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.
First, it makes no sense to seed crypto/rng at boot time and then keep
using it like this, when in fact there's already get_random_bytes_wait,
which can ensure there's enough entropy and be a much more standard way
of generating keys.
Second, 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.
So, to summarize, this commit fixes the following vulnerabilities:
* Low entropy key generation, allowing an attacker to potentially
guess or predict keys.
* 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.
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Eric Biggers <ebiggers3@xxxxxxxxx>
Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: Kirill Marinushkin <k.marinushkin@xxxxxxxxx>
Cc: security@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
---
This commit has been compile-tested only, so I'd appreciate it if
somebody else who actually uses this would test it for functionality, or
if somebody into the kernel's crypto API would check that it's been done
correctly.
security/keys/Kconfig | 5 +-
security/keys/big_key.c | 120 +++++++++++++++++++++++-------------------------
2 files changed, 59 insertions(+), 66 deletions(-)
diff --git a/security/keys/Kconfig b/security/keys/Kconfig
index 6fd95f76bfae..2f91f2368d29 100644
--- a/security/keys/Kconfig
+++ b/security/keys/Kconfig
@@ -41,10 +41,9 @@ 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
+ select CRYPTO_AEAD
help
This option provides support for holding large keys within the kernel
(for example Kerberos ticket caches). The data may be stored out to
diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index 835c1ab30d01..40066cb6b8f4 100644
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c
@@ -1,5 +1,6 @@
/* Large capacity key type
*
+ * Copyright (C) 2017 Jason A. Donenfeld <Jason@xxxxxxxxx>. All Rights Reserved.
* Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@xxxxxxxxxx)
*
@@ -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
@@ -67,24 +73,14 @@ struct key_type key_type_big_key = {
};
/*
- * Crypto names for big_key data encryption
- */
-static const char big_key_rng_name[] = "stdrng";
-static const char big_key_alg_name[] = "ecb(aes)";
-
-/*
- * Crypto algorithms for big_key data encryption
+ * Crypto names for big_key data authenticated encryption
*/
-static struct crypto_rng *big_key_rng;
-static struct crypto_skcipher *big_key_skcipher;
+static const char big_key_alg_name[] = "gcm(aes)";
/*
- * Generate random key to encrypt big_key data
+ * Crypto algorithms for big_key data authenticated encryption
*/
-static inline int big_key_gen_enckey(u8 *key)
-{
- return crypto_rng_get_bytes(big_key_rng, key, ENC_KEY_SIZE);
-}
+static struct crypto_aead *big_key_aead;
/*
* Encrypt/decrypt big_key data
@@ -93,27 +89,40 @@ 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(struct aead_request) +
+ crypto_aead_reqsize(big_key_aead));
+ memset(zero_nonce, 0, crypto_aead_ivsize(big_key_aead));
+ sg_init_one(&sgio, data, datalen + (op == BIG_KEY_ENC ? ENC_AUTHTAG_SIZE : 0));
+
+ 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);
+ 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);
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);
+ memzero_explicit(req_on_stack, sizeof(struct aead_request) +
+ crypto_aead_reqsize(big_key_aead));
error:
return ret;
}
@@ -146,7 +155,7 @@ 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);
@@ -162,13 +171,12 @@ int big_key_preparse(struct key_preparsed_payload *prep)
ret = -ENOMEM;
goto error;
}
-
- ret = big_key_gen_enckey(enckey);
+ ret = get_random_bytes_wait(enckey, ENC_KEY_SIZE);
if (ret)
goto err_enckey;
/* 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;
@@ -294,7 +302,7 @@ 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 +350,33 @@ 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;
}
--
2.13.0