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

Stephan Müller
2021-01-04T21:55:59Z

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);

/*
@@ -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);

void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf);
diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c
index e0ec21055505..f837cb8ec0a5 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -9,7 +9,7 @@
* Copyright 2019 Google LLC
*/

-#include <crypto/hash.h>
+#include <crypto/hkdf.h>
#include <crypto/sha2.h>

#include "fscrypt_private.h"
@@ -37,34 +37,25 @@
* unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No
* salt is used, since fscrypt master keys should already be pseudorandom and
* there's no way to persist a random salt per master key from kernel mode.
- */
-
-/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
-static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
- unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
-{
- static const u8 default_salt[HKDF_HASHLEN];
- int err;
-
- err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
- if (err)
- return err;
-
- return crypto_shash_tfm_digest(hmac_tfm, ikm, ikmlen, prk);
-}
-
-/*
+ *
* 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;

@@ -93,7 +80,6 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
err_free_tfm:
crypto_free_shash(hmac_tfm);
out:
- memzero_explicit(prk, sizeof(prk));
return err;
}

@@ -109,65 +95,25 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
* accidentally repeat an info string when using HKDF for different purposes.)
*/
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);
return err;
}

--
2.26.2