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    Subject[PATCH 4.14 069/237] random: avoid superfluous call to RDRAND in CRNG extraction
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit 2ee25b6968b1b3c66ffa408de23d023c1bce81cf upstream.

    RDRAND is not fast. RDRAND is actually quite slow. We've known this for
    a while, which is why functions like get_random_u{32,64} were converted
    to use batching of our ChaCha-based CRNG instead.

    Yet CRNG extraction still includes a call to RDRAND, in the hot path of
    every call to get_random_bytes(), /dev/urandom, and getrandom(2).

    This call to RDRAND here seems quite superfluous. CRNG is already
    extracting things based on a 256-bit key, based on good entropy, which
    is then reseeded periodically, updated, backtrack-mutated, and so
    forth. The CRNG extraction construction is something that we're already
    relying on to be secure and solid. If it's not, that's a serious
    problem, and it's unlikely that mixing in a measly 32 bits from RDRAND
    is going to alleviate things.

    And in the case where the CRNG doesn't have enough entropy yet, we're
    already initializing the ChaCha key row with RDRAND in
    crng_init_try_arch_early().

    Removing the call to RDRAND improves performance on an i7-11850H by
    370%. In other words, the vast majority of the work done by
    extract_crng() prior to this commit was devoted to fetching 32 bits of
    RDRAND.

    Reviewed-by: Theodore Ts'o <tytso@mit.edu>
    Acked-by: Ard Biesheuvel <ardb@kernel.org>
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/char/random.c | 4 +---
    1 file changed, 1 insertion(+), 3 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -1022,7 +1022,7 @@ static void crng_reseed(struct crng_stat
    static void _extract_crng(struct crng_state *crng,
    __u32 out[CHACHA20_BLOCK_WORDS])
    {
    - unsigned long v, flags, init_time;
    + unsigned long flags, init_time;

    if (crng_ready()) {
    init_time = READ_ONCE(crng->init_time);
    @@ -1032,8 +1032,6 @@ static void _extract_crng(struct crng_st
    &input_pool : NULL);
    }
    spin_lock_irqsave(&crng->lock, flags);
    - if (arch_get_random_long(&v))
    - crng->state[14] ^= v;
    chacha20_block(&crng->state[0], out);
    if (crng->state[12] == 0)
    crng->state[13]++;

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