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

Eric Biggers
2021-01-07T07:20:04Z

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.

>
> /*
> @@ -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.

> -/*
> + *
> * 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);

> 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?

- Eric