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: 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
Theodore Ts'o
parent
27e47a6342
commit
b7e7cf7a66
7 changed files with 175 additions and 59 deletions
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@ -7,6 +7,7 @@ config FS_ENCRYPTION
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select CRYPTO_XTS
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select CRYPTO_CTS
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select CRYPTO_CTR
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select CRYPTO_SHA256
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select KEYS
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help
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Enable encryption of files and directories. This
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@ -26,6 +26,7 @@
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#include <linux/ratelimit.h>
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#include <linux/dcache.h>
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#include <linux/namei.h>
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#include <crypto/aes.h>
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#include "fscrypt_private.h"
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static unsigned int num_prealloc_crypto_pages = 32;
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@ -147,8 +148,8 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
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{
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struct {
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__le64 index;
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u8 padding[FS_XTS_TWEAK_SIZE - sizeof(__le64)];
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} xts_tweak;
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u8 padding[FS_IV_SIZE - sizeof(__le64)];
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} iv;
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struct skcipher_request *req = NULL;
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DECLARE_FS_COMPLETION_RESULT(ecr);
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struct scatterlist dst, src;
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@ -158,6 +159,16 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
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BUG_ON(len == 0);
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BUILD_BUG_ON(sizeof(iv) != FS_IV_SIZE);
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BUILD_BUG_ON(AES_BLOCK_SIZE != FS_IV_SIZE);
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iv.index = cpu_to_le64(lblk_num);
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memset(iv.padding, 0, sizeof(iv.padding));
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if (ci->ci_essiv_tfm != NULL) {
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crypto_cipher_encrypt_one(ci->ci_essiv_tfm, (u8 *)&iv,
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(u8 *)&iv);
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}
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req = skcipher_request_alloc(tfm, gfp_flags);
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if (!req) {
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printk_ratelimited(KERN_ERR
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@ -170,15 +181,11 @@ int fscrypt_do_page_crypto(const struct inode *inode, fscrypt_direction_t rw,
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req, CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
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page_crypt_complete, &ecr);
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BUILD_BUG_ON(sizeof(xts_tweak) != FS_XTS_TWEAK_SIZE);
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xts_tweak.index = cpu_to_le64(lblk_num);
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memset(xts_tweak.padding, 0, sizeof(xts_tweak.padding));
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sg_init_table(&dst, 1);
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sg_set_page(&dst, dest_page, len, offs);
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sg_init_table(&src, 1);
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sg_set_page(&src, src_page, len, offs);
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skcipher_request_set_crypt(req, &src, &dst, len, &xts_tweak);
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skcipher_request_set_crypt(req, &src, &dst, len, &iv);
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if (rw == FS_DECRYPT)
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res = crypto_skcipher_decrypt(req);
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else
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@ -477,6 +484,8 @@ static void __exit fscrypt_exit(void)
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destroy_workqueue(fscrypt_read_workqueue);
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kmem_cache_destroy(fscrypt_ctx_cachep);
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kmem_cache_destroy(fscrypt_info_cachep);
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fscrypt_essiv_cleanup();
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}
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module_exit(fscrypt_exit);
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@ -12,10 +12,13 @@
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#define _FSCRYPT_PRIVATE_H
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#include <linux/fscrypt_supp.h>
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#include <crypto/hash.h>
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/* Encryption parameters */
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#define FS_XTS_TWEAK_SIZE 16
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#define FS_IV_SIZE 16
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#define FS_AES_128_ECB_KEY_SIZE 16
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#define FS_AES_128_CBC_KEY_SIZE 16
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#define FS_AES_128_CTS_KEY_SIZE 16
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#define FS_AES_256_GCM_KEY_SIZE 32
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#define FS_AES_256_CBC_KEY_SIZE 32
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#define FS_AES_256_CTS_KEY_SIZE 32
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@ -54,6 +57,7 @@ struct fscrypt_info {
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u8 ci_filename_mode;
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u8 ci_flags;
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struct crypto_skcipher *ci_ctfm;
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struct crypto_cipher *ci_essiv_tfm;
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u8 ci_master_key[FS_KEY_DESCRIPTOR_SIZE];
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};
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@ -87,4 +91,7 @@ extern int fscrypt_do_page_crypto(const struct inode *inode,
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extern struct page *fscrypt_alloc_bounce_page(struct fscrypt_ctx *ctx,
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gfp_t gfp_flags);
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/* keyinfo.c */
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extern void __exit fscrypt_essiv_cleanup(void);
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#endif /* _FSCRYPT_PRIVATE_H */
