Commit b7e7cf7a authored by Daniel Walter's avatar Daniel Walter Committed by Theodore Ts'o

fscrypt: add support for AES-128-CBC

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: default avatarDaniel Walter <dwalter@sigma-star.at>
[david@sigma-star.at: addressed review comments]
Signed-off-by: David Gstir's avatarDavid Gstir <david@sigma-star.at>
Reviewed-by: default avatarEric Biggers <ebiggers@google.com>
Signed-off-by: default avatarTheodore Ts'o <tytso@mit.edu>
parent 27e47a63
......@@ -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
......
......@@ -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);
......
......@@ -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 */
......@@ -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:
......
......@@ -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)
......
......@@ -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)
......
......@@ -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;
......
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