[RFC][PATCH v3 5/6] encrypted-keys: add ecryptfs format support

Roberto Sassu
2011-01-24T16:45:44Z

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The 'encrypted' key type defines its own payload format which contains a
symmetric key randomly generated that cannot be used directly to mount
an eCryptfs filesystem, because it expects an authentication token
structure.

This patch introduces the new format 'ecryptfs' that allows to store an
authentication token structure inside the encrypted key payload containing
a randomly generated symmetric key, as the same for the format 'default'.

More details about the usage of encrypted keys with the eCryptfs
filesystem can be found in the file 'Documentation/keys-ecryptfs.txt'.

Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Acked-by: Gianluca Ramunno <ramunno@polito.it>
---
Documentation/keys-ecryptfs.txt | 68 +++++++++++++++++++++++++
Documentation/keys-trusted-encrypted.txt | 6 ++-
security/keys/Makefile | 2 +-
security/keys/encrypted.c | 75 +++++++++++++++++++++++++---
security/keys/keys_ecryptfs.c | 81 ++++++++++++++++++++++++++++++
security/keys/keys_ecryptfs.h | 30 +++++++++++
6 files changed, 252 insertions(+), 10 deletions(-)
create mode 100644 Documentation/keys-ecryptfs.txt
create mode 100644 security/keys/keys_ecryptfs.c
create mode 100644 security/keys/keys_ecryptfs.h

diff --git a/Documentation/keys-ecryptfs.txt b/Documentation/keys-ecryptfs.txt
new file mode 100644
index 0000000..c3bbeba
--- /dev/null
+++ b/Documentation/keys-ecryptfs.txt
@@ -0,0 +1,68 @@
+ Encrypted keys for the eCryptfs filesystem
+
+ECryptfs is a stacked filesystem which transparently encrypts and decrypts each
+file using a randomly generated File Encryption Key (FEK).
+
+Each FEK is in turn encrypted with a File Encryption Key Encryption Key (FEFEK)
+either in kernel space or in user space with a daemon called 'ecryptfsd'. In
+the former case the operation is performed directly by the kernel CryptoAPI
+using a key, the FEFEK, derived from a user prompted passphrase; in the latter
+the FEK is encrypted by 'ecryptfsd' with the help of external libraries in order
+to support other mechanisms like public key cryptography, PKCS#11 and TPM based
+operations.
+
+The data structure defined by eCryptfs to contain information required for the
+FEK decryption is called authentication token and, currently, can be stored in a
+kernel key of the 'user' type, inserted in the user's session specific keyring
+by the userspace utility 'mount.ecryptfs' shipped with the package
+'ecryptfs-utils'.
+
+The 'encrypted' key type has been extended with the introduction of the new
+format 'ecryptfs' in order to be used in conjunction with the eCryptfs
+filesystem. Encrypted keys of the newly introduced format store an
+authentication token in its payload with a FEFEK randomly generated by the
+kernel and protected by the parent master key.
+
+In order to avoid known-plaintext attacks, the datablob obtained through
+commands 'keyctl print' or 'keyctl pipe' does not contain the overall
+authentication token, which content is well known, but only the FEFEK in
+encrypted form.
+
+The eCryptfs filesystem may really benefit from using encrypted keys in that the
+required key can be securely generated by an Administrator and provided at boot
+time after the unsealing of a 'trusted' key in order to perform the mount in a
+controlled environment. Another advantage is that the key is not exposed to
+threats of malicious software, because it is available in clear form only at
+kernel level.
+
+Usage:
+ keyctl add encrypted name "new ecryptfs key-type:master-key-name keylen" ring
+ keyctl add encrypted name "load hex_blob" ring
+ keyctl update keyid "update key-type:master-key-name"
+
+name:= '<16 hexadecimal characters>'
+key-type:= 'trusted' | 'user'
+keylen:= 64
+
+
+Example of encrypted key usage with the eCryptfs filesystem:
+
+Create an encrypted key "1000100010001000" of length 64 bytes with format
+'ecryptfs' and save it using a previously loaded user key "test":
+
+ $ keyctl add encrypted 1000100010001000 "new ecryptfs user:test 64" @u
+ 19184530
+
+ $ keyctl print 19184530
+ ecryptfs user:test 64 490045d4bfe48c99f0d465fbbbb79e7500da954178e2de0697
+ dd85091f5450a0511219e9f7cd70dcd498038181466f78ac8d4c19504fcc72402bfc41c2
+ f253a41b7507ccaa4b2b03fff19a69d1cc0b16e71746473f023a95488b6edfd86f7fdd40
+ 9d292e4bacded1258880122dd553a661
+
+ $ keyctl pipe 19184530 > ecryptfs.blob
+
+Mount an eCryptfs filesystem using the created encrypted key "1000100010001000"
+into the '/secret' directory:
+
+ $ mount -i -t ecryptfs -oecryptfs_sig=1000100010001000,\
+ ecryptfs_cipher=aes,ecryptfs_key_bytes=32 /secret /secret
diff --git a/Documentation/keys-trusted-encrypted.txt b/Documentation/keys-trusted-encrypted.txt
index 0afcb50..5f50cca 100644
--- a/Documentation/keys-trusted-encrypted.txt
+++ b/Documentation/keys-trusted-encrypted.txt
@@ -63,7 +63,7 @@ Usage:
keyctl add encrypted name "load hex_blob" ring
keyctl update keyid "update key-type:master-key-name"

-format:= 'default'
+format:= 'default | ecryptfs'
key-type:= 'trusted' | 'user'

