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

Greg KH
2023-08-23T10:25:56Z

On Wed, Aug 23, 2023 at 12:08:35PM +0200, Babis Chalios wrote:
>
>
> On 23/8/23 12:06, Greg KH wrote:
> > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> >
> >
> >
> > On Wed, Aug 23, 2023 at 11:27:11AM +0200, Babis Chalios wrote:
> > > Hi Greg,
> > >
> > > On 23/8/23 11:08, Greg KH wrote:
> > > > CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
> > > >
> > > >
> > > >
> > > > 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 time cached entropy is no longer valid, hence PRNGs need to reseed.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?
> > What 32bit platforms care about this type of interface at all?
>
> I think, any 32bit platform that gets random bytes from the kernel.

You are making a new api, for some new functionality, for what I thought
was virtual machines (hence the virtio driver), none of which work in a
32bit system.

I thought this was an ioctl for userspace, which can handle 64bits at
once (or 2 32bit numbers).

For internal kernel stuff, a lock should be fine, or better yet, a 64bit
atomic value read (horrible on 32bit platforms, I know...)

Just asking, it feels odd to pack bits in these days for when 90% of the
cpus really don't need it.

greg k-h