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

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Message-ID: <ff6aa77e-4e5c-488e-bd45-319fc09720c3@amazon.es>
Date:   Wed, 23 Aug 2023 11:27:11 +0200
From:   Babis Chalios <bchalios@...zon.es>
To:     Greg KH <gregkh@...uxfoundation.org>
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

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 time cached entropy is no longer valid, hence PRNGs need to reseed.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

Cheers,
Babis