random: introduce drain_entropy() helper to declutter crng_reseed()

In preparation for separating responsibilities, break out the entropy
count management part of crng_reseed() into its own function.

No functional changes.

Cc: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: NDominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -260,6 +260,7 @@ static struct {
 };
 
 static void extract_entropy(void *buf, size_t nbytes);
+static bool drain_entropy(void *buf, size_t nbytes);
 
 static void crng_reseed(void);
 
@@ -456,23 +457,13 @@ static void crng_slow_load(const void *cp, size_t len)
 static void crng_reseed(void)
 {
 	unsigned long flags;
-	int entropy_count;
 	unsigned long next_gen;
 	u8 key[CHACHA_KEY_SIZE];
 	bool finalize_init = false;
 
-	/*
-	 * First we make sure we have POOL_MIN_BITS of entropy in the pool,
-	 * and then we drain all of it. Only then can we extract a new key.
-	 */
-	do {
-		entropy_count = READ_ONCE(input_pool.entropy_count);
-		if (entropy_count < POOL_MIN_BITS)
-			return;
-	} while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
-	extract_entropy(key, sizeof(key));
-	wake_up_interruptible(&random_write_wait);
-	kill_fasync(&fasync, SIGIO, POLL_OUT);
+	/* Only reseed if we can, to prevent brute forcing a small amount of new bits. */
+	if (!drain_entropy(key, sizeof(key)))
+		return;
 
 	/*
 	 * We copy the new key into the base_crng, overwriting the old one,
@@ -900,6 +891,25 @@ static void extract_entropy(void *buf, size_t nbytes)
 	memzero_explicit(&block, sizeof(block));
 }
 
+/*
+ * First we make sure we have POOL_MIN_BITS of entropy in the pool, and then we
+ * set the entropy count to zero (but don't actually touch any data). Only then
+ * can we extract a new key with extract_entropy().
+ */
+static bool drain_entropy(void *buf, size_t nbytes)
+{
+	unsigned int entropy_count;
+	do {
+		entropy_count = READ_ONCE(input_pool.entropy_count);
+		if (entropy_count < POOL_MIN_BITS)
+			return false;
+	} while (cmpxchg(&input_pool.entropy_count, entropy_count, 0) != entropy_count);
+	extract_entropy(buf, nbytes);
+	wake_up_interruptible(&random_write_wait);
+	kill_fasync(&fasync, SIGIO, POLL_OUT);
+	return true;
+}
+
 #define warn_unseeded_randomness(previous) \
 	_warn_unseeded_randomness(__func__, (void *)_RET_IP_, (previous))