[RFC PATCH 20/41] random: provide min_crng_reseed_pool_entropy()

[Nicolai Stange]

Currently, the current minimum entropy required from the input_pool for
reseeding the primary_crng() is 16 bytes == 128 bits. A future patch will
introduce support for obtaining up to a certain fraction thereof from the
architecture's RNG, if available.

This will effectively lower the minimum input_pool ->entropy_count required
for a successful reseed of the primary_crng.

As this value is used at a couple of places, namely crng_reseed() itself
as well as dispatch_queued_entropy() and __dispatch_queued_entropy_fast(),
introduce min_crng_reseed_pool_entropy() to ensure consistency among
these.

min_crng_reseed_pool_entropy() returns the minimum amount of entropy in
bytes required from the input_pool for a successful reseed of the
primary_crng. Currently it's hardcoded to 16.

Use it in place of the hardcoded constants in crng_reseed(),
dispatch_queued_entropy() and __dispatch_queued_entropy_fast().

Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
 drivers/char/random.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 1945249597e0..424de1565927 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -516,6 +516,8 @@ static ssize_t extract_entropy(struct entropy_store *r, void *buf,
 static ssize_t _extract_entropy(struct entropy_store *r, void *buf,
 				size_t nbytes, int fips);
 
+static int min_crng_reseed_pool_entropy(void);
+
 static void crng_reseed(struct crng_state *crng, struct entropy_store *r);
 static __u32 input_pool_data[INPUT_POOL_WORDS] __latent_entropy;
 
@@ -916,7 +918,7 @@ static bool __dispatch_queued_entropy_fast(struct entropy_store *r,
 	if (unlikely(r == &input_pool && crng_init < 2)) {
 		const int entropy_bits = entropy_count >> ENTROPY_SHIFT;
 
-		return (entropy_bits >= 128);
+		return (entropy_bits >= min_crng_reseed_pool_entropy() * 8);
 	}
 
 	return false;
@@ -965,7 +967,7 @@ static void dispatch_queued_entropy(struct entropy_store *r,
 		if (crng_init < 2) {
 			const int entropy_bits = entropy_count >> ENTROPY_SHIFT;
 
-			if (entropy_bits < 128)
+			if (entropy_bits < min_crng_reseed_pool_entropy() * 8)
 				return;
 			crng_reseed(&primary_crng, r);
 		}
@@ -1182,6 +1184,15 @@ static int crng_slow_load(const char *cp, size_t len)
 	return 1;
 }
 
+/*
+ * Minimum amount of entropy in bytes required from the input_pool for
+ * a successful reseed of the primary_crng.
+ */
+static int min_crng_reseed_pool_entropy(void)
+{
+	return 16;
+}
+
 static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 {
 	unsigned long	flags;
@@ -1192,7 +1203,8 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
 	} buf;
 
 	if (r) {
-		num = extract_entropy(r, &buf, 32, 16);
+		num = extract_entropy(r, &buf, 32,
+				      min_crng_reseed_pool_entropy());
 		if (num == 0)
 			return;
 	} else {
-- 
2.26.2