[PATCH v5] fscrypt: Add support for AES-128-CBC

From: David Gstir
Date: Mon Jun 19 2017 - 03:28:17 EST

From: Daniel Walter <dwalter@xxxxxxxxxxxxx>

fscrypt provides facilities to use different encryption algorithms which
are selectable by userspace when setting the encryption policy. Currently,
only AES-256-XTS for file contents and AES-256-CBC-CTS for file names are
implemented. This is a clear case of kernel offers the mechanism and
userspace selects a policy. Similar to what dm-crypt and ecryptfs have.

This patch adds support for using AES-128-CBC for file contents and
AES-128-CBC-CTS for file name encryption. To mitigate watermarking
attacks, IVs are generated using the ESSIV algorithm. While AES-CBC is
actually slightly less secure than AES-XTS from a security point of view,
there is more widespread hardware support. Using AES-CBC gives us the
acceptable performance while still providing a moderate level of security
for persistent storage.

Especially low-powered embedded devices with crypto accelerators such as
CAAM or CESA often only support AES-CBC. Since using AES-CBC over AES-XTS
is basically thought of a last resort, we use AES-128-CBC over AES-256-CBC
since it has less encryption rounds and yields noticeable better
performance starting from a file size of just a few kB.

Signed-off-by: Daniel Walter <dwalter@xxxxxxxxxxxxx>
[david@xxxxxxxxxxxxx: addressed review comments]
Signed-off-by: David Gstir <david@xxxxxxxxxxxxx>
Reviewed-by: Eric Biggers <ebiggers@xxxxxxxxxx>
---
fs/crypto/Kconfig | 1 +
fs/crypto/crypto.c | 23 ++++--
fs/crypto/fscrypt_private.h | 9 ++-
fs/crypto/keyinfo.c | 173 ++++++++++++++++++++++++++++++++---------
fs/crypto/policy.c | 8 +-
include/linux/fscrypt_common.h | 16 ++--
include/uapi/linux/fs.h | 2 +
7 files changed, 174 insertions(+), 58 deletions(-)

diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig
index 08b46e6e3995..02b7d91c9231 100644
--- a/fs/crypto/Kconfig
+++ b/fs/crypto/Kconfig
@@ -7,6 +7,7 @@ config FS_ENCRYPTION
select CRYPTO_XTS
select CRYPTO_CTS
select CRYPTO_CTR
+ select CRYPTO_SHA256
select KEYS
help
Enable encryption of files and directories. This
diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 6d6eca394d4d..c7835df7e7b8 100644
--- a/fs/crypto/crypto.c
+++ b/fs/crypto/crypto.c
@@ -26,6 +26,7 @@
#include <linux/ratelimit.h>
#include <linux/dcache.h>
#include <linux/namei.h>
+#include <crypto/aes.h>
#include "fscrypt_private.h"

static unsigned int num_prealloc_crypto_pages = 32;
@@ -147,8 +148,8 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
{
struct {
__le64 index;
- u8 padding[FS_XTS_TWEAK_SIZE - sizeof(__le64)];
- } xts_tweak;
+ u8 padding[FS_IV_SIZE - sizeof(__le64)];
+ } iv;
struct skcipher_request *req = NULL;
DECLARE_FS_COMPLETION_RESULT(ecr);
struct scatterlist dst, src;
@@ -158,6 +159,16 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,

BUG_ON(len == 0);

+ BUILD_BUG_ON(sizeof(iv) != FS_IV_SIZE);
+ BUILD_BUG_ON(AES_BLOCK_SIZE != FS_IV_SIZE);
+ iv.index = cpu_to_le64(lblk_num);
+ memset(iv.padding, 0, sizeof(iv.padding));
+
+ if (ci->ci_essiv_tfm != NULL) {
+ crypto_cipher_encrypt_one(ci->ci_essiv_tfm, (u8 *)&iv,
+ (u8 *)&iv);
+ }
+
req = skcipher_request_alloc(tfm, gfp_flags);
if (!req) {
printk_ratelimited(KERN_ERR
@@ -170,15 +181,11 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
req, CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
page_crypt_complete, &ecr);

- BUILD_BUG_ON(sizeof(xts_tweak) != FS_XTS_TWEAK_SIZE);
- xts_tweak.index = cpu_to_le64(lblk_num);
- memset(xts_tweak.padding, 0, sizeof(xts_tweak.padding));
-
sg_init_table(&dst, 1);
sg_set_page(&dst, dest_page, len, offs);
sg_init_table(&src, 1);
sg_set_page(&src, src_page, len, offs);
- skcipher_request_set_crypt(req, &src, &dst, len, &xts_tweak);
+ skcipher_request_set_crypt(req, &src, &dst, len, &iv);
if (rw == FS_DECRYPT)
res = crypto_skcipher_decrypt(req);
else
@@ -477,6 +484,8 @@ static void __exit fscrypt_exit(void)
destroy_workqueue(fscrypt_read_workqueue);
kmem_cache_destroy(fscrypt_ctx_cachep);
kmem_cache_destroy(fscrypt_info_cachep);
+
+ fscrypt_essiv_cleanup();
}
module_exit(fscrypt_exit);

