From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Ard Biesheuvel <ardb@kernel.org>
Cc:     Hannes Frederic Sowa <hannes@stressinduktion.org>,
        netdev@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@gmail.com>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Erik Kline <ek@google.com>,
        Fernando Gont <fgont@si6networks.com>,
        Lorenzo Colitti <lorenzo@google.com>,
        YOSHIFUJI Hideaki <hideaki.yoshifuji@miraclelinux.com>
Subject: Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address
 calculation
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Date:   Thu, 13 Jan 2022 17:18:15 +0100
Message-ID: <878rvjfw3c.fsf@toke.dk>
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"Jason A. Donenfeld" <Jason@zx2c4.com> writes:

> On Thu, Jan 13, 2022 at 2:50 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>> > Then, at some point down the road, we can talk about removing
>> > CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH too.
>> >
>>
>> What is the point of having CONFIG_OLD_N_CRUSTY if all distros are
>> going to enable it indefinitely?
>
> I think there's probably some combination of
> CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH and CONFIG_OLD_N_CRUSTY and
> maybe even a CONFIG_GOD_MURDERS_KITTENS that might be sufficiently
> disincentivizing? Or this ties into other general ideas on a gradual
> obsolescence->removal flow for things.

Making it a compile-time switch doesn't really solve anything, though.
It'll need to be a runtime switch for people to be able to opt-in to the
new behaviour; otherwise there would still be a flag day when
distributions switch on the new config option.

I don't think there's any reason to offload this decision on
distributions either: there's clearly a "best option" here, absent any
backwards compatibility concerns. So it's on us to design a proper
transition mechanism. Defaulting to SHA1 when stable_secret is set, as
Ard suggested, sounds like a reasonable default; then we only need a
single new value for addr_gen_mode to opt-in to using blake2s even when
setting the stable_secret.

-Toke