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    Subject[PATCH 4.19 115/234] random: do crng pre-init loading in worker rather than irq
    Date
    From: "Jason A. Donenfeld" <Jason@zx2c4.com>

    commit c2a7de4feb6e09f23af7accc0f882a8fa92e7ae5 upstream.

    Taking spinlocks from IRQ context is generally problematic for
    PREEMPT_RT. That is, in part, why we take trylocks instead. However, a
    spin_try_lock() is also problematic since another spin_lock() invocation
    can potentially PI-boost the wrong task, as the spin_try_lock() is
    invoked from an IRQ-context, so the task on CPU (random task or idle) is
    not the actual owner.

    Additionally, by deferring the crng pre-init loading to the worker, we
    can use the cryptographic hash function rather than xor, which is
    perhaps a meaningful difference when considering this data has only been
    through the relatively weak fast_mix() function.

    The biggest downside of this approach is that the pre-init loading is
    now deferred until later, which means things that need random numbers
    after interrupts are enabled, but before workqueues are running -- or
    before this particular worker manages to run -- are going to get into
    trouble. Hopefully in the real world, this window is rather small,
    especially since this code won't run until 64 interrupts had occurred.

    Cc: Sultan Alsawaf <sultan@kerneltoast.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Eric Biggers <ebiggers@kernel.org>
    Cc: Theodore Ts'o <tytso@mit.edu>
    Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Reviewed-by: 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 | 65 ++++++++++++++------------------------------------
    1 file changed, 19 insertions(+), 46 deletions(-)

    --- a/drivers/char/random.c
    +++ b/drivers/char/random.c
    @@ -441,10 +441,6 @@ static void crng_make_state(u32 chacha_s
    * boot time when it's better to have something there rather than
    * nothing.
    *
    - * There are two paths, a slow one and a fast one. The slow one
    - * hashes the input along with the current key. The fast one simply
    - * xors it in, and should only be used from interrupt context.
    - *
    * If account is set, then the crng_init_cnt counter is incremented.
    * This shouldn't be set by functions like add_device_randomness(),
    * where we can't trust the buffer passed to it is guaranteed to be
    @@ -453,19 +449,15 @@ static void crng_make_state(u32 chacha_s
    * Returns the number of bytes processed from input, which is bounded
    * by CRNG_INIT_CNT_THRESH if account is true.
    */
    -static size_t crng_pre_init_inject(const void *input, size_t len,
    - bool fast, bool account)
    +static size_t crng_pre_init_inject(const void *input, size_t len, bool account)
    {
    static int crng_init_cnt = 0;
    + struct blake2s_state hash;
    unsigned long flags;

    - if (fast) {
    - if (!spin_trylock_irqsave(&base_crng.lock, flags))
    - return 0;
    - } else {
    - spin_lock_irqsave(&base_crng.lock, flags);
    - }
    + blake2s_init(&hash, sizeof(base_crng.key));

    + spin_lock_irqsave(&base_crng.lock, flags);
    if (crng_init != 0) {
    spin_unlock_irqrestore(&base_crng.lock, flags);
    return 0;
    @@ -474,21 +466,9 @@ static size_t crng_pre_init_inject(const
    if (account)
    len = min_t(size_t, len, CRNG_INIT_CNT_THRESH - crng_init_cnt);

    - if (fast) {
    - const u8 *src = input;
    - size_t i;
    -
    - for (i = 0; i < len; ++i)
    - base_crng.key[(crng_init_cnt + i) %
    - sizeof(base_crng.key)] ^= src[i];
    - } else {
    - struct blake2s_state hash;
    -
    - blake2s_init(&hash, sizeof(base_crng.key));
    - blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));
    - blake2s_update(&hash, input, len);
    - blake2s_final(&hash, base_crng.key);
    - }
    + blake2s_update(&hash, base_crng.key, sizeof(base_crng.key));
    + blake2s_update(&hash, input, len);
    + blake2s_final(&hash, base_crng.key);

    if (account) {
    crng_init_cnt += len;
    @@ -1029,7 +1009,7 @@ void add_device_randomness(const void *b
    unsigned long flags, now = jiffies;

    if (crng_init == 0 && size)
    - crng_pre_init_inject(buf, size, false, false);
    + crng_pre_init_inject(buf, size, false);

    spin_lock_irqsave(&input_pool.lock, flags);
    _mix_pool_bytes(&cycles, sizeof(cycles));
    @@ -1150,7 +1130,7 @@ void add_hwgenerator_randomness(const vo
    size_t entropy)
    {
    if (unlikely(crng_init == 0)) {
    - size_t ret = crng_pre_init_inject(buffer, count, false, true);
    + size_t ret = crng_pre_init_inject(buffer, count, true);
    mix_pool_bytes(buffer, ret);
    count -= ret;
    buffer += ret;
    @@ -1290,8 +1270,14 @@ static void mix_interrupt_randomness(str
    fast_pool->last = jiffies;
    local_irq_enable();

    - mix_pool_bytes(pool, sizeof(pool));
    - credit_entropy_bits(1);
    + if (unlikely(crng_init == 0)) {
    + crng_pre_init_inject(pool, sizeof(pool), true);
    + mix_pool_bytes(pool, sizeof(pool));
    + } else {
    + mix_pool_bytes(pool, sizeof(pool));
    + credit_entropy_bits(1);
    + }
    +
    memzero_explicit(pool, sizeof(pool));
    }

    @@ -1324,24 +1310,11 @@ void add_interrupt_randomness(int irq)
    fast_mix(fast_pool->pool32);
    new_count = ++fast_pool->count;

    - if (unlikely(crng_init == 0)) {
    - if (new_count >= 64 &&
    - crng_pre_init_inject(fast_pool->pool32, sizeof(fast_pool->pool32),
    - true, true) > 0) {
    - fast_pool->count = 0;
    - fast_pool->last = now;
    - if (spin_trylock(&input_pool.lock)) {
    - _mix_pool_bytes(&fast_pool->pool32, sizeof(fast_pool->pool32));
    - spin_unlock(&input_pool.lock);
    - }
    - }
    - return;
    - }
    -
    if (new_count & MIX_INFLIGHT)
    return;

    - if (new_count < 64 && !time_after(now, fast_pool->last + HZ))
    + if (new_count < 64 && (!time_after(now, fast_pool->last + HZ) ||
    + unlikely(crng_init == 0)))
    return;

    if (unlikely(!fast_pool->mix.func))

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