diff mbox

[v3,3/4] crypto: exynos - Reseed PRNG after generating 2^16 random bytes

Message ID 20171212163607.2985-4-l.stelmach@samsung.com (mailing list archive)
State Not Applicable
Headers show

Commit Message

Lukasz Stelmach Dec. 12, 2017, 4:36 p.m. UTC 
Reseed PRNG after reading 65 kB of randomness. Although this may reduce
performance, in most cases the loss is not noticeable. Also the time
based threshold for reseeding is changed to one second. Reseeding is
performed whenever either limit is exceeded.

Reseeding of a PRNG does not increase entropy, but it helps preventing
backtracking the internal state of the device from its output sequence,
and hence, prevents potential attacker from predicting numbers to be
generated.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
---
 drivers/crypto/exynos-rng.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

Comments

Krzysztof Kozlowski Dec. 13, 2017, 8:12 a.m. UTC | #1 
On Tue, Dec 12, 2017 at 5:36 PM, Łukasz Stelmach <l.stelmach@samsung.com> wrote:
> Reseed PRNG after reading 65 kB of randomness. Although this may reduce
> performance, in most cases the loss is not noticeable. Also the time
> based threshold for reseeding is changed to one second. Reseeding is
> performed whenever either limit is exceeded.
>
> Reseeding of a PRNG does not increase entropy, but it helps preventing
> backtracking the internal state of the device from its output sequence,
> and hence, prevents potential attacker from predicting numbers to be
> generated.
>
> Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
> Reviewed-by: Stephan Mueller <smueller@chronox.de>
> ---
>  drivers/crypto/exynos-rng.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>

Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>

Best regards,
Krzysztof
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diff mbox

Patch

diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
index dcdd444d0b3b..825ed7bfd881 100644
--- a/drivers/crypto/exynos-rng.c
+++ b/drivers/crypto/exynos-rng.c
@@ -55,12 +55,14 @@  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.
  */
@@ -82,6 +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;
@@ -126,6 +130,7 @@  static int exynos_rng_set_seed(struct exynos_rng_dev *rng,
 	}
 
 	rng->last_seeding = jiffies;
+	rng->bytes_seeding = 0;
 
 	return 0;
 }
@@ -164,6 +169,7 @@  static int exynos_rng_get_random(struct exynos_rng_dev *rng,
 			  EXYNOS_RNG_STATUS);
 	*read = min_t(size_t, dlen, EXYNOS_RNG_SEED_SIZE);
 	memcpy_fromio(dst, rng->mem + EXYNOS_RNG_OUT_BASE, *read);
+	rng->bytes_seeding += *read;
 
 	return 0;
 }
@@ -177,7 +183,8 @@  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))