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

[Babis Chalios]

Hi Greg,

On 23/8/23 11:08, Greg KH wrote:
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> 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 time cached entropy is no longer valid, hence PRNGs need to reseed.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?

We made it 32 bits so that we can read/write it atomically in all 32bit 
architectures.
Do you think that's not a problem?

Cheers,
Babis