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

Babis Chalios
2023-08-23T09:52:10Z

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 time cached entropy is no longer valid, hence PRNGs need to reseed.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