From b7e7cf7a66a27e62c5f873a0068cee34094bf5d7 Mon Sep 17 00:00:00 2001 From: Daniel Walter Date: Mon, 19 Jun 2017 09:27:58 +0200 Subject: 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 [david@sigma-star.at: addressed review comments] Signed-off-by: David Gstir Reviewed-by: Eric Biggers Signed-off-by: Theodore Ts'o --- fs/crypto/keyinfo.c | 173 ++++++++++++++++++++++++++++++++++++++++------------ 1 file changed, 135 insertions(+), 38 deletions(-) (limited to 'fs/crypto/keyinfo.c') diff --git a/fs/crypto/keyinfo.c b/fs/crypto/keyinfo.c index 179e578b875b..018c588c7ac3 100644 --- a/fs/crypto/keyinfo.c +++ b/fs/crypto/keyinfo.c @@ -10,8 +10,13 @@ #include #include +#include +#include +#include #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 (!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 +178,76 @@ 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: -- cgit v1.2.3