linux-kernel - Re: [PATCH RFC v1 1/2] random: add mechanism for VM forks to reinitialize crng

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Date:   Wed, 23 Feb 2022 17:27:41 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        qemu-devel@...gnu.org, kvm@...r.kernel.org,
        linux-s390@...r.kernel.org, adrian@...ity.io, dwmw@...zon.co.uk,
        acatan@...zon.com, graf@...zon.com, colmmacc@...zon.com,
        sblbir@...zon.com, raduweis@...zon.com, jannh@...gle.com,
        gregkh@...uxfoundation.org, tytso@....edu
Subject: Re: [PATCH RFC v1 1/2] random: add mechanism for VM forks to
 reinitialize crng

On Thu, Feb 24, 2022 at 01:54:54AM +0100, Jason A. Donenfeld wrote:
> On 2/24/22, Eric Biggers <ebiggers@...nel.org> wrote:
> > I think we should be removing cases where the base_crng key is changed
> > directly
> > besides extraction from the input_pool, not adding new ones.  Why not
> > implement
> > this as add_device_randomness() followed by crng_reseed(force=true), where
> > the
> > 'force' argument forces a reseed to occur even if the entropy_count is too
> > low?
> 
> Because that induces a "premature next" condition which can let that
> entropy, potentially newly acquired by a storm of IRQs at power-on, be
> bruteforced by unprivileged userspace. I actually had it exactly the
> way you describe at first, but decided that this here is the lesser of
> evils and doesn't really complicate things the way an intentional
> premature next would. The only thing we care about here is branching
> the crng stream, and so this does explicitly that, without having to
> interfere with how we collect entropy. Of course we *also* add it as
> non-credited "device randomness" so that it's part of the next
> reseeding, whenever that might occur.

Can you make sure to properly explain this in the code?

- Eric