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

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: default 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

fs/crypto/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -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

fs/crypto/crypto.c

+16 −7
Original line number Diff line number Diff line
@@ -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);

fs/crypto/fscrypt_private.h

+8 −1
Original line number Diff line number Diff line
@@ -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 */

fs/crypto/keyinfo.c

+135 −38
Original line number Diff line number Diff line
@@ -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 @@ out: 


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 (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;

	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;

	}

		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:

fs/crypto/policy.c

+2 −6
Original line number Diff line number Diff line
@@ -38,11 +38,7 @@ 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;

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