_random(rng, seed, sizeof(seed), &read))
@@ -206,6 +215,7 @@ static int exynos_rng_generate(struct crypto_rng *tfm,
 
 		dlen -= read;
 		dst += read;
+		rng->bytes_seeding += read;
 
 		exynos_rng_reseed(rng);
 	} while (dlen > 0);
-- 
2.11.0