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

On Thu, 5 Nov 2020 16:29:44 +0100, 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.

[...]

Applied to arm64 (for-next/random), thanks!

[1/1] random: avoid arch_get_random_seed_long() when collecting IRQ randomness
      https://git.kernel.org/arm64/c/390596c9959c

Ted -- please shout if you would prefer me not to carry this in the arm64
tree. I've haven't seen a response from you on this thread, but this patch
is currently blocking support for the TRNG firmware call on arm64 [1], so
I've pulled it in as a dependency. The branch above is stable, so you can
pull it in as well if necessary.

[1] https://lore.kernel.org/r/20210106103453.152275-1-andre.przywara@arm.com (local)

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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