@@ -41,10 +41,8 @@ config BIG_KEYS
bool "Large payload keys"
depends on KEYS
depends on TMPFS
- depends on (CRYPTO_ANSI_CPRNG = y || CRYPTO_DRBG = y)
select CRYPTO_AES
- select CRYPTO_ECB
- select CRYPTO_RNG
+ select CRYPTO_GCM
help
This option provides support for holding large keys within the kernel
(for example Kerberos ticket caches). The data may be stored out to
@@ -1,5 +1,6 @@
/* Large capacity key type
*
+ * Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
* Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*
@@ -16,10 +17,10 @@
#include <linux/shmem_fs.h>
#include <linux/err.h>
#include <linux/scatterlist.h>
+#include <linux/random.h>
#include <keys/user-type.h>
#include <keys/big_key-type.h>
-#include <crypto/rng.h>
-#include <crypto/skcipher.h>
+#include <crypto/aead.h>
/*
* Layout of key payload words.
@@ -49,7 +50,12 @@ enum big_key_op {
/*
* Key size for big_key data encryption
*/
-#define ENC_KEY_SIZE 16
+#define ENC_KEY_SIZE 32
+
+/*
+ * Authentication tag length
+ */
+#define ENC_AUTHTAG_SIZE 16
/*
* big_key defined keys take an arbitrary string as the description and an
@@ -64,27 +70,23 @@ struct key_type key_type_big_key = {
.destroy = big_key_destroy,
.describe = big_key_describe,
.read = big_key_read,
+ /* no ->update(); don't add it without changing big_key_crypt() nonce */
};
/*
- * Crypto names for big_key data encryption
+ * Crypto names for big_key data authenticated encryption
*/
-static const char big_key_rng_name[] = "stdrng";
-static const char big_key_alg_name[] = "ecb(aes)";
+static const char big_key_alg_name[] = "gcm(aes)";
/*
- * Crypto algorithms for big_key data encryption
+ * Crypto algorithms for big_key data authenticated encryption
*/
-static struct crypto_rng *big_key_rng;
-static struct crypto_skcipher *big_key_skcipher;
+static struct crypto_aead *big_key_aead;
/*
- * Generate random key to encrypt big_key data
+ * Since changing the key affects the entire object, we need a mutex.
*/
-static inline int big_key_gen_enckey(u8 *key)
-{
- return crypto_rng_get_bytes(big_key_rng, key, ENC_KEY_SIZE);
-}
+static DEFINE_MUTEX(big_key_aead_lock);
/*
* Encrypt/decrypt big_key data
@@ -93,28 +95,37 @@ static int big_key_crypt(enum big_key_op op, u8 *data, size_t datalen, u8 *key)
{
int ret = -EINVAL;
struct scatterlist sgio;
- SKCIPHER_REQUEST_ON_STACK(req, big_key_skcipher);
-
- if (crypto_skcipher_setkey(big_key_skcipher, key, ENC_KEY_SIZE)) {
+ u8 req_on_stack[sizeof(struct aead_request) +
+ crypto_aead_reqsize(big_key_aead)];
+ struct aead_request *aead_req = (struct aead_request *)req_on_stack;
+
+ /* We always use a zero nonce. The reason we can get away with this is
+ * because we're using a different randomly generated key for every
+ * different encryption. Notably, too, key_type_big_key doesn't define
+ * an .update function, so there's no chance we'll wind up reusing the
+ * key to encrypt updated data. Simply put: one key, one encryption.
+ */
+ u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
+
+ memset(req_on_stack, 0, sizeof(req_on_stack));
+ memset(zero_nonce, 0, sizeof(zero_nonce));
+ sg_init_one(&sgio, data, datalen + (op == BIG_KEY_ENC ? ENC_AUTHTAG_SIZE : 0));
+ aead_request_set_tfm(aead_req, big_key_aead);
+ aead_request_set_crypt(aead_req, &sgio, &sgio, datalen, zero_nonce);
+ aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
+
+ mutex_lock(&big_key_aead_lock);
+ if (crypto_aead_setkey(big_key_aead, key, ENC_KEY_SIZE)) {
ret = -EAGAIN;
goto error;
}
-
- skcipher_request_set_tfm(req, big_key_skcipher);
- skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP,
- NULL, NULL);
-
- sg_init_one(&sgio, data, datalen);
- skcipher_request_set_crypt(req, &sgio, &sgio, datalen, NULL);
-
if (op == BIG_KEY_ENC)
- ret = crypto_skcipher_encrypt(req);
+ ret = crypto_aead_encrypt(aead_req);
else
- ret = crypto_skcipher_decrypt(req);
-
- skcipher_request_zero(req);
-
+ ret = crypto_aead_decrypt(aead_req);
error:
+ mutex_unlock(&big_key_aead_lock);
+ memzero_explicit(req_on_stack, sizeof(req_on_stack));
return ret;
}
@@ -146,15 +157,12 @@ int big_key_preparse(struct key_preparsed_payload *prep)
*
* File content is stored encrypted with randomly generated key.
