Groups > linux.kernel > #1643275 > unrolled thread

[PATCH v3] fscrypt: Add support for AES-128-CBC

Started by	David Gstir <david@sigma-star.at>
First post	2017-05-17 13:30 +0200
Last post	2017-05-23 21:10 +0200
Articles	5 — 2 participants

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  [PATCH v3] fscrypt: Add support for AES-128-CBC David Gstir <david@sigma-star.at> - 2017-05-17 13:30 +0200
    Re: [PATCH v3] fscrypt: Add support for AES-128-CBC Eric Biggers <ebiggers3@gmail.com> - 2017-05-17 20:10 +0200
      Re: [PATCH v3] fscrypt: Add support for AES-128-CBC David Gstir <david@sigma-star.at> - 2017-05-18 16:00 +0200
      [PATCH v4] fscrypt: Add support for AES-128-CBC David Gstir <david@sigma-star.at> - 2017-05-23 07:20 +0200
        Re: [PATCH v4] fscrypt: Add support for AES-128-CBC Eric Biggers <ebiggers3@gmail.com> - 2017-05-23 21:10 +0200

#1643275 — [PATCH v3] fscrypt: Add support for AES-128-CBC

From	David Gstir <david@sigma-star.at>
Date	2017-05-17 13:30 +0200
Subject	[PATCH v3] fscrypt: Add support for AES-128-CBC
Message-ID	<tI5js-2pQ-11@gated-at.bofh.it>

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: addressed review comments]
Signed-off-by: David Gstir <david@sigma-star.at>
---
 fs/crypto/Kconfig              |   1 +
 fs/crypto/crypto.c             |  23 +++++--
 fs/crypto/fscrypt_private.h    |   9 ++-
 fs/crypto/keyinfo.c            | 147 ++++++++++++++++++++++++++++++++++-------
 fs/crypto/policy.c             |   8 +--
 include/linux/fscrypt_common.h |  13 ++--
 include/uapi/linux/fs.h        |   2 +
 7 files changed, 157 insertions(+), 46 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..0d4582c3aef1 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
@@ -388,6 +395,8 @@ static void fscrypt_destroy(void)
 	INIT_LIST_HEAD(&fscrypt_free_ctxs);
 	mempool_destroy(fscrypt_bounce_page_pool);
 	fscrypt_bounce_page_pool = NULL;
+
+	fscrypt_essiv_cleanup();
 }
 
 /**
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 1e1f8a361b75..68e605613352 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 fscrypt_essiv_cleanup(void);
+
 #endif /* _FSCRYPT_PRIVATE_H */
diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c
index 179e578b875b..a09a4fa5ed52 100644
--- a/fs/crypto/keyinfo.c
+++ b/fs/crypto/keyinfo.c
@@ -10,8 +10,14 @@
 
 #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 DEFINE_MUTEX(essiv_hash_lock);
+
 static void derive_crypt_complete(struct crypto_async_request *req, int rc)
 {
 	struct fscrypt_completion_result *ecr = req->data;
@@ -27,13 +33,13 @@ 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 +66,10 @@ 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 +83,7 @@ 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 +117,15 @@ 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 +136,37 @@ 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",
@@ -163,9 +180,75 @@ 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 *key, int keysize, u8 *salt)
+{
+	int err;
+
+	/* init hash transform on demand */
+	if (unlikely(essiv_hash_tfm == NULL)) {
+		mutex_lock(&essiv_hash_lock);
+		if (essiv_hash_tfm == NULL) {
+			essiv_hash_tfm = crypto_alloc_shash("sha256", 0, 0);
+			if (IS_ERR(essiv_hash_tfm)) {
+				pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld\n",
+						    PTR_ERR(essiv_hash_tfm));
+				err = PTR_ERR(essiv_hash_tfm);
+				essiv_hash_tfm = NULL;
+				mutex_unlock(&essiv_hash_lock);
+				return err;
+			}
+		}
+		mutex_unlock(&essiv_hash_lock);
+	}
+
+	{
+		SHASH_DESC_ON_STACK(desc, essiv_hash_tfm);
+		desc->tfm = essiv_hash_tfm;
+		desc->flags = 0;
+
+		return crypto_shash_digest(desc, key, keysize, salt);
+	}
+}
+
+static int init_essiv_generator(struct fscrypt_info *ci, 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;
+
+	err = crypto_cipher_setkey(essiv_tfm, salt, SHA256_DIGEST_SIZE);
+	if (err)
+		goto out;
+
+out:
+	memzero_explicit(salt, sizeof(salt));
+	return err;
+}
+
+void fscrypt_essiv_cleanup(void)
+{
+	crypto_free_shash(essiv_hash_tfm);
+	essiv_hash_tfm = NULL;
+}
+
 int fscrypt_get_encryption_info(struct inode *inode)
 {
 	struct fscrypt_info *crypt_info;
@@ -204,6 +287,10 @@ 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;
@@ -212,6 +299,7 @@ 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 +316,12 @@ 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 +333,8 @@ 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 +344,14 @@ 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..982c08c4f2ac 100644
--- a/include/linux/fscrypt_common.h
+++ b/include/linux/fscrypt_common.h
@@ -91,14 +91,13 @@ 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 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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#1643562

