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

[Krzysztof Kozlowski]

On Tue, Dec 5, 2017 at 1:35 PM, Łukasz Stelmach  wrote:
> Reseed PRNG after reading 65 kB of randomness. Although this may reduce
> performance, in most casese the loss is not noticable.
s/casese/cases/
s/noticable/noticeable/

Please explain why you want to reseed after 65 kB (as opposite to
current implementation). Mention also why you are changing the time of
reseed.

>
> Signed-off-by: Łukasz Stelmach 
> ---
>  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_BYTES65536
> +
> +

Just one empty line.

>  /*
>   * In polling mode, do not wait infinitely for the engine to finish the work.
>   */
> @@ -81,6 +84,8 @@ struct exynos_rng_dev {
> unsigned intseed_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;
> +
> +

No need for these lines.

Best regards,
Krzysztof

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

[Stephan Mueller]

Am Dienstag, 5. Dezember 2017, 13:35:58 CET schrieb Łukasz Stelmach:

Hi Łukasz,

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

Please add to the log that you also increase the timer-based reseed to 1 
second?!

Another suggestion: maybe you want to add a comment to the reseed function to 
indicate it is for enhanced backtracking resistance. Otherwise a lot of folks 
would scratch their head why such code exists in the first place. :-)

Other than that:

Reviewed-by: Stephan Mueller 

Ciao
Stephan

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

[Łukasz Stelmach]

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 
---
 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_BYTES65536
+
+
 /*
  * In polling mode, do not wait infinitely for the engine to finish the work.
  */
@@ -81,6 +84,8 @@ struct exynos_rng_dev {
unsigned intseed_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