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 1e1f8a361b75..a1d5021c31ef 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -12,10 +12,13 @@
#define _FSCRYPT_PRIVATE_H

#include <linux/fscrypt_supp.h>
+#include <crypto/hash.h>

/* Encryption parameters */
-#define FS_XTS_TWEAK_SIZE 16
+#define FS_IV_SIZE 16
#define FS_AES_128_ECB_KEY_SIZE 16
+#define FS_AES_128_CBC_KEY_SIZE 16
+#define FS_AES_128_CTS_KEY_SIZE 16
#define FS_AES_256_GCM_KEY_SIZE 32
#define FS_AES_256_CBC_KEY_SIZE 32
#define FS_AES_256_CTS_KEY_SIZE 32
@@ -54,6 +57,7 @@ struct fscrypt_info {
u8 ci_filename_mode;
u8 ci_flags;
struct crypto_skcipher *ci_ctfm;
+ struct crypto_cipher *ci_essiv_tfm;
u8 ci_master_key[FS_KEY_DESCRIPTOR_SIZE];
};

@@ -87,4 +91,7 @@ extern int fscrypt_do_page_crypto(const struct inode *inode,
extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
gfp_t gfp_flags);

+/* keyinfo.c */
+extern void __exit fscrypt_essiv_cleanup(void);
+
#endif /* _FSCRYPT_PRIVATE_H */
diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 179e578b875b..018c588c7ac3 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -10,8 +10,13 @@

#include <keys/user-type.h>
#include <linux/scatterlist.h>
+#include <linux/ratelimit.h>
+#include <crypto/aes.h>
+#include <crypto/sha.h>
#include "fscrypt_private.h"

+static struct crypto_shash *essiv_hash_tfm;
+
static void derive_crypt_complete(struct crypto_async_request *req, int rc)
{
struct fscrypt_completion_result *ecr = req->data;
@@ -27,13 +32,13 @@ static void derive_crypt_complete(struct crypto_async_request *req, int rc)
* derive_key_aes() - Derive a key using AES-128-ECB
* @deriving_key: Encryption key used for derivation.
* @source_key: Source key to which to apply derivation.
- * @derived_key: Derived key.
+ * @derived_raw_key: Derived raw key.
*
* Return: Zero on success; non-zero otherwise.
*/
static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
- u8 source_key[FS_AES_256_XTS_KEY_SIZE],
- u8 derived_key[FS_AES_256_XTS_KEY_SIZE])
+ const struct fscrypt_key *source_key,
+ u8 derived_raw_key[FS_MAX_KEY_SIZE])
{
int res = 0;
struct skcipher_request *req = NULL;
@@ -60,10 +65,10 @@ static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
if (res < 0)
goto out;

- sg_init_one(&src_sg, source_key, FS_AES_256_XTS_KEY_SIZE);
- sg_init_one(&dst_sg, derived_key, FS_AES_256_XTS_KEY_SIZE);
- skcipher_request_set_crypt(req, &src_sg, &dst_sg,
- FS_AES_256_XTS_KEY_SIZE, NULL);
+ sg_init_one(&src_sg, source_key->raw, source_key->size);
+ sg_init_one(&dst_sg, derived_raw_key, source_key->size);
+ skcipher_request_set_crypt(req, &src_sg, &dst_sg, source_key->size,
+ NULL);
res = crypto_skcipher_encrypt(req);
if (res == -EINPROGRESS || res == -EBUSY) {
wait_for_completion(&ecr.completion);
@@ -77,7 +82,7 @@ static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],

static int validate_user_key(struct fscrypt_info *crypt_info,
struct fscrypt_context *ctx, u8 *raw_key,
- const char *prefix)
+ const char *prefix, int min_keysize)
{
char *description;
struct key *keyring_key;
@@ -111,50 +116,60 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
master_key = (struct fscrypt_key *)ukp->data;
BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);

- if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
+ if (master_key->size < min_keysize || master_key->size > FS_MAX_KEY_SIZE
+ || master_key->size % AES_BLOCK_SIZE != 0) {
printk_once(KERN_WARNING
"%s: key size incorrect: %d\n",
__func__, master_key->size);
res = -ENOKEY;
goto out;
}
- res = derive_key_aes(ctx->nonce, master_key->raw, raw_key);
+ res = derive_key_aes(ctx->nonce, master_key, raw_key);
out:
up_read(&keyring_key->sem);
key_put(keyring_key);
return res;
}