From	Eric Biggers <ebiggers3@gmail.com>
Date	2017-05-17 20:10 +0200
Message-ID	<tIbyy-6u4-21@gated-at.bofh.it>
In reply to	#1643275

Hi David, thanks for the update!

On Wed, May 17, 2017 at 01:21:04PM +0200, David Gstir wrote:
> 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.

You covered this briefly in an email, but can you include more detail in the
commit message on the reasoning behind choosing AES-128 instead of AES-256?
Note that this is independent of the decision of CBC vs. XTS.

> @@ -129,27 +136,37 @@ 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;
>  	}

With the added call to fscrypt_valid_enc_modes() earlier, the warnings about
unsupported encryption modes are no longer reachable.  IMO, the
fscrypt_valid_enc_modes() check should be moved into this function, a proper
warning message added for it, and the redundant warnings removed.  Also now that
there will be more modes I think it would be appropriate to put the algorithm
names and key sizes in a table, to avoid the ugly switch statements.  Here's
what I came up with:

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)
{
	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_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;
	}

	*cipher_str_ret = available_modes[mode].cipher_str;
	*keysize_ret = available_modes[mode].keysize;
	return 0;
}


Note that I changed the 'invalid file type' warning to a WARN_ONCE(), since it
indicates a filesystem bug, unlike the 'unsupported encryption modes' warning
which can be triggered by unrecognized stuff on-disk.

>  
>  	pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
> @@ -163,9 +180,75 @@ 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 *key, int keysize, u8 *salt)
> +{

const u8 *key

> +	int err;
> +
> +	/* init hash transform on demand */
> +	if (unlikely(essiv_hash_tfm == NULL)) {
> +		mutex_lock(&essiv_hash_lock);
> +		if (essiv_hash_tfm == NULL) {
> +			essiv_hash_tfm = crypto_alloc_shash("sha256", 0, 0);
> +			if (IS_ERR(essiv_hash_tfm)) {
> +				pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld\n",
> +						    PTR_ERR(essiv_hash_tfm));
> +				err = PTR_ERR(essiv_hash_tfm);
> +				essiv_hash_tfm = NULL;
> +				mutex_unlock(&essiv_hash_lock);
> +				return err;
> +			}
> +		}
> +		mutex_unlock(&essiv_hash_lock);
> +	}

There is a bug here: a thread can set essiv_hash_tfm to an ERR_PTR(), and
another thread can use it before it's set back to NULL.  Did you consider using
a cmpxchg-based solution instead, similar to what fscrypt_get_encryption_info()
does with ->i_crypt_info?  Then there would be no need for a mutex.  Something
like this:

static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
{
        /* init hash transform on demand */
        struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);

        if (unlikely(!tfm)) {
                struct crypto_shash *prev;

                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 = cmpxchg(&essiv_hash_tfm, NULL, tfm);
                if (prev) {
                        crypto_free_shash(tfm);
                        tfm = prev;
                }
        }

        {
                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, u8 *raw_key,
> +				int keysize)

const u8 *raw_key

> +{
> +	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;
> +
> +	err = crypto_cipher_setkey(essiv_tfm, salt, SHA256_DIGEST_SIZE);
> +	if (err)
> +		goto out;

sizeof(salt) instead of hardcoding SHA256_DIGEST_SIZE.

