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Subject[PATCH] fscrypt: Add support for AES-128-CBC
Date
From: Daniel Walter <dwalter@sigma-star.at>

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.
Which 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. Especially low-powered embedded devices crypto
accelerators such as CAAM or CESA support only AES-CBC-128 with an acceptable
speed. Using AES-CBC gives us the acceptable performance while still providing
a moderate level of security for persistent storage.

[david: massaged commit message]

Signed-off-by: Daniel Walter <dwalter@sigma-star.at>
Signed-off-by: David Gstir <david@sigma-star.at>
---
fs/crypto/crypto.c | 25 ++++++++----
fs/crypto/fscrypt_private.h | 5 ++-
fs/crypto/keyinfo.c | 87 ++++++++++++++++++++++++++++++++++++------
include/linux/fscrypt_common.h | 6 ++-
include/uapi/linux/fs.h | 2 +
5 files changed, 103 insertions(+), 22 deletions(-)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 6d6eca3..210b586 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;
@@ -147,17 +148,31 @@ 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)];
+ } blk_desc;
struct skcipher_request *req = NULL;
DECLARE_FS_COMPLETION_RESULT(ecr);
struct scatterlist dst, src;
struct fscrypt_info *ci = inode->i_crypt_info;
struct crypto_skcipher *tfm = ci->ci_ctfm;
+ u8 iv[FS_IV_SIZE];
int res = 0;

BUG_ON(len == 0);

+ BUILD_BUG_ON(sizeof(blk_desc) != FS_IV_SIZE);
+ BUILD_BUG_ON(AES_BLOCK_SIZE != FS_IV_SIZE);
+ blk_desc.index = cpu_to_le64(lblk_num);
+ memset(blk_desc.padding, 0, sizeof(blk_desc.padding));
+
+ if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
+ BUG_ON(ci->ci_essiv_tfm == NULL);
+ memset(iv, 0, sizeof(iv));
+ crypto_cipher_encrypt_one(ci->ci_essiv_tfm, iv, (u8 *)&blk_desc);
+ } else {
+ memcpy(iv, &blk_desc, sizeof(iv));
+ }
+
req = skcipher_request_alloc(tfm, gfp_flags);
if (!req) {
printk_ratelimited(KERN_ERR
@@ -170,15 +185,11 @@ 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
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index e39696e..81ce9af 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -16,8 +16,10 @@
#define FS_FNAME_CRYPTO_DIGEST_SIZE 32

/* 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
@@ -67,6 +69,7 @@ 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];
};

diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 8cdfddc..cc515e9 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -10,6 +10,7 @@

#include <keys/user-type.h>
#include <linux/scatterlist.h>
+#include <linux/cryptohash.h>
#include "fscrypt_private.h"

static void derive_crypt_complete(struct crypto_async_request *req, int rc)
@@ -27,13 +28,13 @@ 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_key_raw: 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])
+ struct fscrypt_key *source_key,
+ u8 derived_raw_key[FS_MAX_KEY_SIZE])
{
int res = 0;
struct skcipher_request *req = NULL;
@@ -60,10 +61,10 @@ 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);
@@ -75,9 +76,28 @@ static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
return res;
}

