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Subject[PATCH v2] 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. 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. Especially low-powered embedded
devices with 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.

Signed-off-by: Daniel Walter <dwalter@sigma-star.at>
[david@sigma-star.at: massaged commit message]
Signed-off-by: David Gstir <david@sigma-star.at>
---
v2: Compute ESSIV salt using SHA256 instead of SHA1 and improve style
as pointed out by Eric Biggers [1].

[1] https://lkml.kernel.org/r/20170331062149.GA32409%20()%20zzz

fs/crypto/crypto.c | 22 ++++--
fs/crypto/fscrypt_private.h | 5 +-
fs/crypto/keyinfo.c | 165 ++++++++++++++++++++++++++++++++++-------
fs/crypto/policy.c | 7 +-
include/linux/fscrypt_common.h | 13 ++--
include/uapi/linux/fs.h | 2 +
6 files changed, 169 insertions(+), 45 deletions(-)

diff --git a/fs/crypto/crypto.c b/fs/crypto/crypto.c
index 6d6eca394d4d..9dd5f23a55a5 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 +148,28 @@ 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;
int res = 0;
+ u8 *iv = (u8 *)&blk_desc;

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_essiv_tfm != NULL) {
+ memset(iv, 0, sizeof(blk_desc));
+ crypto_cipher_encrypt_one(ci->ci_essiv_tfm, iv, iv);
+ }
+
req = skcipher_request_alloc(tfm, gfp_flags);
if (!req) {
printk_ratelimited(KERN_ERR
@@ -170,15 +182,11 @@ 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 e39696e64494..81ce9af449cb 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 8cdfddce2b34..590efb0a891d 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -10,6 +10,8 @@

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

static void derive_crypt_complete(struct crypto_async_request *req, int rc)
@@ -27,13 +29,13 @@ 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])
+ const struct fscrypt_key *source_key,
+ u8 derived_raw_key[FS_MAX_KEY_SIZE])
{
int res = 0;
struct skcipher_request *req = NULL;
@@ -60,10 +62,10 @@ 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 +79,7 @@ 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;
@@ -111,14 +113,15 @@ 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);
@@ -129,27 +132,37 @@ 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) {
+ switch (ci->ci_data_mode) {
+ case FS_ENCRYPTION_MODE_AES_256_XTS:
*cipher_str_ret = "xts(aes)";
*keysize_ret = FS_AES_256_XTS_KEY_SIZE;
return 0;
+ case FS_ENCRYPTION_MODE_AES_128_CBC:
+ *cipher_str_ret = "cbc(aes)";
+ *keysize_ret = FS_AES_128_CBC_KEY_SIZE;
+ return 0;
+ default:
+ pr_warn_once("fscrypto: unsupported contents encryption mode %d for inode %lu\n",
+ ci->ci_data_mode, inode->i_ino);
+ return -ENOKEY;
}
- pr_warn_once("fscrypto: unsupported contents encryption mode "
- "%d for inode %lu\n",
- ci->ci_data_mode, inode->i_ino);
- return -ENOKEY;
}

if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
- if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS) {
+ switch (ci->ci_filename_mode) {
+ case FS_ENCRYPTION_MODE_AES_256_CTS:
*cipher_str_ret = "cts(cbc(aes))";
*keysize_ret = FS_AES_256_CTS_KEY_SIZE;
return 0;
+ case FS_ENCRYPTION_MODE_AES_128_CTS:
+ *cipher_str_ret = "cts(cbc(aes))";
+ *keysize_ret = FS_AES_128_CTS_KEY_SIZE;
+ return 0;
+ default:
+ pr_warn_once("fscrypto: unsupported filenames encryption mode %d for inode %lu\n",
+ ci->ci_filename_mode, inode->i_ino);
+ return -ENOKEY;
}
- pr_warn_once("fscrypto: unsupported filenames encryption mode "
- "%d for inode %lu\n",
- ci->ci_filename_mode, inode->i_ino);
- return -ENOKEY;
}

pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
@@ -163,14 +176,100 @@ 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(u8 *raw_key, int keysize, u8 **salt_out,
+ unsigned int *saltsize)
+{
+ int res;
+ struct crypto_ahash *hash_tfm;
+ unsigned int digestsize;
+ u8 *salt = NULL;
+ struct scatterlist sg;
+ struct ahash_request *req = NULL;
+
+ hash_tfm = crypto_alloc_ahash("sha256", 0, 0);
+ if (IS_ERR(hash_tfm))
+ return PTR_ERR(hash_tfm);
+
+ digestsize = crypto_ahash_digestsize(hash_tfm);
+ salt = kzalloc(digestsize, GFP_NOFS);
+ if (!salt) {
+ res = -ENOMEM;
+ goto out;
+ }
+
+ req = ahash_request_alloc(hash_tfm, GFP_NOFS);
+ if (!req) {
+ kfree(salt);
+ res = -ENOMEM;
+ goto out;
+ }
+
+ sg_init_one(&sg, raw_key, keysize);
+ ahash_request_set_callback(req,
+ CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
+ NULL, NULL);
+ ahash_request_set_crypt(req, &sg, salt, keysize);
+
+ res = crypto_ahash_digest(req);
+ if (res) {
+ kfree(salt);
+ goto out;
+ }
+
+ *salt_out = salt;
+ *saltsize = digestsize;
+
+out:
+ crypto_free_ahash(hash_tfm);
+ ahash_request_free(req);
+ return res;
+}
+
+static int init_essiv_generator(struct fscrypt_info *ci, u8 *raw_key,
+ int keysize)
+{
+ int res;
+ struct crypto_cipher *essiv_tfm;
+ u8 *salt = NULL;
+ unsigned int saltsize;
+
+ /* 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;
+ goto err;
+ }
+
+ res = derive_essiv_salt(raw_key, keysize, &salt, &saltsize);
+ if (res)
+ goto err;
+
+ res = crypto_cipher_setkey(essiv_tfm, salt, saltsize);
+ if (res)
+ goto err;
+
+ ci->ci_essiv_tfm = essiv_tfm;
+
+ return 0;
+
+err:
+ if (essiv_tfm && !IS_ERR(essiv_tfm))
+ crypto_free_cipher(essiv_tfm);
+
+ kzfree(salt);
+ return res;
+}
+
int fscrypt_get_encryption_info(struct inode *inode)
{
struct fscrypt_info *crypt_info;
struct fscrypt_context ctx;
struct crypto_skcipher *ctfm;
+
const char *cipher_str;
int keysize;
u8 *raw_key = NULL;
@@ -207,6 +306,10 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
return -EINVAL;

+ if (!fscrypt_valid_enc_modes(ctx.contents_encryption_mode,
+ ctx.filenames_encryption_mode))
+ return -EINVAL;
+
crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
if (!crypt_info)
return -ENOMEM;
@@ -215,6 +318,7 @@ 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 +335,12 @@ 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;
@@ -246,9 +352,9 @@ 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 %u) allocating crypto tfm\n",
+ __func__, res, (unsigned int) inode->i_ino);
goto out;
}
crypt_info->ci_ctfm = ctfm;
@@ -258,6 +364,15 @@ int fscrypt_get_encryption_info(struct inode *inode)
if (res)
goto out;

+ if (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 %u) allocating essiv tfm\n",
+ __func__, res, (unsigned int) 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 4908906d54d5..bac8009245f2 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -41,11 +41,8 @@ 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(
+ if (!fscrypt_valid_enc_modes(
+ policy->contents_encryption_mode,
policy->filenames_encryption_mode))
return -EINVAL;

diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h
index 10c1abfbac6c..a91ce99f6e19 100644
--- a/include/linux/fscrypt_common.h
+++ b/include/linux/fscrypt_common.h
@@ -102,14 +102,13 @@ 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);
-}
-
-static inline bool fscrypt_valid_filenames_enc_mode(u32 mode)
-{
- return (mode == FS_ENCRYPTION_MODE_AES_256_CTS);
+ return ((contents_mode == FS_ENCRYPTION_MODE_AES_128_CBC &&
+ filenames_mode == FS_ENCRYPTION_MODE_AES_128_CTS) ||
+ (contents_mode == FS_ENCRYPTION_MODE_AES_256_XTS &&
+ filenames_mode == FS_ENCRYPTION_MODE_AES_256_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 048a85e9f017..231b15d4a90e 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;
--
2.12.0
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