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@ -10,8 +10,13 @@
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#include <keys/user-type.h>
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#include <linux/scatterlist.h>
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#include <linux/ratelimit.h>
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#include <crypto/aes.h>
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#include <crypto/sha.h>
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#include "fscrypt_private.h"
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static struct crypto_shash *essiv_hash_tfm;
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static void derive_crypt_complete(struct crypto_async_request *req, int rc)
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{
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struct fscrypt_completion_result *ecr = req->data;
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@ -27,13 +32,13 @@ static void derive_crypt_complete(struct crypto_async_request *req, int rc)
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* derive_key_aes() - Derive a key using AES-128-ECB
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* @deriving_key: Encryption key used for derivation.
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* @source_key: Source key to which to apply derivation.
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* @derived_key: Derived key.
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* @derived_raw_key: Derived raw key.
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*
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* Return: Zero on success; non-zero otherwise.
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*/
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static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
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u8 source_key[FS_AES_256_XTS_KEY_SIZE],
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u8 derived_key[FS_AES_256_XTS_KEY_SIZE])
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const struct fscrypt_key *source_key,
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u8 derived_raw_key[FS_MAX_KEY_SIZE])
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{
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int res = 0;
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struct skcipher_request *req = NULL;
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@ -60,10 +65,10 @@ static int derive_key_aes(u8 deriving_key[FS_AES_128_ECB_KEY_SIZE],
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if (res < 0)
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goto out;
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sg_init_one(&src_sg, source_key, FS_AES_256_XTS_KEY_SIZE);
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sg_init_one(&dst_sg, derived_key, FS_AES_256_XTS_KEY_SIZE);
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skcipher_request_set_crypt(req, &src_sg, &dst_sg,
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FS_AES_256_XTS_KEY_SIZE, NULL);
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sg_init_one(&src_sg, source_key->raw, source_key->size);
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sg_init_one(&dst_sg, derived_raw_key, source_key->size);
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skcipher_request_set_crypt(req, &src_sg, &dst_sg, source_key->size,
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NULL);
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res = crypto_skcipher_encrypt(req);
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if (res == -EINPROGRESS || res == -EBUSY) {
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wait_for_completion(&ecr.completion);
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@ -77,7 +82,7 @@ out:
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static int validate_user_key(struct fscrypt_info *crypt_info,
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struct fscrypt_context *ctx, u8 *raw_key,
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const char *prefix)
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const char *prefix, int min_keysize)
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{
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char *description;
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struct key *keyring_key;
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@ -111,50 +116,60 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
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master_key = (struct fscrypt_key *)ukp->data;
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BUILD_BUG_ON(FS_AES_128_ECB_KEY_SIZE != FS_KEY_DERIVATION_NONCE_SIZE);
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if (master_key->size != FS_AES_256_XTS_KEY_SIZE) {
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if (master_key->size < min_keysize || master_key->size > FS_MAX_KEY_SIZE
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|| master_key->size % AES_BLOCK_SIZE != 0) {
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printk_once(KERN_WARNING
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"%s: key size incorrect: %d\n",
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__func__, master_key->size);
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res = -ENOKEY;
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goto out;
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}
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res = derive_key_aes(ctx->nonce, master_key->raw, raw_key);
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res = derive_key_aes(ctx->nonce, master_key, raw_key);
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out:
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up_read(&keyring_key->sem);
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key_put(keyring_key);
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return res;
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}
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static const struct {
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const char *cipher_str;
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int keysize;
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} available_modes[] = {
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[FS_ENCRYPTION_MODE_AES_256_XTS] = { "xts(aes)",
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FS_AES_256_XTS_KEY_SIZE },
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[FS_ENCRYPTION_MODE_AES_256_CTS] = { "cts(cbc(aes))",
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FS_AES_256_CTS_KEY_SIZE },
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[FS_ENCRYPTION_MODE_AES_128_CBC] = { "cbc(aes)",
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FS_AES_128_CBC_KEY_SIZE },
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[FS_ENCRYPTION_MODE_AES_128_CTS] = { "cts(cbc(aes))",
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FS_AES_128_CTS_KEY_SIZE },
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};
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static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
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const char **cipher_str_ret, int *keysize_ret)
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{
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u32 mode;