+static bool valid_key_size(struct fscrypt_info *ci, struct fscrypt_key *key,
+ int reg_file)
+{
+ if (reg_file) {
+ switch(ci->ci_data_mode) {
+ case FS_ENCRYPTION_MODE_AES_256_XTS:
+ return key->size >= FS_AES_256_XTS_KEY_SIZE;
+ case FS_ENCRYPTION_MODE_AES_128_CBC:
+ return key->size >= FS_AES_128_CBC_KEY_SIZE;
+ }
+ } else {
+ if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS)
+ return key->size >= FS_AES_256_CTS_KEY_SIZE;
+ if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_128_CTS)
+ return key->size >= FS_AES_128_CTS_KEY_SIZE;
+ }
+ return false;
+}
+
static int validate_user_key(struct fscrypt_info *crypt_info,
struct fscrypt_context *ctx, u8 *raw_key,
- const char *prefix)
+ const char *prefix, int reg_file)
{
char *description;
struct key *keyring_key;
@@ -111,14 +131,14 @@ 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 (!valid_key_size(crypt_info, master_key, reg_file)) {
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);
@@ -134,6 +154,11 @@ static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
*keysize_ret = FS_AES_256_XTS_KEY_SIZE;
return 0;
}
+ if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
+ *cipher_str_ret = "cbc(aes)";
+ *keysize_ret = FS_AES_128_CBC_KEY_SIZE;
+ return 0;
+ }
pr_warn_once("fscrypto: unsupported contents encryption mode "
"%d for inode %lu\n",
ci->ci_data_mode, inode->i_ino);
@@ -146,6 +171,11 @@ static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
*keysize_ret = FS_AES_256_CTS_KEY_SIZE;
return 0;
}
+ if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_128_CTS) {
+ *cipher_str_ret = "cts(cbc(aes))";
+ *keysize_ret = FS_AES_128_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);
@@ -163,6 +193,8 @@ static void put_crypt_info(struct fscrypt_info *ci)
return;

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

@@ -171,6 +203,10 @@ int fscrypt_get_encryption_info(struct inode *inode)
struct fscrypt_info *crypt_info;
struct fscrypt_context ctx;
struct crypto_skcipher *ctfm;
+ struct crypto_cipher *essiv_tfm;
+ __u32 sha_ws[SHA_WORKSPACE_WORDS];
+ __u32 essiv_key[SHA_DIGEST_WORDS];
+
const char *cipher_str;
int keysize;
u8 *raw_key = NULL;
@@ -207,6 +243,10 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
return -EINVAL;

+ if (ctx.contents_encryption_mode == FS_ENCRYPTION_MODE_AES_128_CBC &&
+ ctx.filenames_encryption_mode != FS_ENCRYPTION_MODE_AES_128_CTS)
+ return -EINVAL;
+
crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
if (!crypt_info)
return -ENOMEM;
@@ -215,6 +255,7 @@ 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));

@@ -231,10 +272,12 @@ 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,
+ S_ISREG(inode->i_mode));
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,
+ S_ISREG(inode->i_mode));
if (res2) {
if (res2 == -ENOKEY)
res = -ENOKEY;
@@ -258,6 +301,26 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (res)
goto out;

+ if (crypt_info->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
+ /* init ESSIV generator */
+ essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
+ if (!essiv_tfm || IS_ERR(essiv_tfm)) {
+ res = essiv_tfm ? PTR_ERR(essiv_tfm) : -ENOMEM;
+ printk(KERN_DEBUG
+ "%s: error %d (inode %u) allocating essiv tfm\n",
+ __func__, res, (unsigned) inode->i_ino);
+ goto out;
+ }
+ /* calc sha of key for essiv generation */
+ memset(sha_ws, 0, sizeof(sha_ws));
+ sha_init(essiv_key);
+ sha_transform(essiv_key, raw_key, sha_ws);
+ res = crypto_cipher_setkey(essiv_tfm, (u8 *)essiv_key, keysize);
+ if (res)
+ goto out;
+
+ crypt_info->ci_essiv_tfm = essiv_tfm;
+ }
if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
crypt_info = NULL;
out:
diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h
index 10c1abf..83d32a6 100644
--- a/include/linux/fscrypt_common.h
+++ b/include/linux/fscrypt_common.h
@@ -104,12 +104,14 @@ static inline bool fscrypt_dummy_context_enabled(struct inode *inode)

static inline bool fscrypt_valid_contents_enc_mode(u32 mode)
{
- return (mode == FS_ENCRYPTION_MODE_AES_256_XTS);
+ return (mode == FS_ENCRYPTION_MODE_AES_256_XTS ||
+ mode == FS_ENCRYPTION_MODE_AES_128_CBC);
}

static inline bool fscrypt_valid_filenames_enc_mode(u32 mode)
{
- return (mode == FS_ENCRYPTION_MODE_AES_256_CTS);
+ return (mode == FS_ENCRYPTION_MODE_AES_256_CTS ||
+ mode == FS_ENCRYPTION_MODE_AES_128_CTS);
}

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 048a85e..231b15d 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;
--
2.10.2
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