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    Subject[PATCH 4.19 123/234] random: skip fast_init if hwrng provides large chunk of entropy
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit af704c856e888fb044b058d731d61b46eeec499d upstream.

    At boot time, EFI calls add_bootloader_randomness(), which in turn calls
    add_hwgenerator_randomness(). Currently add_hwgenerator_randomness()
    feeds the first 64 bytes of randomness to the "fast init"
    non-crypto-grade phase. But if add_hwgenerator_randomness() gets called
    with more than POOL_MIN_BITS of entropy, there's no point in passing it
    off to the "fast init" stage, since that's enough entropy to bootstrap
    the real RNG. The "fast init" stage is just there to provide _something_
    in the case where we don't have enough entropy to properly bootstrap the
    RNG. But if we do have enough entropy to bootstrap the RNG, the current
    logic doesn't serve a purpose. So, in the case where we're passed
    greater than or equal to POOL_MIN_BITS of entropy, this commit makes us
    skip the "fast init" phase.

    Cc: Dominik Brodowski <linux@dominikbrodowski.net>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1120,7 +1120,7 @@ void rand_initialize_disk(struct gendisk
    void add_hwgenerator_randomness(const void *buffer, size_t count,
    size_t entropy)
    {
    - if (unlikely(crng_init == 0)) {
    + if (unlikely(crng_init == 0 && entropy < POOL_MIN_BITS)) {
    size_t ret = crng_pre_init_inject(buffer, count, true);
    mix_pool_bytes(buffer, ret);
    count -= ret;

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