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Date: Thu, 15 Jun 2017 13:41:29 -0700
From: Michael Halcrow <mhalcrow@...gle.com>
To: David Gstir <david@...ma-star.at>
Cc: tytso@....edu, jaegeuk@...nel.org, ebiggers3@...il.com,
richard@...ma-star.at, herbert@...dor.apana.org.au,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-fscrypt@...r.kernel.org,
Daniel Walter <dwalter@...ma-star.at>
Subject: Re: [PATCH v4] fscrypt: Add support for AES-128-CBC
On Tue, May 23, 2017 at 07:11:20AM +0200, David Gstir wrote:
> From: Daniel Walter <dwalter@...ma-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. 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@...ma-star.at>
> [david@...ma-star.at: addressed review comments]
> Signed-off-by: David Gstir <david@...ma-star.at>
> ---
> fs/crypto/Kconfig | 1 +
> fs/crypto/crypto.c | 23 ++++--
> fs/crypto/fscrypt_private.h | 9 ++-
> fs/crypto/keyinfo.c | 171 ++++++++++++++++++++++++++++++++---------
> fs/crypto/policy.c | 8 +-
> include/linux/fscrypt_common.h | 16 ++--
> include/uapi/linux/fs.h | 2 +
> 7 files changed, 172 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..b8f5e1d5a3cc 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) {
I suggest validating the provided key size directly against the mode.
Else, it looks to me that this code will accept a 128-bit key for
AES-256.
> 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 +178,79 @@ 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];
> +
> + if (WARN_ON_ONCE(keysize > sizeof(salt)))
> + return -EINVAL;
> +
> + 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
It would probably be fine to just truncate the generated salt and
stick with AES-128 throughout. However that's just a footnote, and
I think you're fine to keep it the way it is now.
> + * 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 +297,7 @@ 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 +314,12 @@ 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;
> @@ -243,9 +331,8 @@ 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;
> @@ -255,6 +342,14 @@ 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 %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;
> --
> 2.12.0
>
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