*/
- size_t enclen = ALIGN(datalen, crypto_skcipher_blocksize(big_key_skcipher));
+ size_t enclen = datalen + ENC_AUTHTAG_SIZE;
- /* prepare aligned data to encrypt */
data = kmalloc(enclen, GFP_KERNEL);
if (!data)
return -ENOMEM;
-
memcpy(data, prep->data, datalen);
- memset(data + datalen, 0x00, enclen - datalen);
/* generate random key */
enckey = kmalloc(ENC_KEY_SIZE, GFP_KERNEL);
@@ -162,13 +170,10 @@ int big_key_preparse(struct key_preparsed_payload *prep)
ret = -ENOMEM;
goto error;
}
-
- ret = big_key_gen_enckey(enckey);
- if (ret)
- goto err_enckey;
+ get_random_bytes(enckey, ENC_KEY_SIZE);
/* encrypt aligned data */
- ret = big_key_crypt(BIG_KEY_ENC, data, enclen, enckey);
+ ret = big_key_crypt(BIG_KEY_ENC, data, datalen, enckey);
if (ret)
goto err_enckey;
@@ -212,7 +217,7 @@ int big_key_preparse(struct key_preparsed_payload *prep)
err_enckey:
kfree(enckey);
error:
- kfree(data);
+ kzfree(data);
return ret;
}
@@ -294,7 +299,7 @@ long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
struct file *file;
u8 *data;
u8 *enckey = (u8 *)key->payload.data[big_key_data];
- size_t enclen = ALIGN(datalen, crypto_skcipher_blocksize(big_key_skcipher));
+ size_t enclen = datalen + ENC_AUTHTAG_SIZE;
data = kmalloc(enclen, GFP_KERNEL);
if (!data)
@@ -342,47 +347,33 @@ long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
*/
static int __init big_key_init(void)
{
- struct crypto_skcipher *cipher;
- struct crypto_rng *rng;
+ struct crypto_aead *tfm;
int ret;
- rng = crypto_alloc_rng(big_key_rng_name, 0, 0);
- if (IS_ERR(rng)) {
- pr_err("Can't alloc rng: %ld\n", PTR_ERR(rng));
- return PTR_ERR(rng);
- }
-
- big_key_rng = rng;
-
- /* seed RNG */
- ret = crypto_rng_reset(rng, NULL, crypto_rng_seedsize(rng));
- if (ret) {
- pr_err("Can't reset rng: %d\n", ret);
- goto error_rng;
- }
-
/* init block cipher */
- cipher = crypto_alloc_skcipher(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
- if (IS_ERR(cipher)) {
- ret = PTR_ERR(cipher);
+ tfm = crypto_alloc_aead(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
+ if (IS_ERR(tfm)) {
+ ret = PTR_ERR(tfm);
pr_err("Can't alloc crypto: %d\n", ret);
- goto error_rng;
+ return ret;
+ }
+ ret = crypto_aead_setauthsize(tfm, ENC_AUTHTAG_SIZE);
+ if (ret < 0) {
+ pr_err("Can't set crypto auth tag len: %d\n", ret);
+ goto free_aead;
}
-
- big_key_skcipher = cipher;
ret = register_key_type(&key_type_big_key);
if (ret < 0) {
pr_err("Can't register type: %d\n", ret);
- goto error_cipher;
+ goto free_aead;
}
+ big_key_aead = tfm;
return 0;
-error_cipher:
- crypto_free_skcipher(big_key_skcipher);
-error_rng:
- crypto_free_rng(big_key_rng);
+free_aead:
+ crypto_free_aead(tfm);
return ret;
}
This started out as just replacing the use of crypto/rng with get_random_bytes, so that we wouldn't use bad randomness at boot time. But, upon looking further, it appears that there were even deeper underlying cryptographic problems, and that this seems to have been committed with very little crypto review. So, I rewrote the whole thing, trying to keep to the conventions introduced by the previous author, to fix these cryptographic flaws. Since this sensitive material is being stored untrusted, using ECB and no authentication is simply not okay at all. I find it surprising that this code even made it past basic crypto review, though perhaps there's something I'm missing. This patch moves from using AES-ECB to using AES-GCM. Since keys are uniquely generated each time, we can set the nonce to zero. There was also a race condition in which the same key would be reused at the same time in different threads. A mutex fixes this issue now. And, some error paths forgot to zero out sensitive material, so this patch changes a kfree into a kzfree. So, to summarize, this commit fixes the following vulnerabilities: * Unauthenticated encryption, allowing an attacker to modify the cipher text in particular ways in order to manipulate the plaintext, which is is even more frightening considering the next point. * Use of ECB mode, allowing an attacker to trivially swap blocks or compare identical plaintext blocks. * Key re-use. * Faulty memory zeroing. Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> Cc: David Howells <dhowells@redhat.com> Cc: Eric Biggers <ebiggers3@gmail.com> Cc: Herbert Xu <herbert@gondor.apana.org.au> Cc: Kirill Marinushkin <k.marinushkin@gmail.com> Cc: security@kernel.org --- Changes v1->v2: - The code has now been tested, and it works. - Style fixups. - Memory zeroring. - Use mutex to prevent key reuse race condition. - Useful comments. - Kconfig simplification. - We no longer rely on get_random_bytes_wait, even though we really should, because that hasn't yet been merged. When it is merged, I'll submit a follow-up patch. security/keys/Kconfig | 4 +- security/keys/big_key.c | 129 ++++++++++++++++++++++-------------------------- 2 files changed, 61 insertions(+), 72 deletions(-)