On Fri, Mar 24, 2017 at 03:46:44PM +0100, Stephan M?ller wrote:

Am Freitag, 24. M?rz 2017, 15:43:48 CET schrieb Krzysztof Kozlowski:

Hi Krzysztof,

quoted

On Fri, Mar 24, 2017 at 03:37:59PM +0100, Stephan M?ller wrote:

quoted

Am Freitag, 24. M?rz 2017, 15:24:44 CET schrieb Krzysztof Kozlowski:

Hi Krzysztof,

quoted

+
+static int exynos_rng_set_seed(struct exynos_rng_dev *rng,
+			       const u8 *seed, unsigned int slen)
+{
+	int ret, i;
+	u32 val;
+
+	dev_dbg(rng->dev, "Seeding with %u bytes\n", slen);
+
+	ret = clk_prepare_enable(rng->clk);
+	if (ret)
+		return ret;
+
+	if (slen < EXYNOS_RNG_SEED_SIZE) {
+		dev_warn(rng->dev, "Seed too short (only %u bytes)\n", slen);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	for (i = 0 ; i < EXYNOS_RNG_SEED_REGS ; i++) {
+		val = seed[i * 4] << 24;
+		val |= seed[i * 4 + 1] << 16;
+		val |= seed[i * 4 + 2] << 8;
+		val |= seed[i * 4 + 3] << 0;
+
+		exynos_rng_writel(rng, val, EXYNOS_RNG_SEED(i));
+	}
+
+	val = exynos_rng_readl(rng, EXYNOS_RNG_STATUS);
+	if (!(val & EXYNOS_RNG_STATUS_SEED_SETTING_DONE)) {
+		dev_warn(rng->dev, "Seed setting not finished\n");
+		ret = -EIO;
+		goto out;
+	}
+
+	ret = 0;
+	/* Save seed for suspend */
+	memcpy(rng->seed_save, seed, slen);

Is this really necessary? If you need to save some seed, shouldn't that be
an output of the DRNG and not the real seed?

When suspended to RAM device will loose the contents of registers
(including SEED registers) so it has to be initialized with something on
resume.

The seed registers are write-only so I cannot read them and store
contents just before suspend.

I understand that real seed should not be stored... but then if I am not
able to re-seed it with same values, I will loose the continuous and
reproducible pseudo-random generation after suspend. Aren't this
expected out of PRNG?

An RNG has to be stateless from the perspective of the caller -- this is the 
core implication of entropy.

Then, if you add the initial seed after the RNG lost its state implies that 
the same sequence of random numbers starts again. I.e. where is the 
randomness?

Ahhh, yes, you are right. The device would not continue on its random
generation because all state is lost anyway.

I'll use the generated random numbers.

quoted

quoted

Besides, how do you know that slen is not larger than
EXYNOS_RNG_SEED_SIZE?

Right, there is a overflow here. It should be sizeof(rng->seed_save);

shouldn't it be min(rng->seed_save, slen)?

Now it is irrelevant but anyway I am not allowing the seed to be less
then EXYNOS_RNG_SEED_SIZE (== sizeof(rng->seed_save)). The device
expects all five channels (registers) to be seeded.

Best regards,
Krzysztof