crypto: jitter - Allow configuration of oversampling rate

The oversampling rate used by the Jitter RNG allows the configuration of
the heuristically implied entropy in one timing measurement. This
entropy rate is (1 / OSR) bits of entropy per time stamp.

Considering that the Jitter RNG now support APT/RCT health tests for
different OSRs, allow this value to be configured at compile time to
support systems with limited amount of entropy in their timer.

The allowed range of OSR values complies with the APT/RCT cutoff health
test values which range from 1 through 15.

The default value of the OSR selection support is left at 1 which is the
current default. Thus, the addition of the configuration support does
not alter the default Jitter RNG behavior.
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/Kconfig

@@ -1339,6 +1339,23 @@ config CRYPTO_JITTERENTROPY_MEMORY_BLOCKSIZE
 	default 1024 if CRYPTO_JITTERENTROPY_MEMSIZE_1024
 	default 2048 if CRYPTO_JITTERENTROPY_MEMSIZE_8192
 
+config CRYPTO_JITTERENTROPY_OSR
+	int "CPU Jitter RNG Oversampling Rate"
+	range 1 15
+	default 1
+	depends on CRYPTO_JITTERENTROPY
+	help
+	  The Jitter RNG allows the specification of an oversampling rate (OSR).
+	  The Jitter RNG operation requires a fixed amount of timing
+	  measurements to produce one output block of random numbers. The
+	  OSR value is multiplied with the amount of timing measurements to
+	  generate one output block. Thus, the timing measurement is oversampled
+	  by the OSR factor. The oversampling allows the Jitter RNG to operate
+	  on hardware whose timers deliver limited amount of entropy (e.g.
+	  the timer is coarse) by setting the OSR to a higher value. The
+	  trade-off, however, is that the Jitter RNG now requires more time
+	  to generate random numbers.
+
 config CRYPTO_JITTERENTROPY_TESTINTERFACE
 	bool "CPU Jitter RNG Test Interface"
 	depends on CRYPTO_JITTERENTROPY

crypto/jitterentropy-kcapi.c

@@ -256,7 +256,9 @@ static int jent_kcapi_init(struct crypto_tfm *tfm)
 	crypto_shash_init(sdesc);
 	rng->sdesc = sdesc;
 
-	rng->entropy_collector = jent_entropy_collector_alloc(0, 0, sdesc);
+	rng->entropy_collector =
+		jent_entropy_collector_alloc(CONFIG_CRYPTO_JITTERENTROPY_OSR, 0,
+					     sdesc);
 	if (!rng->entropy_collector) {
 		ret = -ENOMEM;
 		goto err;
@@ -345,7 +347,7 @@ static int __init jent_mod_init(void)
 
 	desc->tfm = tfm;
 	crypto_shash_init(desc);
-	ret = jent_entropy_init(0, 0, desc);
+	ret = jent_entropy_init(CONFIG_CRYPTO_JITTERENTROPY_OSR, 0, desc);
 	shash_desc_zero(desc);
 	crypto_free_shash(tfm);
 	if (ret) {