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

From: Alexander Graf
Date: Wed Aug 23 2023 - 06:37:41 EST


On 23.08.23 12:25, Greg KH wrote:
On Wed, Aug 23, 2023 at 12:08:35PM +0200, Babis Chalios wrote:

On 23/8/23 12:06, Greg KH wrote:
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On Wed, Aug 23, 2023 at 11:27:11AM +0200, Babis Chalios wrote:
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time cached entropy is no longer valid, hence PRNGs need to reseed.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?
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.


There should be 2 use cases of this that I'm aware of:

  * Virtual machine clones. Most 64bit VMs can execute 32bit user space.
  * Bare metal rng time-to-live. An easy mechanism to tell every PRNG in the system to reseed every 5 minutes. This applies to all architectures Linux supports.


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


The ioctl is only to create a file descriptor that you can use to mmap() a shared page between kernel and user space which you can then atomically access to understand if you're in the old epoch (keep using previous RNG values) or in the new epoch (discard any old cached RNG values).


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.


I agree, but we're really not really bit constrained for this value and by making it 32bit always, we can guarantee that there will never be muckery for 32-on-64 compatibility.


Alex





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