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    SubjectRe: [PATCH] random: avoid arch_get_random_seed_long() when collecting IRQ randomness
    (+ Eric)

    On Thu, 5 Nov 2020 at 16:29, Ard Biesheuvel <ardb@kernel.org> wrote:
    >
    > When reseeding the CRNG periodically, arch_get_random_seed_long() is
    > called to obtain entropy from an architecture specific source if one
    > is implemented. In most cases, these are special instructions, but in
    > some cases, such as on ARM, we may want to back this using firmware
    > calls, which are considerably more expensive.
    >
    > Another call to arch_get_random_seed_long() exists in the CRNG driver,
    > in add_interrupt_randomness(), which collects entropy by capturing
    > inter-interrupt timing and relying on interrupt jitter to provide
    > random bits. This is done by keeping a per-CPU state, and mixing in
    > the IRQ number, the cycle counter and the return address every time an
    > interrupt is taken, and mixing this per-CPU state into the entropy pool
    > every 64 invocations, or at least once per second. The entropy that is
    > gathered this way is credited as 1 bit of entropy. Every time this
    > happens, arch_get_random_seed_long() is invoked, and the result is
    > mixed in as well, and also credited with 1 bit of entropy.
    >
    > This means that arch_get_random_seed_long() is called at least once
    > per second on every CPU, which seems excessive, and doesn't really
    > scale, especially in a virtualization scenario where CPUs may be
    > oversubscribed: in cases where arch_get_random_seed_long() is backed
    > by an instruction that actually goes back to a shared hardware entropy
    > source (such as RNDRRS on ARM), we will end up hitting it hundreds of
    > times per second.
    >
    > So let's drop the call to arch_get_random_seed_long() from
    > add_interrupt_randomness(), and instead, rely on crng_reseed() to call
    > the arch hook to get random seed material from the platform.
    >
    > Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
    > ---
    > drivers/char/random.c | 15 +--------------
    > 1 file changed, 1 insertion(+), 14 deletions(-)
    >
    > diff --git a/drivers/char/random.c b/drivers/char/random.c
    > index 2a41b21623ae..a9c393c1466d 100644
    > --- a/drivers/char/random.c
    > +++ b/drivers/char/random.c
    > @@ -1261,8 +1261,6 @@ void add_interrupt_randomness(int irq, int irq_flags)
    > cycles_t cycles = random_get_entropy();
    > __u32 c_high, j_high;
    > __u64 ip;
    > - unsigned long seed;
    > - int credit = 0;
    >
    > if (cycles == 0)
    > cycles = get_reg(fast_pool, regs);
    > @@ -1298,23 +1296,12 @@ void add_interrupt_randomness(int irq, int irq_flags)
    >
    > fast_pool->last = now;
    > __mix_pool_bytes(r, &fast_pool->pool, sizeof(fast_pool->pool));
    > -
    > - /*
    > - * If we have architectural seed generator, produce a seed and
    > - * add it to the pool. For the sake of paranoia don't let the
    > - * architectural seed generator dominate the input from the
    > - * interrupt noise.
    > - */
    > - if (arch_get_random_seed_long(&seed)) {
    > - __mix_pool_bytes(r, &seed, sizeof(seed));
    > - credit = 1;
    > - }
    > spin_unlock(&r->lock);
    >
    > fast_pool->count = 0;
    >
    > /* award one bit for the contents of the fast pool */
    > - credit_entropy_bits(r, credit + 1);
    > + credit_entropy_bits(r, 1);
    > }
    > EXPORT_SYMBOL_GPL(add_interrupt_randomness);
    >
    > --
    > 2.17.1
    >

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