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if (!fscrypt_valid_enc_modes(ci->ci_data_mode, ci->ci_filename_mode)) {
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pr_warn_ratelimited("fscrypt: inode %lu uses unsupported encryption modes (contents mode %d, filenames mode %d)\n",
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inode->i_ino,
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ci->ci_data_mode, ci->ci_filename_mode);
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return -EINVAL;
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}
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if (S_ISREG(inode->i_mode)) {
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if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_256_XTS) {
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*cipher_str_ret = "xts(aes)";
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*keysize_ret = FS_AES_256_XTS_KEY_SIZE;
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return 0;
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}
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pr_warn_once("fscrypto: unsupported contents encryption mode "
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"%d for inode %lu\n",
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ci->ci_data_mode, inode->i_ino);
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return -ENOKEY;
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mode = ci->ci_data_mode;
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} else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
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mode = ci->ci_filename_mode;
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} else {
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WARN_ONCE(1, "fscrypt: filesystem tried to load encryption info for inode %lu, which is not encryptable (file type %d)\n",
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inode->i_ino, (inode->i_mode & S_IFMT));
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return -EINVAL;
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}
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if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
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if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS) {
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*cipher_str_ret = "cts(cbc(aes))";
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*keysize_ret = FS_AES_256_CTS_KEY_SIZE;
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return 0;
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}
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pr_warn_once("fscrypto: unsupported filenames encryption mode "
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"%d for inode %lu\n",
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ci->ci_filename_mode, inode->i_ino);
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return -ENOKEY;
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}
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pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
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(inode->i_mode & S_IFMT), inode->i_ino);
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return -ENOKEY;
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*cipher_str_ret = available_modes[mode].cipher_str;
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*keysize_ret = available_modes[mode].keysize;
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return 0;
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}
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static void put_crypt_info(struct fscrypt_info *ci)
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@ -163,9 +178,76 @@ static void put_crypt_info(struct fscrypt_info *ci)
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return;
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crypto_free_skcipher(ci->ci_ctfm);
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crypto_free_cipher(ci->ci_essiv_tfm);
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kmem_cache_free(fscrypt_info_cachep, ci);
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}
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static int derive_essiv_salt(const u8 *key, int keysize, u8 *salt)
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{
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struct crypto_shash *tfm = READ_ONCE(essiv_hash_tfm);
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/* init hash transform on demand */
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if (unlikely(!tfm)) {
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struct crypto_shash *prev_tfm;
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tfm = crypto_alloc_shash("sha256", 0, 0);
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if (IS_ERR(tfm)) {
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pr_warn_ratelimited("fscrypt: error allocating SHA-256 transform: %ld\n",
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PTR_ERR(tfm));
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return PTR_ERR(tfm);
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}
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prev_tfm = cmpxchg(&essiv_hash_tfm, NULL, tfm);
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if (prev_tfm) {
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crypto_free_shash(tfm);
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tfm = prev_tfm;
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}
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}
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{
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SHASH_DESC_ON_STACK(desc, tfm);
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desc->tfm = tfm;
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desc->flags = 0;
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return crypto_shash_digest(desc, key, keysize, salt);
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}
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}
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static int init_essiv_generator(struct fscrypt_info *ci, const u8 *raw_key,
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int keysize)
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{
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int err;
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struct crypto_cipher *essiv_tfm;
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u8 salt[SHA256_DIGEST_SIZE];
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essiv_tfm = crypto_alloc_cipher("aes", 0, 0);
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if (IS_ERR(essiv_tfm))
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return PTR_ERR(essiv_tfm);
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ci->ci_essiv_tfm = essiv_tfm;
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err = derive_essiv_salt(raw_key, keysize, salt);
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if (err)
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goto out;
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/*
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* Using SHA256 to derive the salt/key will result in AES-256 being
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* used for IV generation. File contents encryption will still use the
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* configured keysize (AES-128) nevertheless.
