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    Subject[PATCH 5.15 063/145] random: introduce drain_entropy() helper to declutter crng_reseed()
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    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);

    @@ -456,23 +457,13 @@ 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,
    @@ -900,6 +891,25 @@ 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))


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