superrob1500
/
mirror-linux
mirror of https://github.com/torvalds/linux
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

fscrypt: use HMAC-SHA512 library for HKDF 

For the HKDF-SHA512 key derivation needed by fscrypt, just use the
HMAC-SHA512 library functions directly.  These functions were introduced
in v6.17, and they provide simple and efficient direct support for
HMAC-SHA512.  This ends up being quite a bit simpler and more efficient
than using crypto/hkdf.c, as it avoids the generic crypto layer:

- The HMAC library can't fail, so callers don't need to handle errors
- No inefficient indirect calls
- No inefficient and error-prone dynamic allocations
- No inefficient and error-prone loading of algorithm by name
- Less stack usage

Benchmarks on x86_64 show that deriving a per-file key gets about 30%
faster, and FS_IOC_ADD_ENCRYPTION_KEY gets nearly twice as fast.

The only small downside is the HKDF-Expand logic gets duplicated again.
Then again, even considering that, the new fscrypt_hkdf_expand() is only
7 lines longer than the version that called hkdf_expand().  Later we
could add HKDF support to lib/crypto/, but for now let's just do this.

Link: https://lore.kernel.org/r/20250906035913.1141532-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
pull/1354/merge
Eric Biggers 2025-09-05 20:59:13 -07:00
parent 0e6608d493
commit 19591f7e78
8 changed files with 80 additions and 158 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
fs/crypto/Kconfig
View File

@ -2,10 +2,9 @@
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 CRYPTO_LIB_SHA512
select KEYS
help
Enable encryption of files and directories. This
@ -32,8 +31,6 @@ config FS_ENCRYPTION_ALGS
select CRYPTO_CBC
select CRYPTO_CTS
select CRYPTO_ECB
select CRYPTO_HMAC
select CRYPTO_SHA512
select CRYPTO_XTS
config FS_ENCRYPTION_INLINE_CRYPT

1
fs/crypto/fname.c
View File

@ -11,7 +11,6 @@
* This has not yet undergone a rigorous security audit.
*/
#include <crypto/hash.h>
#include <crypto/sha2.h>
#include <crypto/skcipher.h>
#include <linux/export.h>

26
fs/crypto/fscrypt_private.h
View File

@ -11,10 +11,10 @@
#ifndef _FSCRYPT_PRIVATE_H
#define _FSCRYPT_PRIVATE_H
#include <crypto/sha2.h>
#include <linux/fscrypt.h>
#include <linux/minmax.h>
#include <linux/siphash.h>
#include <crypto/hash.h>
#include <linux/blk-crypto.h>
#define CONST_STRLEN(str) (sizeof(str) - 1)
@ -381,12 +381,8 @@ bool __fscrypt_fname_encrypted_size(const union fscrypt_policy *policy,
u32 *encrypted_len_ret);
/* hkdf.c */
struct fscrypt_hkdf {
struct crypto_shash *hmac_tfm;
};
int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
unsigned int master_key_size);
void fscrypt_init_hkdf(struct hmac_sha512_key *hkdf, const u8 *master_key,
unsigned int master_key_size);
/*
* The list of contexts in which fscrypt uses HKDF. These values are used as
@ -405,11 +401,9 @@ int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key,
#define HKDF_CONTEXT_KEY_IDENTIFIER_FOR_HW_WRAPPED_KEY \
8 /* info=<empty> */
int fscrypt_hkdf_expand(const struct fscrypt_hkdf *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen);
void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf);
void fscrypt_hkdf_expand(const struct hmac_sha512_key *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen);
/* inline_crypt.c */
#ifdef CONFIG_FS_ENCRYPTION_INLINE_CRYPT
@ -517,7 +511,7 @@ struct fscrypt_master_key_secret {
* ->is_hw_wrapped=false, or by the "software secret" that hardware
* derived from this master key if ->is_hw_wrapped=true.
*/
struct fscrypt_hkdf hkdf;
struct hmac_sha512_key hkdf;
/*
* True if this key is a hardware-wrapped key; false if this key is a
@ -696,7 +690,7 @@ struct fscrypt_master_key *
fscrypt_find_master_key(struct super_block *sb,
const struct fscrypt_key_specifier *mk_spec);
int fscrypt_get_test_dummy_key_identifier(
void fscrypt_get_test_dummy_key_identifier(
u8 key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE]);
int fscrypt_add_test_dummy_key(struct super_block *sb,
@ -732,8 +726,8 @@ void fscrypt_destroy_prepared_key(struct super_block *sb,
int fscrypt_set_per_file_enc_key(struct fscrypt_inode_info *ci,
const u8 *raw_key);
int fscrypt_derive_dirhash_key(struct fscrypt_inode_info *ci,
const struct fscrypt_master_key *mk);
void fscrypt_derive_dirhash_key(struct fscrypt_inode_info *ci,
const struct fscrypt_master_key *mk);
void fscrypt_hash_inode_number(struct fscrypt_inode_info *ci,
const struct fscrypt_master_key *mk);

