Messages in this thread |  | | Date | Fri, 20 Nov 2020 15:27:14 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH] random: avoid arch_get_random_seed_long() when collecting IRQ randomness |
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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>
M. -- Jazz is not dead. It just smells funny...
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