Re: [PATCH 1/5 v2] PM / hibernate: Create snapshot keys handler

Eric Biggers
2019-01-09T08:21:11Z

On Wed, Jan 09, 2019 at 08:05:21AM +0100, Stephan Mueller wrote:
> Am Mittwoch, 9. Januar 2019, 07:58:28 CET schrieb James Bottomley:
>
> Hi James,
>
> > On Wed, 2019-01-09 at 07:45 +0100, Stephan Mueller wrote:
> > > Am Mittwoch, 9. Januar 2019, 01:44:31 CET schrieb James Bottomley:
> > >
> > > Hi James,
> > >
> > > > Actually, it would be enormously helpful if we could reuse these
> > > > pieces for the TPM as well.
> > >
> > > Could you please help me understand whether the KDFs in TPM are
> > > directly usable as a standalone cipher primitive or does it go
> > > together with additional key generation operations?
> >
> > They're used as generators ... which means they deterministically
> > produce keys from what the TPM calls seeds so we can get crypto agility
> > of TPM 2.0 ... well KDFa does. KDFe is simply what NIST recommends you
> > do when using EC for a shared key agreement ... and really we shouldn't
> > be using ECDH in the kernel without it.
> >
>
> Thanks for clarifying. That would mean that indeed we would have hardware-
> provided KDF implementations that may be usable with the kernel crypto API.
>
> [...]
> >
> > > Would it be appropriate, to implement a type cast to a structure from
> > > the u8 pointer?
> > >
> > > E.g. for the aforementioned label/context data, we could define
> > > something like
> > >
> > > struct crypto_kdf_ctr {
> > >
> > > char *label;
> > > size_t label_len;
> > > u8 *contextU;
> > > size_t contextU_len;
> > > u8 *contextV;
> > > size_t contextV_len;
> > >
> > > };
> > >
> > > And the implementation of the generate function for CTR KDF would
> > >
> > > then have a type cast along the following lines:
> > > if (slen != sizeof(struct crypto_kdf_ctr))
> > >
> > > return -EINVAL;
> > >
> > > const struct crypto_kdf_ctr *kdf_ctr_input = (struct
> > >
> > > crypto_kdf_ctr *)src;
> > >
> > >
> > > For different KDFs, different structs would be needed.
> >
> > Actually, we probably just need the input key (or secret material), the
> > concatenation and the number of output bits.
>
> Thanks for confirming. Though, when it comes to HKDF (not that I see it being
> needed or required in the kernel), there is a need to split up the src pointer
> since the mentioned input is used in different ways.
>
> In order to try to get the implementation and thus the interface right, I
> would suggest to at least have a consensus on how to handle such situations.
>
> Thus, would the proposal be acceptable for such KDFs that may need to have
> different components communicated as input to the KDF?
>

FWIW, it's been very slow going since I've been working on other projects and I
also need to be very sure to get the API changes right, but I still plan to
change the KDF in fscrypt (a.k.a. ext4/f2fs/ubifs encryption) to HKDF-SHA512 as
part of a larger set of improvements to how fscrypt encryption keys are managed.
I sent the last patchset a year ago
(https://marc.info/?l=linux-fsdevel&m=150879493206257) but I'm working to revive
it. In the work-in-progress version in my git tree, this is the commit that
adds a HKDF implementation as fs/crypto/hkdf.c:
https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/commit/?id=e8a78767131c9717ee838f0c4e307948d65a4427
It basically just wraps a crypto_shash for "hmac(sha512)".

I'd be fine with using a common implementation instead, provided that it gives
the same functionality, including supporting user-specified salt and
application-specific info strings, and isn't slower or more complex to use.

(This comment is solely on the tangential discussion about KDF implementations;
I've not looked at the hibernation image encryption stuff yet.)

- Eric