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fscrypt: add support for AES-128-CBC

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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@sigma-star.at>
[david@sigma-star.at: addressed review comments]
Signed-off-by: David Gstir <david@sigma-star.at>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Daniel Walter
2017-06-19 09:27:58 +02:00
committed by Theodore Ts'o
parent 27e47a6342
commit b7e7cf7a66
7 changed files with 175 additions and 59 deletions
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23
fs/crypto/crypto.c
View File

@@ -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);