Re: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation

[Ard Biesheuvel]

On Thu, 13 Jan 2022 at 14:30, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
>
> Ard Biesheuvel <ardb@xxxxxxxxxx> writes:
>
> > On Thu, 13 Jan 2022 at 12:15, Hannes Frederic Sowa
> > <hannes@xxxxxxxxxxxxxxxxxxx> wrote:
> >>
> >> Hello,
> >>
> >> On 13.01.22 00:31, Jason A. Donenfeld wrote:
> >> > On 1/13/22, Toke Høiland-Jørgensen <toke@xxxxxxxxxx> wrote:
> >> >> However, if we make this change, systems setting a stable_secret and
> >> >> using addr_gen_mode 2 or 3 will come up with a completely different
> >> >> address after a kernel upgrade. Which would be bad for any operator
> >> >> expecting to be able to find their machine again after a reboot,
> >> >> especially if it is accessed remotely.
> >> >>
> >> >> I haven't ever used this feature myself, though, or seen it in use. So I
> >> >> don't know if this is purely a theoretical concern, or if the
> >> >> stable_address feature is actually used in this way in practice. If it
> >> >> is, I guess the switch would have to be opt-in, which kinda defeats the
> >> >> purpose, no (i.e., we'd have to keep the SHA1 code around
> >>
> >> Yes, it is hard to tell if such a change would have real world impact
> >> due to not knowing its actual usage in the field - but I would avoid
> >> such a change. The reason for this standard is to have stable addresses
> >> across reboots. The standard is widely used but most servers or desktops
> >> might get their stable privacy addresses being generated by user space
> >> network management systems (NetworkManager/networkd) nowadays. I would
> >> guess it could be used in embedded installations.
> >>
> >> The impact of this change could be annoying though: users could suddenly
> >> lose connectivity due to e.g. changes to the default gateway after an
> >> upgrade.
> >>
> >> > I'm not even so sure that's true. That was my worry at first, but
> >> > actually, looking at this more closely, DAD means that the address can
> >> > be changed anyway - a byte counter is hashed in - so there's no
> >> > gurantee there.
> >>
> >> The duplicate address detection counter is a way to merely provide basic
> >> network connectivity in case of duplicate addresses on the network
> >> (maybe some kind misconfiguration or L2 attack). Such detected addresses
> >> would show up in the kernel log and an administrator should investigate
> >> and clean up the situation. Afterwards bringing the interface down and
> >> up again should revert the interface to its initial (dad_counter == 0)
> >> address.
> >>
> >> > There's also the other aspect that open coding sha1_transform like
> >> > this and prepending it with the secret (rather than a better
> >> > construction) isn't so great... Take a look at the latest version of
> >> > this in my branch to see a really nice simplification and security
> >> > improvement:
> >> >
> >> > https://git.zx2c4.com/linux-dev/log/?h=remove-sha1
> >>
> >> All in all, I consider the hash produced here as being part of uAPI
> >> unfortunately and thus cannot be changed. It is unfortunate that it
> >> can't easily be improved (I assume a separate mode for this is not
> >> reasonable). The patches definitely look like a nice cleanup.
> >>
> >> Would this be the only user of sha_transform left?
> >>
> >
> > The question is not whether but when we can/will change this.
> >
> > SHA-1 is broken and should be removed at *some* point, so unless the
> > feature itself is going to be obsolete, its implementation will need
> > to switch to a PRF that fulfils the requirements in RFC7217 once SHA-1
> > ceases to do so.
> >
> > And I should also point out that the current implementation does not
> > even use SHA-1 correctly, as it omits the finalization step. This may
> > or may not matter in practice, but it deviates from crypto best
> > practices, as well as from RFC7217
>
> Right, but that implies we need to work on a transition mechanism. For
> newly deployed systems changing the hash is obviously fine, it's the
> "reboot and you have a new address" problem.
>
> We could introduce new values to the addr_gen_mode? I.e. values of 4 and
> 5 would be equivalent to 2 and 3 (respectively), but with the new
> hashing algorithm? And then document that 2 and 3 are considered
> deprecated to be removed at some point in the future...
>

I guess that for the time being, we could use assignments of
stable_secret by user space as a hint that we should switch to the old
scheme. We'd also need a knob to opt into the new scheme in that case,
and maybe print a warning otherwise? That would at least give us a
path forward where we can rip it out /some/ point in the future.