I think there should also be a brief comment explaining that the ESSIV cipher
uses AES-256 so that its key size matches the size of the hash, even though the
"real" encryption may use AES-128.

> +void fscrypt_essiv_cleanup(void)
> +{
> +	crypto_free_shash(essiv_hash_tfm);
> +	essiv_hash_tfm = NULL;
> +}

This is called from fscrypt_destroy(), which is a little weird because
fscrypt_destroy() is meant to clean up only from "fscrypt_initialize()", which
only allocates certain things.  I think it should be called from
"fscrypt_exit()" instead.  Then you could also add the __exit annotation, and
remove setting essiv_hash_tfm to NULL which would clearly be unnecessary.

> +
>  int fscrypt_get_encryption_info(struct inode *inode)
>  {
>  	struct fscrypt_info *crypt_info;
> @@ -204,6 +287,10 @@ 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;
> +

As noted earlier I think this should be moved into determine_cipher_type(), to
avoid redundancy when interpreting the encryption modes.

> diff --git a/include/linux/fscrypt_common.h b/include/linux/fscrypt_common.h
> index 0a30c106c1e5..982c08c4f2ac 100644
> --- a/include/linux/fscrypt_common.h
> +++ b/include/linux/fscrypt_common.h
> @@ -91,14 +91,13 @@ 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));
>  }
>  

IMO, make these 'if' statements, to discourage people from turning this
expression into more of a mess when they add more modes:

static inline bool fscrypt_valid_enc_modes(u32 contents_mode,
                                        u32 filenames_mode)
{
        if (contents_mode == FS_ENCRYPTION_MODE_AES_256_XTS &&
            filenames_mode == FS_ENCRYPTION_MODE_AES_256_CTS)
                return true;

        if (contents_mode == FS_ENCRYPTION_MODE_AES_128_CBC &&
            filenames_mode == FS_ENCRYPTION_MODE_AES_128_CTS)
                return true;

        return false;
}

Eric

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#1644568

From	David Gstir <david@sigma-star.at>
Date	2017-05-18 16:00 +0200
Message-ID	<tIu89-3sl-3@gated-at.bofh.it>
In reply to	#1643562

[resend without the HTML crap - sorry about that!]

Hi Eric!

Thanks for the thorough review! :)

> On 17 May 2017, at 20:08, Eric Biggers <ebiggers3@gmail.com> wrote:
> 
> Hi David, thanks for the update!
> 
> On Wed, May 17, 2017 at 01:21:04PM +0200, David Gstir wrote:
>> 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.
> 
> You covered this briefly in an email, but can you include more detail in the
> commit message on the reasoning behind choosing AES-128 instead of AES-256?
> Note that this is independent of the decision of CBC vs. XTS.

Sure, I'll extend the commit message to include that.


> 
>> @@ -129,27 +136,37 @@ 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;
>> 	}
> 
> With the added call to fscrypt_valid_enc_modes() earlier, the warnings about
> unsupported encryption modes are no longer reachable.  IMO, the
> fscrypt_valid_enc_modes() check should be moved into this function, a proper
> warning message added for it, and the redundant warnings removed.  Also now that
> there will be more modes I think it would be appropriate to put the algorithm
> names and key sizes in a table, to avoid the ugly switch statements.  

I agree. I'll clean this up.



>> +	int err;
>> +
>> +	/* init hash transform on demand */
>> +	if (unlikely(essiv_hash_tfm == NULL)) {
>> +		mutex_lock(&essiv_hash_lock);
>> +		if (essiv_hash_tfm == NULL) {
>> +			essiv_hash_tfm = crypto_alloc_shash("sha256", 0, 0);
>> +			if (IS_ERR(essiv_hash_tfm)) {
>> +				pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld\n",
>> +						    PTR_ERR(essiv_hash_tfm));
>> +				err = PTR_ERR(essiv_hash_tfm);
>> +				essiv_hash_tfm = NULL;
>> +				mutex_unlock(&essiv_hash_lock);
>> +				return err;
>> +			}
>> +		}
>> +		mutex_unlock(&essiv_hash_lock);
>> +	}
> 
> There is a bug here: a thread can set essiv_hash_tfm to an ERR_PTR(), and
> another thread can use it before it's set back to NULL.  