+static const struct {
+ const char *cipher_str;
+ int keysize;
+} available_modes[] = {
+ [FS_ENCRYPTION_MODE_AES_256_XTS] = { "xts(aes)",
+ FS_AES_256_XTS_KEY_SIZE },
+ [FS_ENCRYPTION_MODE_AES_256_CTS] = { "cts(cbc(aes))",
+ FS_AES_256_CTS_KEY_SIZE },
+ [FS_ENCRYPTION_MODE_AES_128_CBC] = { "cbc(aes)",
+ FS_AES_128_CBC_KEY_SIZE },
+ [FS_ENCRYPTION_MODE_AES_128_CTS] = { "cts(cbc(aes))",
+ FS_AES_128_CTS_KEY_SIZE },
+};
+
static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
const char **cipher_str_ret, int *keysize_ret)
{
- if (S_ISREG(inode->i_mode)) {
- if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_256_XTS) {
- *cipher_str_ret = "xts(aes)";
- *keysize_ret = FS_AES_256_XTS_KEY_SIZE;
- return 0;
- }
- pr_warn_once("fscrypto: unsupported contents encryption mode "
- "%d for inode %lu\n",
- ci->ci_data_mode, inode->i_ino);
- return -ENOKEY;
+ u32 mode;
+
+ if (!fscrypt_valid_enc_modes(ci->ci_data_mode, ci->ci_filename_mode)) {
+ pr_warn_ratelimited("fscrypt: inode %lu uses unsupported encryption modes (contents mode %d, filenames mode %d)\n",
+ inode->i_ino,
+ ci->ci_data_mode, ci->ci_filename_mode);
+ return -EINVAL;
}

- if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
- if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS) {
- *cipher_str_ret = "cts(cbc(aes))";
- *keysize_ret = FS_AES_256_CTS_KEY_SIZE;
- return 0;
- }
- pr_warn_once("fscrypto: unsupported filenames encryption mode "
- "%d for inode %lu\n",
- ci->ci_filename_mode, inode->i_ino);
- return -ENOKEY;
+ if (S_ISREG(inode->i_mode)) {
+ mode = ci->ci_data_mode;
+ } else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
+ mode = ci->ci_filename_mode;
+ } else {
+ WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
+ inode->i_ino, (inode->i_mode & S_IFMT));
+ return -EINVAL;
}

- pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
- (inode->i_mode & S_IFMT), inode->i_ino);
- return -ENOKEY;
+ *cipher_str_ret = available_modes[mode].cipher_str;
+ *keysize_ret = available_modes[mode].keysize;
+ return 0;
}

static void put_crypt_info(struct fscrypt_info *ci)
@@ -163,9 +178,76 @@ static void put_crypt_info(struct fscrypt_info *ci)
return;

crypto_free_skcipher(ci->ci_ctfm);
+ crypto_free_cipher(ci->ci_essiv_tfm);
kmem_cache_free(fscrypt_info_cachep, ci);
}

+static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
+{
+ struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);
+
+ /* init hash transform on demand */
+ if (unlikely(!tfm)) {
+ struct crypto_shash *prev_tfm;
+
+ tfm = crypto_alloc_shash("sha256", 0, 0);
+ if (IS_ERR(tfm)) {
+ pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld\n",
+ PTR_ERR(tfm));
+ return PTR_ERR(tfm);
+ }
+ prev_tfm = cmpxchg(&essiv_hash_tfm, NULL, tfm);
+ if (prev_tfm) {
+ crypto_free_shash(tfm);
+ tfm = prev_tfm;
+ }
+ }
+
+ {
+ SHASH_DESC_ON_STACK(desc, tfm);
+ desc->tfm = tfm;
+ desc->flags = 0;
+
+ return crypto_shash_digest(desc, key, keysize, salt);
+ }
+}
+
+static int init_essiv_generator(struct fscrypt_info *ci, const u8 *raw_key,
+ int keysize)
+{
+ int err;
+ struct crypto_cipher *essiv_tfm;
+ u8 salt[SHA256_DIGEST_SIZE];
+
+ essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
+ if (IS_ERR(essiv_tfm))
+ return PTR_ERR(essiv_tfm);
+
+ ci->ci_essiv_tfm = essiv_tfm;
+
+ err = derive_essiv_salt(raw_key, keysize, salt);
+ if (err)
+ goto out;
+
+ /*
+ * Using SHA256 to derive the salt/key will result in AES-256 being
+ * used for IV generation. File contents encryption will still use the
+ * configured keysize (AES-128) nevertheless.
+ */
+ err = crypto_cipher_setkey(essiv_tfm, salt, sizeof(salt));
+ if (err)
+ goto out;
+
+out:
+ memzero_explicit(salt, sizeof(salt));
+ return err;
+}
+
+void __exit fscrypt_essiv_cleanup(void)
+{
+ crypto_free_shash(essiv_hash_tfm);
+}
+
int fscrypt_get_encryption_info(struct inode *inode)
{
struct fscrypt_info *crypt_info;
@@ -212,6 +294,7 @@ int fscrypt_get_encryption_info(struct inode *inode)
crypt_info->ci_data_mode = ctx.contents_encryption_mode;
crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
crypt_info->ci_ctfm = NULL;
+ crypt_info->ci_essiv_tfm = NULL;
memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
sizeof(crypt_info->ci_master_key));

