Re: [RFC PATCH 1/2] random: emit reseed notifications for PRNGs

Greg KH
2023-08-23T09:21:02Z

On Wed, Aug 23, 2023 at 11:01:05AM +0200, Babis Chalios wrote:
> Sometimes, PRNGs need to reseed. For example, on a regular timer
> interval, to ensure nothing consumes a random value for longer than e.g.
> 5 minutes, or when VMs get cloned, to ensure seeds don't leak in to
> clones.
>
> The notification happens through a 32bit epoch value that changes every
> time cached entropy is no longer valid, hence PRNGs need to reseed. User
> space applications can get hold of a pointer to this value through
> /dev/(u)random. We introduce a new ioctl() that returns an anonymous
> file descriptor. From this file descriptor we can mmap() a single page
> which includes the epoch at offset 0.
>
> random.c maintains the epoch value in a global shared page. It exposes
> a registration API for kernel subsystems that are able to notify when
> reseeding is needed. Notifiers register with random.c and receive a
> unique 8bit ID and a pointer to the epoch. When they need to report a
> reseeding event they write a new epoch value which includes the
> notifier ID in the first 8 bits and an increasing counter value in the
> remaining 24 bits:
>
> RNG epoch
> *-------------*---------------------*
> | notifier id | epoch counter value |
> *-------------*---------------------*
> 8 bits 24 bits

Why not just use 32/32 for a full 64bit value, or better yet, 2
different variables? Why is 32bits and packing things together here
somehow simpler?

thanks,

greg k-h