Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
fscrypt: use AES library for v1 key derivation
Convert the implementation of the v1 (original / deprecated) fscrypt
per-file key derivation algorithm to use the AES library instead of an
"ecb(aes)" crypto_skcipher. This is much simpler.
While the AES library doesn't support AES-ECB directly yet, we can still
simply call aes_encrypt() in a loop. While that doesn't explicitly
parallelize the AES encryptions, it doesn't really matter in this case,
where a new key is used each time and only 16 to 64 bytes are encrypted.
In fact, a quick benchmark (AMD Ryzen 9 9950X) shows that this commit
actually greatly improves performance, from ~7000 cycles per key derived
to ~1500. The times don't differ much between 32 bytes and 64 bytes
either, so clearly the bottleneck is API stuff and key expansion.
Granted, performance of the v1 key derivation is no longer very
relevant: most users have moved onto v2 encryption policies. The v2 key
derivation uses HKDF-SHA512 (which is ~3500 cycles on the same CPU).
Still, it's nice that the simpler solution is much faster as well.
Compatibility verified with xfstests generic/548.
Link: https://lore.kernel.org/r/20260321075338.99809-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
+29
-60
+1
-1
fs/crypto/Kconfig
+1
-1
fs/crypto/Kconfig
···
3
3
bool "FS Encryption (Per-file encryption)"
4
4
select CRYPTO
5
5
select CRYPTO_SKCIPHER
6
+
select CRYPTO_LIB_AES
6
7
select CRYPTO_LIB_SHA256
7
8
select CRYPTO_LIB_SHA512
8
9
select KEYS
···
31
30
select CRYPTO_AES
32
31
select CRYPTO_CBC
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32
select CRYPTO_CTS
34
-
select CRYPTO_ECB
35
33
select CRYPTO_XTS
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34
37
35
config FS_ENCRYPTION_INLINE_CRYPT
+28
-59
fs/crypto/keysetup_v1.c
+28
-59
fs/crypto/keysetup_v1.c
···
20
20
* managed alongside the master keys in the filesystem-level keyring)
21
21
*/
22
22
23
-
#include <crypto/skcipher.h>
23
+
#include <crypto/aes.h>
24
24
#include <crypto/utils.h>
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25
#include <keys/user-type.h>
26
26
#include <linux/hashtable.h>
27
-
#include <linux/scatterlist.h>
28
27
29
28
#include "fscrypt_private.h"
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30
/* Table of keys referenced by DIRECT_KEY policies */
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static DEFINE_HASHTABLE(fscrypt_direct_keys, 6); /* 6 bits = 64 buckets */
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static DEFINE_SPINLOCK(fscrypt_direct_keys_lock);
34
-
35
-
/*
36
-
* v1 key derivation function. This generates the derived key by encrypting the
37
-
* master key with AES-128-ECB using the nonce as the AES key. This provides a
38
-
* unique derived key with sufficient entropy for each inode. However, it's
39
-
* nonstandard, non-extensible, doesn't evenly distribute the entropy from the
40
-
* master key, and is trivially reversible: an attacker who compromises a
41
-
* derived key can "decrypt" it to get back to the master key, then derive any
42
-
* other key. For all new code, use HKDF instead.
43
-
*
44
-
* The master key must be at least as long as the derived key. If the master
45
-
* key is longer, then only the first 'derived_keysize' bytes are used.
