Messages in this thread |  | | From | Ard Biesheuvel <> | Date | Fri, 27 Nov 2020 13:08:37 +0100 | Subject | Re: [PATCH] random: avoid arch_get_random_seed_long() when collecting IRQ randomness |
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On Fri, 20 Nov 2020 at 16:27, Marc Zyngier <maz@kernel.org> wrote: > > On 2020-11-05 15:29, Ard Biesheuvel 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> > > Looks sensible. Having this on the interrupt path looks quite > heavy handed, and my understanding of the above is that it has > an adverse effect on the entropy pool. > > Acked-by: Marc Zyngier <maz@kernel.org> >
Thanks Marc.
Ted, any thoughts?
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