On Mon, 4 Jan 2021 at 20:09, Ard Biesheuvel <a...@kernel.org> wrote: > > On Tue, 17 Nov 2020 at 14:33, Ard Biesheuvel <a...@kernel.org> wrote: > > > > On Wed, 11 Nov 2020 at 09:19, Ard Biesheuvel <a...@kernel.org> wrote: > > > > > > (+ Eric) > > > > > > On Thu, 5 Nov 2020 at 16:29, Ard Biesheuvel <a...@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 <a...@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 > > > > > > > > Ping? > > Ping?
Ping again? Ted, Acceptance of this patch is now blocking RNG related work that is in flight for ARM and arm64. [0] So please shout if you have any objections to this patch, or if you don't, please ack it so it can be taken through one of the ARM trees. Thanks, Ard. [0] https://lore.kernel.org/kvmarm/20210106103453.152275-1-andre.przyw...@arm.com/