46
-
*/
47
-
static int derive_key_aes(const u8 *master_key,
48
-
const u8 nonce[FSCRYPT_FILE_NONCE_SIZE],
49
-
u8 *derived_key, unsigned int derived_keysize)
50
-
{
51
-
struct crypto_sync_skcipher *tfm;
52
-
int err;
53
-
54
-
tfm = crypto_alloc_sync_skcipher("ecb(aes)", 0, FSCRYPT_CRYPTOAPI_MASK);
55
-
if (IS_ERR(tfm))
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-
return PTR_ERR(tfm);
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-
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-
err = crypto_sync_skcipher_setkey(tfm, nonce, FSCRYPT_FILE_NONCE_SIZE);
59
-
if (err == 0) {
60
-
SYNC_SKCIPHER_REQUEST_ON_STACK(req, tfm);
61
-
struct scatterlist src_sg, dst_sg;
62
-
63
-
skcipher_request_set_callback(req,
64
-
CRYPTO_TFM_REQ_MAY_BACKLOG |
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-
CRYPTO_TFM_REQ_MAY_SLEEP,
66
-
NULL, NULL);
67
-
sg_init_one(&src_sg, master_key, derived_keysize);
68
-
sg_init_one(&dst_sg, derived_key, derived_keysize);
69
-
skcipher_request_set_crypt(req, &src_sg, &dst_sg,
70
-
derived_keysize, NULL);
71
-
err = crypto_skcipher_encrypt(req);
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-
}
73
-
crypto_free_sync_skcipher(tfm);
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-
return err;
75
-
}
76
33
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34
/*
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35
* Search the current task's subscribed keyrings for a "logon" key with
···
212
255
return 0;
213
256
}
214
257
215
-
/* v1 policy, !DIRECT_KEY: derive the file's encryption key */
258
+
/*
259
+
* v1 policy, !DIRECT_KEY: derive the file's encryption key.
260
+
*
261
+
* The v1 key derivation function generates the derived key by encrypting the
262
+
* master key with AES-128-ECB using the file's nonce as the AES key. This
263
+
* provides a unique derived key with sufficient entropy for each inode.
264
+
* However, it's nonstandard, non-extensible, doesn't evenly distribute the
265
+
* entropy from the master key, and is trivially reversible: an attacker who
266
+
* compromises a derived key can "decrypt" it to get back to the master key,
267
+
* then derive any other key. For all new code, use HKDF instead.
268
+
*
269
+
* The master key must be at least as long as the derived key. If the master
270
+
* key is longer, then only the first ci->ci_mode->keysize bytes are used.
271
+
*/
216
272
static int setup_v1_file_key_derived(struct fscrypt_inode_info *ci,
217
273
const u8 *raw_master_key)
218
274
{
219
-
u8 *derived_key;
275
+
const unsigned int derived_keysize = ci->ci_mode->keysize;
276
+
u8 derived_key[FSCRYPT_MAX_RAW_KEY_SIZE];
277
+
struct aes_enckey aes;
220
278
int err;
221
279
222
-
/*
223
-
* This cannot be a stack buffer because it will be passed to the
224
-
* scatterlist crypto API during derive_key_aes().
225
-
*/
226
-
derived_key = kmalloc(ci->ci_mode->keysize, GFP_KERNEL);
227
-
if (!derived_key)
228
-
return -ENOMEM;
280
+
if (WARN_ON_ONCE(derived_keysize > FSCRYPT_MAX_RAW_KEY_SIZE ||
281
+
derived_keysize % AES_BLOCK_SIZE != 0))
282
+
return -EINVAL;
229
283
230
-
err = derive_key_aes(raw_master_key, ci->ci_nonce,
231
-
derived_key, ci->ci_mode->keysize);
232
-
if (err)
233
-
goto out;
284
+
static_assert(FSCRYPT_FILE_NONCE_SIZE == AES_KEYSIZE_128);
285
+
aes_prepareenckey(&aes, ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE);
286
+
for (unsigned int i = 0; i < derived_keysize; i += AES_BLOCK_SIZE)
287
+
aes_encrypt(&aes, &derived_key[i], &raw_master_key[i]);
234
288
235
289
err = fscrypt_set_per_file_enc_key(ci, derived_key);
236
-
out:
237
-
kfree_sensitive(derived_key);
290
+
291
+
memzero_explicit(derived_key, derived_keysize);
292
+
/* No need to zeroize 'aes', as its key is not secret. */
238
293
return err;
239
294
}
240
295