@@ -228,10 +311,12 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (!raw_key)
goto out;

- res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX);
+ res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX,
+ keysize);
if (res && inode->i_sb->s_cop->key_prefix) {
int res2 = validate_user_key(crypt_info, &ctx, raw_key,
- inode->i_sb->s_cop->key_prefix);
+ inode->i_sb->s_cop->key_prefix,
+ keysize);
if (res2) {
if (res2 == -ENOKEY)
res = -ENOKEY;
@@ -243,18 +328,30 @@ int fscrypt_get_encryption_info(struct inode *inode)
ctfm = crypto_alloc_skcipher(cipher_str, 0, 0);
if (!ctfm || IS_ERR(ctfm)) {
res = ctfm ? PTR_ERR(ctfm) : -ENOMEM;
- printk(KERN_DEBUG
- "%s: error %d (inode %u) allocating crypto tfm\n",
- __func__, res, (unsigned) inode->i_ino);
+ pr_debug("%s: error %d (inode %lu) allocating crypto tfm\n",
+ __func__, res, inode->i_ino);
goto out;
}
crypt_info->ci_ctfm = ctfm;
crypto_skcipher_clear_flags(ctfm, ~0);
crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
+ /*
+ * if the provided key is longer than keysize, we use the first
+ * keysize bytes of the derived key only
+ */
res = crypto_skcipher_setkey(ctfm, raw_key, keysize);
if (res)
goto out;

+ if (S_ISREG(inode->i_mode) &&
+ crypt_info->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
+ res = init_essiv_generator(crypt_info, raw_key, keysize);
+ if (res) {
+ pr_debug("%s: error %d (inode %lu) allocating essiv tfm\n",
+ __func__, res, inode->i_ino);
+ goto out;
+ }
+ }
if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
crypt_info = NULL;
out:
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 210976e7a269..9914d51dff86 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -38,12 +38,8 @@ static int create_encryption_context_from_policy(struct inode *inode,
memcpy(ctx.master_key_descriptor, policy->master_key_descriptor,
FS_KEY_DESCRIPTOR_SIZE);

- if (!fscrypt_valid_contents_enc_mode(
- policy->contents_encryption_mode))
- return -EINVAL;
-
- if (!fscrypt_valid_filenames_enc_mode(
- policy->filenames_encryption_mode))
+ if (!fscrypt_valid_enc_modes(policy->contents_encryption_mode,
+ policy->filenames_encryption_mode))
return -EINVAL;

if (policy->flags & ~FS_POLICY_FLAGS_VALID)
diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h
index 0a30c106c1e5..4022c61f7e9b 100644
--- a/include/linux/fscrypt_common.h
+++ b/include/linux/fscrypt_common.h
@@ -91,14 +91,18 @@ static inline bool fscrypt_dummy_context_enabled(struct inode *inode)
return false;
}

-static inline bool fscrypt_valid_contents_enc_mode(u32 mode)
+static inline bool fscrypt_valid_enc_modes(u32 contents_mode,
+ u32 filenames_mode)
{
- return (mode == FS_ENCRYPTION_MODE_AES_256_XTS);
-}
+ if (contents_mode == FS_ENCRYPTION_MODE_AES_128_CBC &&
+ filenames_mode == FS_ENCRYPTION_MODE_AES_128_CTS)
+ return true;

-static inline bool fscrypt_valid_filenames_enc_mode(u32 mode)
-{
- return (mode == FS_ENCRYPTION_MODE_AES_256_CTS);
+ if (contents_mode == FS_ENCRYPTION_MODE_AES_256_XTS &&
+ filenames_mode == FS_ENCRYPTION_MODE_AES_256_CTS)
+ return true;
+
+ return false;
}

static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 24e61a54feaa..a2a3ffb06038 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -272,6 +272,8 @@ struct fsxattr {
#define FS_ENCRYPTION_MODE_AES_256_GCM 2
#define FS_ENCRYPTION_MODE_AES_256_CBC 3
#define FS_ENCRYPTION_MODE_AES_256_CTS 4
+#define FS_ENCRYPTION_MODE_AES_128_CBC 5
+#define FS_ENCRYPTION_MODE_AES_128_CTS 6

struct fscrypt_policy {
__u8 version;
--
2.12.3