crypto,fs: Separate out hkdf_extract() and hkdf_expand()

Separate out the HKDF functions into a separate module to
to make them available to other callers.
And add a testsuite to the module with test vectors
from RFC 5869 (and additional vectors for SHA384 and SHA512)
to ensure the integrity of the algorithm.

Signed-off-by: Hannes Reinecke <hare@kernel.org>
Acked-by: Eric Biggers <ebiggers@kernel.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Keith Busch <kbusch@kernel.org>

2 files changed, 16 insertions, 70 deletions

diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
index 5aff5934baa1..0ea56f588a4a 100644
--- a/fs/crypto/Kconfig
+++ b/fs/crypto/Kconfig
@@ -3,6 +3,7 @@ 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/hkdf.c b/fs/crypto/hkdf.c
index 5a384dad2c72..855a0f4b7318 100644
--- a/fs/crypto/hkdf.c
+++ b/fs/crypto/hkdf.c
@@ -1,9 +1,5 @@
 // SPDX-License-Identifier: GPL-2.0
 /*
- * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation
- * Function"), aka RFC 5869.  See also the original paper (Krawczyk 2010):
- * "Cryptographic Extraction and Key Derivation: The HKDF Scheme".
- *
  * This is used to derive keys from the fscrypt master keys.
  *
  * Copyright 2019 Google LLC
@@ -11,6 +7,7 @@
 
 #include <crypto/hash.h>
 #include <crypto/sha2.h>
+#include <crypto/hkdf.h>
 
 #include "fscrypt_private.h"
 
@@ -44,20 +41,6 @@
  * 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.
@@ -69,6 +52,7 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 		      unsigned int master_key_size)
 {
 	struct crypto_shash *hmac_tfm;
+	static const u8 default_salt[HKDF_HASHLEN];
 	u8 prk[HKDF_HASHLEN];
 	int err;
 
@@ -84,7 +68,8 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
 		goto err_free_tfm;
 	}
 
-	err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk);
+	err = hkdf_extract(hmac_tfm, master_key, master_key_size,
+			   default_salt, HKDF_HASHLEN, prk);
 	if (err)
 		goto err_free_tfm;
 
@@ -118,61 +103,21 @@ int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
 			u8 *okm, unsigned int okmlen)
 {
 	SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
-	u8 prefix[9];
-	unsigned int i;
+	u8 *full_info;
 	int err;
-	const u8 *prev = NULL;
-	u8 counter = 1;
-	u8 tmp[HKDF_HASHLEN];
-
-	if (WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN))
-		return -EINVAL;
 
+	full_info = kzalloc(infolen + 9, GFP_KERNEL);
+	if (!full_info)
+		return -ENOMEM;
 	desc->tfm = hkdf->hmac_tfm;
 
-	memcpy(prefix, "fscrypt\0", 8);
-	prefix[8] = context;
-
-	for (i = 0; i < okmlen; i += HKDF_HASHLEN) {
-
-		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);
+	memcpy(full_info, "fscrypt\0", 8);
+	full_info[8] = context;
+	memcpy(full_info + 9, info, infolen);
+
+	err = hkdf_expand(hkdf->hmac_tfm, full_info, infolen + 9,
+			  okm, okmlen);
+	kfree_sensitive(full_info);
 	return err;
 }