105
fs/crypto/hkdf.c
View File

@ -1,5 +1,9 @@
// 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 (or from the
* "software secrets" which hardware derives from the fscrypt master keys, in
* the case that the fscrypt master keys are hardware-wrapped keys).
@ -7,10 +11,6 @@
* Copyright 2019 Google LLC
*/
#include <crypto/hash.h>
#include <crypto/hkdf.h>
#include <crypto/sha2.h>
#include "fscrypt_private.h"
/*
@ -24,7 +24,6 @@
* HKDF-SHA512 being much faster than HKDF-SHA256, as the longer digest size of
* SHA-512 causes HKDF-Expand to only need to do one iteration rather than two.
*/
#define HKDF_HMAC_ALG "hmac(sha512)"
#define HKDF_HASHLEN SHA512_DIGEST_SIZE
/*
@ -44,54 +43,24 @@
*/
/*
* 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.
* Compute HKDF-Extract using 'master_key' as the input keying material, and
* prepare the resulting HMAC key in 'hkdf'. Afterwards, 'hkdf' 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,
unsigned int master_key_size)
void fscrypt_init_hkdf(struct hmac_sha512_key *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;
hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, FSCRYPT_CRYPTOAPI_MASK);
if (IS_ERR(hmac_tfm)) {
fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld",
PTR_ERR(hmac_tfm));
return PTR_ERR(hmac_tfm);
}
if (WARN_ON_ONCE(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) {
err = -EINVAL;
goto err_free_tfm;
}
err = hkdf_extract(hmac_tfm, master_key, master_key_size,
default_salt, HKDF_HASHLEN, prk);
if (err)
goto err_free_tfm;
err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk));
if (err)
goto err_free_tfm;
hkdf->hmac_tfm = hmac_tfm;
goto out;
err_free_tfm:
crypto_free_shash(hmac_tfm);
out:
hmac_sha512_usingrawkey(default_salt, sizeof(default_salt),
master_key, master_key_size, prk);
hmac_sha512_preparekey(hkdf, prk, sizeof(prk));
memzero_explicit(prk, sizeof(prk));
return err;
}
/*
* HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which
* was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen'
* HKDF-Expand (RFC 5869 section 2.3). Expand the HMAC key 'hkdf' into 'okmlen'
* bytes of output keying material parameterized by the application-specific
* 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context'
* byte. This is thread-safe and may be called by multiple threads in parallel.
@ -100,30 +69,32 @@ out:
* adds to its application-specific info strings to guarantee that it doesn't
* 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 *okm, unsigned int okmlen)
void fscrypt_hkdf_expand(const struct hmac_sha512_key *hkdf, u8 context,
const u8 *info, unsigned int infolen,
u8 *okm, unsigned int okmlen)
{
SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm);
u8 *full_info;
int err;
struct hmac_sha512_ctx ctx;
u8 counter = 1;
u8 tmp[HKDF_HASHLEN];
full_info = kzalloc(infolen + 9, GFP_KERNEL);
if (!full_info)
return -ENOMEM;
desc->tfm = hkdf->hmac_tfm;
WARN_ON_ONCE(okmlen > 255 * HKDF_HASHLEN);
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;
}
void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf)
{
crypto_free_shash(hkdf->hmac_tfm);
for (unsigned int i = 0; i < okmlen; i += HKDF_HASHLEN) {
hmac_sha512_init(&ctx, hkdf);
if (i != 0)
hmac_sha512_update(&ctx, &okm[i - HKDF_HASHLEN],
HKDF_HASHLEN);
hmac_sha512_update(&ctx, "fscrypt\0", 8);
hmac_sha512_update(&ctx, &context, 1);
hmac_sha512_update(&ctx, info, infolen);
hmac_sha512_update(&ctx, &counter, 1);
if (okmlen - i < HKDF_HASHLEN) {
hmac_sha512_final(&ctx, tmp);
memcpy(&okm[i], tmp, okmlen - i);
memzero_explicit(tmp, sizeof(tmp));
} else {
hmac_sha512_final(&ctx, &okm[i]);
}
counter++;
}
}

