[PATCH 5.4 090/240] random: introduce drain_entropy() helper to declutter crng_reseed()

Greg Kroah-Hartman
2022-06-20T13:41:59Z

commit 246c03dd899164d0186b6d685d6387f228c28d93 upstream.

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: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/char/random.c | 36 +++++++++++++++++++++++-------------
1 file changed, 23 insertions(+), 13 deletions(-)

--- a/drivers/char/random.c
+++ b/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);

@@ -454,23 +455,13 @@ static void crng_slow_load(const void *c
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,
@@ -898,6 +889,25 @@ static void extract_entropy(void *buf, s
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))