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*/
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err = crypto_cipher_setkey(essiv_tfm, salt, sizeof(salt));
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if (err)
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goto out;
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out:
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memzero_explicit(salt, sizeof(salt));
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return err;
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}
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void __exit fscrypt_essiv_cleanup(void)
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{
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crypto_free_shash(essiv_hash_tfm);
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}
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int fscrypt_get_encryption_info(struct inode *inode)
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{
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struct fscrypt_info *crypt_info;
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@ -212,6 +294,7 @@ int fscrypt_get_encryption_info(struct inode *inode)
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crypt_info->ci_data_mode = ctx.contents_encryption_mode;
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crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
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crypt_info->ci_ctfm = NULL;
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crypt_info->ci_essiv_tfm = NULL;
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memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
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sizeof(crypt_info->ci_master_key));
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@ -228,10 +311,12 @@ int fscrypt_get_encryption_info(struct inode *inode)
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if (!raw_key)
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goto out;
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res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX);
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res = validate_user_key(crypt_info, &ctx, raw_key, FS_KEY_DESC_PREFIX,
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keysize);
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if (res && inode->i_sb->s_cop->key_prefix) {
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int res2 = validate_user_key(crypt_info, &ctx, raw_key,
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inode->i_sb->s_cop->key_prefix);
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inode->i_sb->s_cop->key_prefix,
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keysize);
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if (res2) {
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if (res2 == -ENOKEY)
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res = -ENOKEY;
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@ -243,18 +328,30 @@ int fscrypt_get_encryption_info(struct inode *inode)
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ctfm = crypto_alloc_skcipher(cipher_str, 0, 0);
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if (!ctfm || IS_ERR(ctfm)) {
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res = ctfm ? PTR_ERR(ctfm) : -ENOMEM;
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printk(KERN_DEBUG
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"%s: error %d (inode %u) allocating crypto tfm\n",
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__func__, res, (unsigned) inode->i_ino);
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pr_debug("%s: error %d (inode %lu) allocating crypto tfm\n",
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__func__, res, inode->i_ino);
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goto out;
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}
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crypt_info->ci_ctfm = ctfm;
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crypto_skcipher_clear_flags(ctfm, ~0);
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crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
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/*
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* if the provided key is longer than keysize, we use the first
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* keysize bytes of the derived key only
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*/
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res = crypto_skcipher_setkey(ctfm, raw_key, keysize);
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if (res)
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goto out;
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if (S_ISREG(inode->i_mode) &&
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crypt_info->ci_data_mode == FS_ENCRYPTION_MODE_AES_128_CBC) {
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res = init_essiv_generator(crypt_info, raw_key, keysize);
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if (res) {
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pr_debug("%s: error %d (inode %lu) allocating essiv tfm\n",
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__func__, res, inode->i_ino);
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goto out;
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}
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}
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if (cmpxchg(&inode->i_crypt_info, NULL, crypt_info) == NULL)
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crypt_info = NULL;
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out:
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@ -38,12 +38,8 @@ static int create_encryption_context_from_policy(struct inode *inode,
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memcpy(ctx.master_key_descriptor, policy->master_key_descriptor,
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FS_KEY_DESCRIPTOR_SIZE);
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if (!fscrypt_valid_contents_enc_mode(
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policy->contents_encryption_mode))
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return -EINVAL;
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|
||||
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;
|
||||
|
|
|
|||
Loading…
Reference in a new issue