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

[Ard Biesheuvel]

On Wed, 11 Nov 2020 at 10:45, André Przywara <andre.przywara at arm.com> wrote:
>
> On 11/11/2020 08:19, Ard Biesheuvel wrote:
>
> Hi,
>
> > (+ Eric)
> >
> > On Thu, 5 Nov 2020 at 16:29, Ard Biesheuvel <ardb at 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.
>
> May I ask why this should be a particular problem? Form what I gathered
> on the web, it seems like most h/w RNGs have a capacity of multiple
> MBit/s. Wikipedia [1] suggests that the x86 CPU instructions generate at
> least 20 Mbit/s (worst case: AMD's 2500 cycles @ 800 MHz), and I
> measured around 78 Mbit/s with the raw entropy source on my Juno
> (possibly even limited by slow MMIO).
> So it seems unlikely that a few kbit/s drain the hardware entropy source.
>
> If we consider this interface comparably cheap, should we then not try
> to plug the Arm firmware interface into this?
>

I'm not sure I follow. Are you saying we should not wire up a
comparatively expensive firmware interface to
arch_get_random_seed_long() because we currently assume it is backed
by something cheap?

Because doing so would add significantly to the cost. Also note that a
firmware interface would permit other ways of gathering entropy that
are not necessarily backed by a dedicated high bandwidth noise source
(and we already have examples of this)


> I am not against this patch, actually am considering this a nice
> cleanup, to separate interrupt generated entropy from other sources.
> Especially since we call arch_get_random_seed_long() under a spinlock here.
> But I am curious about the expectations from arch_get_random in general.
>

I think it is reasonable to clean this up a little bit. A random
*seed* is not the same thing as a random number, and given that we
expose both interfaces, it makes sense to permit the seed variant to
be more costly, and only use it as intended (i.e., to seed a random
number generator)

> >> 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.
>
> So I tested this and it works as expected: I see some calls on
> initialisation, then a handful of calls every few seconds from the
> periodic reseeding. The large number of calls every second are gone.
>

Excellent, thanks for confirming.

> >>
> >> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
>
> Since the above questions are unrelated to this particular patch:
>
> Reviewed-by: Andre Przywara <andre.przywara at arm.com>
> Tested-by: Andre Przywara <andre.przywara at arm.com>
>
> Cheers,
> Andre
>
> [1] https://en.wikipedia.org/wiki/RDRAND#Performance
>
> >> ---
> >>  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
> >>
>