Re: [PATCH 5/5] fs: use HKDF implementation from kernel crypto API

Stephan Mueller
2021-01-07T07:53:04Z

Am Mittwoch, dem 06.01.2021 um 23:19 -0800 schrieb Eric Biggers:
> On Mon, Jan 04, 2021 at 10:50:49PM +0100, Stephan Müller wrote:
> > As the kernel crypto API implements HKDF, replace the
> > file-system-specific HKDF implementation with the generic HKDF
> > implementation.
> >
> > Signed-off-by: Stephan Mueller <smueller@chronox.de>
> > ---
> >  fs/crypto/Kconfig           |   2 +-
> >  fs/crypto/fscrypt_private.h |   4 +-
> >  fs/crypto/hkdf.c            | 108 +++++++++---------------------------
> >  3 files changed, 30 insertions(+), 84 deletions(-)
> >
> > diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
> > index a5f5c30368a2..9450e958f1d1 100644
> > --- a/fs/crypto/Kconfig
> > +++ b/fs/crypto/Kconfig
> > @@ -2,7 +2,7 @@
> >  config FS_ENCRYPTION
> >         bool "FS Encryption (Per-file encryption)"
> >         select CRYPTO
> > -       select CRYPTO_HASH
> > +       select CRYPTO_HKDF
> >         select CRYPTO_SKCIPHER
> >         select CRYPTO_LIB_SHA256
> >         select KEYS
> > diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
> > index 3fa965eb3336..0d6871838099 100644
> > --- a/fs/crypto/fscrypt_private.h
> > +++ b/fs/crypto/fscrypt_private.h
> > @@ -304,7 +304,7 @@ struct fscrypt_hkdf {
> >         struct crypto_shash *hmac_tfm;
> >  };
> >
> > -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> > +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
> >                       unsigned int master_key_size);
>
> It shouldn't be necessary to remove const here.

Unfortunately it is when adding the pointer to struct kvec
>
> >
> >  /*
> > @@ -323,7 +323,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const
> > u8 *master_key,
> >  #define HKDF_CONTEXT_INODE_HASH_KEY    7 /* info=<empty>               */
> >
> >  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> > -                       const u8 *info, unsigned int infolen,
> > +                       u8 *info, unsigned int infolen,
> >                         u8 *okm, unsigned int okmlen);
>
> Likewise.  In fact some callers rely on 'info' not being modified.

Same here.
>
> > -/*
> > + *
> >   * Compute HKDF-Extract using the given master key as the input keying
> > material,
> >   * and prepare an HMAC transform object keyed by the resulting
> > pseudorandom key.
> >   *
> >   * Afterwards, the keyed HMAC transform object can be used for HKDF-
> > Expand many
> >   * times without having to recompute HKDF-Extract each time.
> >   */
> > -int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
> > +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, u8 *master_key,
> >                       unsigned int master_key_size)
> >  {
> > +       /* HKDF-Extract (RFC 5869 section 2.2), unsalted */
> > +       const struct kvec seed[] = { {
> > +               .iov_base = NULL,
> > +               .iov_len = 0
> > +       }, {
> > +               .iov_base = master_key,
> > +               .iov_len = master_key_size
> > +       } };
> >         struct crypto_shash *hmac_tfm;
> > -       u8 prk[HKDF_HASHLEN];
> >         int err;
> >
> >         hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0);
> > @@ -74,16 +65,12 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const
> > u8 *master_key,
> >                 return PTR_ERR(hmac_tfm);
> >         }
> >
> > -       if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
> > +       if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != HKDF_HASHLEN)) {
> >                 err = -EINVAL;
> >                 goto err_free_tfm;
> >         }
> >
> > -       err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
> > -       if (err)
> > -               goto err_free_tfm;
> > -
> > -       err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
> > +       err = crypto_hkdf_setkey(hmac_tfm, seed, ARRAY_SIZE(seed));
> >         if (err)
> >                 goto err_free_tfm;
>
> It's weird that the salt and key have to be passed in a kvec.
> Why not just have normal function parameters like:
>
>         int crypto_hkdf_setkey(struct crypto_shash *hmac_tfm,
>                                const u8 *key, size_t keysize,
>                                const u8 *salt, size_t saltsize);

I wanted to have an identical interface for all types of KDFs to allow turning
them into a template eventually. For example, SP800-108 KDFs only have one
parameter. Hence the use of a kvec.

>
> >  int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
> > -                       const u8 *info, unsigned int infolen,
> > +                       u8 *info, unsigned int infolen,
> >                         u8 *okm, unsigned int okmlen)
> >  {
> > -       SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
> > -       u8 prefix[9];
> > -       unsigned int i;
> > -       int err;
> > -       const u8 *prev = NULL;
> > -       u8 counter = 1;
> > -       u8 tmp[HKDF_HASHLEN];
> > -
> > -       if (WARN_ON(okmlen > 255 * HKDF_HASHLEN))
> > -               return -EINVAL;
> > -
> > -       desc->tfm = hkdf->hmac_tfm;
> > -
> > -       memcpy(prefix, "fscrypt\0", 8);
> > -       prefix[8] = context;
> > -
> > -       for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
> > +       const struct kvec info_iov[] = { {
> > +               .iov_base = "fscrypt\0",
> > +               .iov_len = 8,
> > +       }, {
> > +               .iov_base = &context,
> > +               .iov_len = 1,
> > +       }, {
> > +               .iov_base = info,
> > +               .iov_len = infolen,
> > +       } };
> > +       int err = crypto_hkdf_generate(hkdf->hmac_tfm,
> > +                                      info_iov, ARRAY_SIZE(info_iov),
> > +                                      okm, okmlen);
> >
> > -               err = crypto_shash_init(desc);
> > -               if (err)
> > -                       goto out;
> > -
> > -               if (prev) {
> > -                       err = crypto_shash_update(desc, prev,
> > HKDF_HASHLEN);
> > -                       if (err)
> > -                               goto out;
> > -               }
> > -
> > -               err = crypto_shash_update(desc, prefix, sizeof(prefix));
> > -               if (err)
> > -                       goto out;
> > -
> > -               err = crypto_shash_update(desc, info, infolen);
> > -               if (err)
> > -                       goto out;
> > -
> > -               BUILD_BUG_ON(sizeof(counter) != 1);
> > -               if (okmlen - i < HKDF_HASHLEN) {
> > -                       err = crypto_shash_finup(desc, &counter, 1, tmp);
> > -                       if (err)
> > -                               goto out;
> > -                       memcpy(&okm[i], tmp, okmlen - i);
> > -                       memzero_explicit(tmp, sizeof(tmp));
> > -               } else {
> > -                       err = crypto_shash_finup(desc, &counter, 1,
> > &okm[i]);
> > -                       if (err)
> > -                               goto out;
> > -               }
> > -               counter++;
> > -               prev = &okm[i];
> > -       }
> > -       err = 0;
> > -out:
> >         if (unlikely(err))
> >                 memzero_explicit(okm, okmlen); /* so caller doesn't need
> > to */
> > -       shash_desc_zero(desc);
>
> Shouldn't crypto_hkdf_generate() handle the above memzero_explicit() of the
> output buffer on error, so that all callers don't need to do it?

Yes, I will move it to HKDF (and the SP800-108 KDF as well).

Thanks for the review
Stephan
>
> - Eric