Messages in this thread |  | | From | Ard Biesheuvel <> | Date | Wed, 11 Nov 2020 09:19:37 +0100 | Subject | Re: [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 >
|  |