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

[André Przywara]

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 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.

>> 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.

>>
>> 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
>>