Re: [PATCH 3/4] dm crypt: switch to EBOIV crypto API template

[Gilad Ben-Yossef]

On Mon, Oct 26, 2020 at 9:04 PM Milan Broz <gmazyland@xxxxxxxxx> wrote:
>
>
>
> On 26/10/2020 19:39, Eric Biggers wrote:
> > On Mon, Oct 26, 2020 at 07:29:57PM +0100, Milan Broz wrote:
> >> On 26/10/2020 18:52, Eric Biggers wrote:
> >>> On Mon, Oct 26, 2020 at 03:04:46PM +0200, Gilad Ben-Yossef wrote:
> >>>> Replace the explicit EBOIV handling in the dm-crypt driver with calls
> >>>> into the crypto API, which now possesses the capability to perform
> >>>> this processing within the crypto subsystem.
> >>>>
> >>>> Signed-off-by: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
> >>>>
> >>>> ---
> >>>>  drivers/md/Kconfig    |  1 +
> >>>>  drivers/md/dm-crypt.c | 61 ++++++++++++++-----------------------------
> >>>>  2 files changed, 20 insertions(+), 42 deletions(-)
> >>>>
> >>>> diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
> >>>> index 30ba3573626c..ca6e56a72281 100644
> >>>> --- a/drivers/md/Kconfig
> >>>> +++ b/drivers/md/Kconfig
> >>>> @@ -273,6 +273,7 @@ config DM_CRYPT
> >>>>    select CRYPTO
> >>>>    select CRYPTO_CBC
> >>>>    select CRYPTO_ESSIV
> >>>> +  select CRYPTO_EBOIV
> >>>>    help
> >>>>      This device-mapper target allows you to create a device that
> >>>>      transparently encrypts the data on it. You'll need to activate
> >>>
> >>> Can CRYPTO_EBOIV please not be selected by default?  If someone really wants
> >>> Bitlocker compatibility support, they can select this option themselves.
> >>
> >> Please no! Until this move of IV to crypto API, we can rely on
> >> support in dm-crypt (if it is not supported, it is just a very old kernel).
> >> (Actually, this was the first thing I checked in this patchset - if it is
> >> unconditionally enabled for compatibility once dmcrypt is selected.)
> >>
> >> People already use removable devices with BitLocker.
> >> It was the whole point that it works out-of-the-box without enabling anything.
> >>
> >> If you insist on this to be optional, please better keep this IV inside dmcrypt.
> >> (EBOIV has no other use than for disk encryption anyway.)
> >>
> >> Or maybe another option would be to introduce option under dm-crypt Kconfig that
> >> defaults to enabled (like support for foreign/legacy disk encryption schemes) and that
> >> selects these IVs/modes.
> >> But requiring some random switch in crypto API will only confuse users.
> >
> > CONFIG_DM_CRYPT can either select every weird combination of algorithms anyone
> > can ever be using, or it can select some defaults and require any other needed
> > algorithms to be explicitly selected.
> >
> > In reality, dm-crypt has never even selected any particular block ciphers, even
> > AES.  Nor has it ever selected XTS.  So it's actually always made users (or
> > kernel distributors) explicitly select algorithms.  Why the Bitlocker support
> > suddenly different?
> >
> > I'd think a lot of dm-crypt users don't want to bloat their kernels with random
> > legacy algorithms.
>
> Yes, but IV is in reality not a cryptographic algorithm, it is kind-of a configuration
> "option" of sector encryption mode here.
>
> We had all of disk-IV inside dmcrypt before - but once it is partially moved into crypto API
> (ESSIV, EBOIV for now), it becomes much more complicated for user to select what he needs.
>
> I think we have no way to check that IV is available from userspace - it
> will report the same error as if block cipher is not available, not helping user much
> with the error.
>
> But then I also think we should add abstract dm-crypt options here (Legacy TrueCrypt modes,
> Bitlocker modes) that will select these crypto API configuration switches.
> Otherwise it will be only a complicated matrix of crypto API options...

hm... just thinking out loud, but maybe the right say to go is to not
have a build dependency,
but add some user assistance code in cryptosetup that parses
/proc/crypto after failures to
try and suggest the user with a way forward?

e.g. if eboiv mapping initiation fails, scan /proc/crypto and either
warn of a lack of AES
or, assuming some instance of AES is found, warn of lack of EBOIV.
It's a little messy
and heuristic code for sure, but it lives in a user space utility.

Does that sound sane?
-- 
Gilad Ben-Yossef
Chief Coffee Drinker

values of β will give rise to dom!