2
fs/crypto/hooks.c
View File

@ -205,7 +205,7 @@ int fscrypt_prepare_setflags(struct inode *inode,
mk = ci->ci_master_key;
down_read(&mk->mk_sem);
if (mk->mk_present)
err = fscrypt_derive_dirhash_key(ci, mk);
fscrypt_derive_dirhash_key(ci, mk);
else
err = -ENOKEY;
up_read(&mk->mk_sem);

30
fs/crypto/keyring.c
View File

@ -42,7 +42,6 @@ struct fscrypt_keyring {
static void wipe_master_key_secret(struct fscrypt_master_key_secret *secret)
{
fscrypt_destroy_hkdf(&secret->hkdf);
memzero_explicit(secret, sizeof(*secret));
}
@ -587,21 +586,17 @@ static int add_master_key(struct super_block *sb,
keyid_kdf_ctx =
HKDF_CONTEXT_KEY_IDENTIFIER_FOR_HW_WRAPPED_KEY;
}
err = fscrypt_init_hkdf(&secret->hkdf, kdf_key, kdf_key_size);
fscrypt_init_hkdf(&secret->hkdf, kdf_key, kdf_key_size);
/*
* Now that the KDF context is initialized, the raw KDF key is
* no longer needed.
*/
memzero_explicit(kdf_key, kdf_key_size);
if (err)
return err;
/* Calculate the key identifier */
err = fscrypt_hkdf_expand(&secret->hkdf, keyid_kdf_ctx, NULL, 0,
key_spec->u.identifier,
FSCRYPT_KEY_IDENTIFIER_SIZE);
if (err)
return err;
fscrypt_hkdf_expand(&secret->hkdf, keyid_kdf_ctx, NULL, 0,
key_spec->u.identifier,
FSCRYPT_KEY_IDENTIFIER_SIZE);
}
return do_add_master_key(sb, secret, key_spec);
}
@ -835,24 +830,17 @@ fscrypt_get_test_dummy_secret(struct fscrypt_master_key_secret *secret)
memcpy(secret->bytes, test_key, sizeof(test_key));
}
int fscrypt_get_test_dummy_key_identifier(
void fscrypt_get_test_dummy_key_identifier(
u8 key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE])
{
struct fscrypt_master_key_secret secret;
int err;
fscrypt_get_test_dummy_secret(&secret);
err = fscrypt_init_hkdf(&secret.hkdf, secret.bytes, secret.size);
if (err)
goto out;
err = fscrypt_hkdf_expand(&secret.hkdf,
HKDF_CONTEXT_KEY_IDENTIFIER_FOR_RAW_KEY,
NULL, 0, key_identifier,
FSCRYPT_KEY_IDENTIFIER_SIZE);
out:
fscrypt_init_hkdf(&secret.hkdf, secret.bytes, secret.size);
fscrypt_hkdf_expand(&secret.hkdf,
HKDF_CONTEXT_KEY_IDENTIFIER_FOR_RAW_KEY, NULL, 0,
key_identifier, FSCRYPT_KEY_IDENTIFIER_SIZE);
wipe_master_key_secret(&secret);
return err;
}
/**

