hw_random: use add_hwgenerator_randomness() for early entropy

Rather than calling add_device_randomness(), the add_early_randomness()
function should use add_hwgenerator_randomness(), so that the early
entropy can be potentially credited, which allows for the RNG to
initialize earlier without having to wait for the kthread to come up.

This requires some minor API refactoring, by adding a `sleep_after`
parameter to add_hwgenerator_randomness(), so that we don't hit a
blocking sleep from add_early_randomness().
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/hw_random/core.c

@@ -69,8 +69,10 @@ static void add_early_randomness(struct hwrng *rng)
 	mutex_lock(&reading_mutex);
 	bytes_read = rng_get_data(rng, rng_fillbuf, 32, 0);
 	mutex_unlock(&reading_mutex);
-	if (bytes_read > 0)
-		add_device_randomness(rng_fillbuf, bytes_read);
+	if (bytes_read > 0) {
+		size_t entropy = bytes_read * 8 * rng->quality / 1024;
+		add_hwgenerator_randomness(rng_fillbuf, bytes_read, entropy, false);
+	}
 }
 
 static inline void cleanup_rng(struct kref *kref)
@@ -528,7 +530,7 @@ static int hwrng_fillfn(void *unused)
 
 		/* Outside lock, sure, but y'know: randomness. */
 		add_hwgenerator_randomness((void *)rng_fillbuf, rc,
-					   entropy >> 10);
+					   entropy >> 10, true);
 	}
 	hwrng_fill = NULL;
 	return 0;

drivers/char/random.c

@@ -727,7 +727,7 @@ static void __cold _credit_init_bits(size_t bits)
  * the above entropy accumulation routines:
  *
  *	void add_device_randomness(const void *buf, size_t len);
- *	void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy);
+ *	void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy, bool sleep_after);
  *	void add_bootloader_randomness(const void *buf, size_t len);
  *	void add_vmfork_randomness(const void *unique_vm_id, size_t len);
  *	void add_interrupt_randomness(int irq);
@@ -907,11 +907,11 @@ void add_device_randomness(const void *buf, size_t len)
 EXPORT_SYMBOL(add_device_randomness);
 
 /*
- * Interface for in-kernel drivers of true hardware RNGs.
- * Those devices may produce endless random bits and will be throttled
- * when our pool is full.
+ * Interface for in-kernel drivers of true hardware RNGs. Those devices
+ * may produce endless random bits, so this function will sleep for
+ * some amount of time after, if the sleep_after parameter is true.
  */
-void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy)
+void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy, bool sleep_after)
 {
 	mix_pool_bytes(buf, len);
 	credit_init_bits(entropy);
@@ -920,7 +920,7 @@ void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy)
 	 * Throttle writing to once every reseed interval, unless we're not yet
 	 * initialized or no entropy is credited.
 	 */
-	if (!kthread_should_stop() && (crng_ready() || !entropy))
+	if (sleep_after && !kthread_should_stop() && (crng_ready() || !entropy))
 		schedule_timeout_interruptible(crng_reseed_interval());
 }
 EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);

include/linux/random.h

@@ -17,7 +17,7 @@ void __init add_bootloader_randomness(const void *buf, size_t len);
 void add_input_randomness(unsigned int type, unsigned int code,
 			  unsigned int value) __latent_entropy;
 void add_interrupt_randomness(int irq) __latent_entropy;
-void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy);
+void add_hwgenerator_randomness(const void *buf, size_t len, size_t entropy, bool sleep_after);
 
 #if defined(LATENT_ENTROPY_PLUGIN) && !defined(__CHECKER__)
 static inline void add_latent_entropy(void)