Sorry, I missed that... :-(



>> +	err = crypto_cipher_setkey(essiv_tfm, salt, SHA256_DIGEST_SIZE);
>> +	if (err)
>> +		goto out;
> 
> sizeof(salt) instead of hardcoding SHA256_DIGEST_SIZE.
> 
> I think there should also be a brief comment explaining that the ESSIV cipher
> uses AES-256 so that its key size matches the size of the hash, even though the
> "real" encryption may use AES-128.

Good point!


> 
>> +void fscrypt_essiv_cleanup(void)
>> +{
>> +	crypto_free_shash(essiv_hash_tfm);
>> +	essiv_hash_tfm = NULL;
>> +}
> 
> This is called from fscrypt_destroy(), which is a little weird because
> fscrypt_destroy() is meant to clean up only from "fscrypt_initialize()", which
> only allocates certain things.  I think it should be called from
> "fscrypt_exit()" instead.  Then you could also add the __exit annotation, and
> remove setting essiv_hash_tfm to NULL which would clearly be unnecessary.

fscrypt_destroy() is actually also called from fscrypt_exit(). Thats why I chose it,
but changing this fscrypt_exit() seems the cleaner approach. :)

Thanks,
David

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#1647639 — [PATCH v4] fscrypt: Add support for AES-128-CBC

From	David Gstir <david@sigma-star.at>
Date	2017-05-23 07:20 +0200
Subject	[PATCH v4] fscrypt: Add support for AES-128-CBC
Message-ID	<tKaoF-7aM-13@gated-at.bofh.it>
In reply to	#1643562

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. 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>
---
 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) {
 		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
+	 * 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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#1648349 — Re: [PATCH v4] fscrypt: Add support for AES-128-CBC

From	Eric Biggers <ebiggers3@gmail.com>
Date	2017-05-23 21:10 +0200
Subject	Re: [PATCH v4] fscrypt: Add support for AES-128-CBC
Message-ID	<tKnlT-7qO-3@gated-at.bofh.it>
In reply to	#1647639

Hi David,

On Tue, May 23, 2017 at 07:11:20AM +0200, David Gstir wrote:
> 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. 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>

Overall this looks good now; you can add

Reviewed-by: Eric Biggers <ebiggers@google.com>

I did notice a couple minor improvements that can be made, though:

>  
> +	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;
> +		}
> +	}

Since the ESSIV generator is only needed for contents encryption, it should only
be initialized when both 'S_ISREG(inode->i_mode) && crypt_info->ci_data_mode ==
FS_ENCRYPTION_MODE_AES_128_CBC'.  Otherwise ->ci_essiv_tfm will be allocated for
directories and symlinks too, then never used.

> +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;
> +

The 'keysize > sizeof(salt)' check is now pointless and should be removed, since
we decided not to key the ESSIV cipher with 'keysize' bytes, but rather with
sizeof(salt) bytes.  So this function is compatible with any 'keysize', not just
keysize <= sizeof(salt).

You should also consider how it should be made possible to test these new
encryption modes in xfstests.  Currently, while the "set_encpolicy" xfs_io
command allows specifying different encryption modes and flags, in general the
tests in the "encrypt" group are hardcoded to use AES_256_XTS and AES_256_CTS.
Similarly, those modes are also used with the test_dummy_encryption mount
option, which causes all new files to be automatically encrypted, and is used by
the "encrypt" config for kvm-xfstests and gce-xfstests (currently ext4-specific,
but other filesystems could support it too).

Eric

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csiph-web

[PATCH v3] fscrypt: Add support for AES-128-CBC

Contents

#1643275 — [PATCH v3] fscrypt: Add support for AES-128-CBC

#1643562

#1644568

#1647639 — [PATCH v4] fscrypt: Add support for AES-128-CBC

#1648349 — Re: [PATCH v4] fscrypt: Add support for AES-128-CBC