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

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Date:   Wed, 23 Aug 2023 11:08:05 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Babis Chalios <bchalios@...zon.es>
Cc:     Olivia Mackall <olivia@...enic.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Theodore Ts'o <tytso@....edu>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, graf@...zon.de,
        xmarcalx@...zon.co.uk, aams@...zon.de, dwmw@...zon.co.uk
Subject: Re: [RFC PATCH 1/2] random: emit reseed notifications for PRNGs

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 time cached entropy is no longer valid, hence PRNGs need to reseed.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