65
fs/crypto/keysetup.c
View File

@ -253,11 +253,8 @@ static int setup_per_mode_enc_key(struct fscrypt_inode_info *ci,
sizeof(sb->s_uuid));
hkdf_infolen += sizeof(sb->s_uuid);
}
err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
hkdf_context, hkdf_info, hkdf_infolen,
mode_key, mode->keysize);
if (err)
goto out_unlock;
fscrypt_hkdf_expand(&mk->mk_secret.hkdf, hkdf_context, hkdf_info,
hkdf_infolen, mode_key, mode->keysize);
err = fscrypt_prepare_key(prep_key, mode_key, ci);
memzero_explicit(mode_key, mode->keysize);
if (err)
@ -278,36 +275,25 @@ out_unlock:
* as a pair of 64-bit words. Therefore, on big endian CPUs we have to do an
* endianness swap in order to get the same results as on little endian CPUs.
*/
static int fscrypt_derive_siphash_key(const struct fscrypt_master_key *mk,
u8 context, const u8 *info,
unsigned int infolen, siphash_key_t *key)
static void fscrypt_derive_siphash_key(const struct fscrypt_master_key *mk,
u8 context, const u8 *info,
unsigned int infolen, siphash_key_t *key)
{
int err;
err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf, context, info, infolen,
(u8 *)key, sizeof(*key));
if (err)
return err;
fscrypt_hkdf_expand(&mk->mk_secret.hkdf, context, info, infolen,
(u8 *)key, sizeof(*key));
BUILD_BUG_ON(sizeof(*key) != 16);
BUILD_BUG_ON(ARRAY_SIZE(key->key) != 2);
le64_to_cpus(&key->key[0]);
le64_to_cpus(&key->key[1]);
return 0;
}
int fscrypt_derive_dirhash_key(struct fscrypt_inode_info *ci,
const struct fscrypt_master_key *mk)
void fscrypt_derive_dirhash_key(struct fscrypt_inode_info *ci,
const struct fscrypt_master_key *mk)
{
int err;
err = fscrypt_derive_siphash_key(mk, HKDF_CONTEXT_DIRHASH_KEY,
ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
&ci->ci_dirhash_key);
if (err)
return err;
fscrypt_derive_siphash_key(mk, HKDF_CONTEXT_DIRHASH_KEY,
ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
&ci->ci_dirhash_key);
ci->ci_dirhash_key_initialized = true;
return 0;
}
void fscrypt_hash_inode_number(struct fscrypt_inode_info *ci,
@ -338,17 +324,12 @@ static int fscrypt_setup_iv_ino_lblk_32_key(struct fscrypt_inode_info *ci,
if (mk->mk_ino_hash_key_initialized)
goto unlock;
err = fscrypt_derive_siphash_key(mk,
HKDF_CONTEXT_INODE_HASH_KEY,
NULL, 0, &mk->mk_ino_hash_key);
if (err)
goto unlock;
fscrypt_derive_siphash_key(mk, HKDF_CONTEXT_INODE_HASH_KEY,
NULL, 0, &mk->mk_ino_hash_key);
/* pairs with smp_load_acquire() above */
smp_store_release(&mk->mk_ino_hash_key_initialized, true);
unlock:
mutex_unlock(&fscrypt_mode_key_setup_mutex);
if (err)
return err;
}
/*
@ -402,13 +383,10 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_inode_info *ci,
} else {
u8 derived_key[FSCRYPT_MAX_RAW_KEY_SIZE];
err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
HKDF_CONTEXT_PER_FILE_ENC_KEY,
ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
derived_key, ci->ci_mode->keysize);
if (err)
return err;
fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
HKDF_CONTEXT_PER_FILE_ENC_KEY,
ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
derived_key, ci->ci_mode->keysize);
err = fscrypt_set_per_file_enc_key(ci, derived_key);
memzero_explicit(derived_key, ci->ci_mode->keysize);
}
@ -416,11 +394,8 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_inode_info *ci,
return err;
/* Derive a secret dirhash key for directories that need it. */
if (need_dirhash_key) {
err = fscrypt_derive_dirhash_key(ci, mk);
if (err)
return err;
}
if (need_dirhash_key)
fscrypt_derive_dirhash_key(ci, mk);
return 0;
}

4
fs/crypto/policy.c
View File

@ -826,10 +826,8 @@ int fscrypt_parse_test_dummy_encryption(const struct fs_parameter *param,
policy->version = FSCRYPT_POLICY_V2;
policy->v2.contents_encryption_mode = FSCRYPT_MODE_AES_256_XTS;
policy->v2.filenames_encryption_mode = FSCRYPT_MODE_AES_256_CTS;
err = fscrypt_get_test_dummy_key_identifier(
fscrypt_get_test_dummy_key_identifier(
policy->v2.master_key_identifier);
if (err)
goto out;
} else {
err = -EINVAL;
goto out;