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 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>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/drivers/crypto/exynos-rng.c b/drivers/crypto/exynos-rng.c
index dcdd444d0b3be19450643499ea87817bab9d14f1..825ed7bfd881673bfdd8bdc76901e439c093a527 100644 (file)
--- 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))