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

Eric Biggers
2019-01-09T17:35:00Z

On Wed, Jan 09, 2019 at 11:17:45AM +0100, Stephan Mueller wrote:
> Am Mittwoch, 9. Januar 2019, 09:21:04 CET schrieb Eric Biggers:
>
> Hi Eric,
> >
> > 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.)
>
> Thanks for the clarification. I have started a generic HKDF implementation for
> the kernel crypto API which lead to the questions above. I would then also try
> to provide a HKDF proposal.
>
> To use the (H)KDF, I currently envision 2 calls apart from alloc/free. The
> following code would serve as an example.
>
> * Example without proper error handling:
> * char *keying_material = "\x00\x11\x22\x33\x44\x55\x66\x77";
> * char *label_context = "\xde\xad\xbe\xef\x00\xde\xad\xbe\xef";
> * kdf = crypto_alloc_rng(name, 0, 0);
> * crypto_rng_reset(kdf, keying_material, 8);
> * crypto_rng_generate(kdf, label_context, 9, outbuf, outbuflen);
>
> That hopefully should be simple enough.
>
> For HKDF, as mentioned, I would envision to use a struct instead of a char *
> for the label_context to communicate IKM, Salt, and the label/info
> information.
>
> Ciao
> Stephan
>

That would not meet my performance requirements as I want to precompute
HKDF-Extract, and then do HKDF-Expand many times. Also the HKDF-Expand part
should be thread-safe and not require allocating memory, especially not a whole
crypto_shash tfm every time.

So presumably with crypto_rng, crypto_rng_reset() would need to take the input
keyring material and salt and do HKDF-Extract (like my fscrypt_init_hkdf()), and
crypto_rng_generate() would need to take the application-specific info string
and do HKDF-Expand (like my fscrypt_hkdf_expand()).

It is ugly though. Please also consider just having simple crypto_hkdf_*()
helper functions which wrap a HMAC tfm along the lines of my patch, rather than
shoehorning it into the crypto_rng API.

- Eric