Re: [PATCH] random: invalidate crngs and batches on cpuhp teardown

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-02-14 14:48:38 [+0100], Jason A. Donenfeld wrote:
> Now that we have a cpuhp teardown notifier, we can invalidate the keys
> used by the per-cpu crngs and the batches used by per-cpu batched
> entropy, so that if the cpus come back online, and the generation
> counter happens to have cycled all the way around to where it was
> before, it doesn't mistakenly use the old data. The chances of this
> happening are exceedingly rare, but since we now have the notifier
> setup, doing this is basically free.

Wasn't aware that random bits get bad over time ;)

> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: Sultan Alsawaf <sultan@kerneltoast.com>
> Cc: Dominik Brodowski <linux@dominikbrodowski.net>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  drivers/char/random.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index df5aef93da34..ce199af9bc56 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1225,6 +1225,15 @@ int random_dead_cpu(unsigned int cpu)
>  	 * since the MIX_INFLIGHT flag will be cleared.
>  	 */
>  	per_cpu_ptr(&irq_randomness, cpu)->count = 0;
> +
> +	/*
> +	 * We also want to invalidate per-cpu crngs and batches,
> +	 * so that if the CPU does come back online, it uses
> +	 * fresh entropy.
> +	 */
> +	per_cpu_ptr(&crngs, cpu)->generation = ULONG_MAX;
> +	per_cpu_ptr(&batched_entropy_u32, cpu)->position = UINT_MAX;
> +	per_cpu_ptr(&batched_entropy_u64, cpu)->position = UINT_MAX;

I think if you want to do this, then it would also make sense to put it
into the startup callback. If there is an user doing get_random_u32()
then you would preload the "old" entropy. But on your way "online" you
would preload it with the new entropy.

>  	return 0;
>  }
>  

Sebastian