@@ -154,4 +154,6 @@ Load an encrypted key "evm" from saved blob:
24717c64 5972dcb82ab2dde83376d82b2e3c09ffc

Other uses for trusted and encrypted keys, such as for disk and file encryption
-are anticipated.
+are anticipated. In particular the new format 'ecryptfs' has been defined in
+in order to use encrypted keys to mount an eCryptfs filesystem. More details
+about the usage can be found in the file 'Documentation/keys-ecryptfs.txt'.
diff --git a/security/keys/Makefile b/security/keys/Makefile
index 1bf090a..bf8c878 100644
--- a/security/keys/Makefile
+++ b/security/keys/Makefile
@@ -14,7 +14,7 @@ obj-y := \
user_defined.o

obj-$(CONFIG_TRUSTED_KEYS) += trusted.o
-obj-$(CONFIG_ENCRYPTED_KEYS) += encrypted.o
+obj-$(CONFIG_ENCRYPTED_KEYS) += keys_ecryptfs.o encrypted.o
obj-$(CONFIG_KEYS_COMPAT) += compat.o
obj-$(CONFIG_PROC_FS) += proc.o
obj-$(CONFIG_SYSCTL) += sysctl.o
diff --git a/security/keys/encrypted.c b/security/keys/encrypted.c
index 59f719e..a916ce2 100644
--- a/security/keys/encrypted.c
+++ b/security/keys/encrypted.c
@@ -29,11 +29,13 @@
#include <linux/rcupdate.h>
#include <linux/scatterlist.h>
#include <linux/crypto.h>
+#include <linux/ctype.h>
#include <crypto/hash.h>
#include <crypto/sha.h>
#include <crypto/aes.h>

#include "encrypted.h"
+#include "keys_ecryptfs.h"

static const char KEY_TRUSTED_PREFIX[] = "trusted:";
static const char KEY_USER_PREFIX[] = "user:";
@@ -41,11 +43,13 @@ static const char hash_alg[] = "sha256";
static const char hmac_alg[] = "hmac(sha256)";
static const char blkcipher_alg[] = "cbc(aes)";
static const char key_format_default[] = "default";
+static const char key_format_ecryptfs[] = "ecryptfs";
static unsigned int ivsize;
static int blksize;

#define KEY_TRUSTED_PREFIX_LEN (sizeof (KEY_TRUSTED_PREFIX) - 1)
#define KEY_USER_PREFIX_LEN (sizeof (KEY_USER_PREFIX) - 1)
+#define KEY_ECRYPTFS_DESC_LEN 16
#define HASH_SIZE SHA256_DIGEST_SIZE
#define MAX_DATA_SIZE 4096
#define MIN_DATA_SIZE 20
@@ -63,11 +67,12 @@ enum {
};

enum {
- Opt_error = -1, Opt_default
+ Opt_error = -1, Opt_default, Opt_ecryptfs
};

static const match_table_t key_format_tokens = {
{Opt_default, "default"},
+ {Opt_ecryptfs, "ecryptfs"},
{Opt_error, NULL}
};

@@ -95,6 +100,34 @@ static int aes_get_sizes(void)
}

/*
+ * valid_ecryptfs_desc - verify the description of a new/loaded encrypted key
+ *
+ * The description of a encrypted key with format 'ecryptfs' must contain
+ * exactly 16 hexadecimal characters.
+ *
+ */
+static int valid_ecryptfs_desc(const char *ecryptfs_desc)
+{
+ int i;
+
+ if (strlen(ecryptfs_desc) != KEY_ECRYPTFS_DESC_LEN) {
+ pr_err("encrypted_key: key description must be %d hexadecimal "
+ "characters long\n", KEY_ECRYPTFS_DESC_LEN);
+ return -EINVAL;
+ }
+
+ for (i = 0; i < KEY_ECRYPTFS_DESC_LEN; i++) {
+ if (!isxdigit(ecryptfs_desc[i])) {
+ pr_err("encrypted_key: key description must contain "
+ "only hexadecimal characters\n");
+ return -EINVAL;
+ }
+ }
+
+ return 0;
+}
+
+/*
* valid_master_desc - verify the 'key-type:desc' of a new/updated master-key
*
* key-type:= "trusted:" | "user:"
@@ -158,7 +191,7 @@ static int datablob_parse(char *datablob, const char **format,
}
key_cmd = match_token(keyword, key_tokens, args);

- /* Get optional format: default */
+ /* Get optional format: default | ecryptfs */
p = strsep(&datablob, " \t");
if (!p) {
pr_err("encrypted_key: insufficient parameters specified\n");
@@ -167,6 +200,7 @@ static int datablob_parse(char *datablob, const char **format,

key_format = match_token(p, key_format_tokens, args);
switch (key_format) {
+ case Opt_ecryptfs:
case Opt_default:
*format = p;
*master_desc = strsep(&datablob, " \t");
@@ -598,6 +632,17 @@ static struct encrypted_key_payload *encrypted_key_alloc(struct key *key,
format_len = (!format) ? strlen(key_format_default) : strlen(format);
decrypted_datalen = dlen;
payload_datalen = decrypted_datalen;
+ if (format && !strcmp(format, key_format_ecryptfs)) {
+ if (dlen != ECRYPTFS_MAX_KEY_BYTES) {
+ pr_err("encrypted_key: keylen for the ecryptfs format "
+ "must be equal to %d bytes\n",
+ ECRYPTFS_MAX_KEY_BYTES);
+ return ERR_PTR(-EINVAL);
+ }
+ decrypted_datalen = ECRYPTFS_MAX_KEY_BYTES;
+ payload_datalen = sizeof(struct ecryptfs_auth_tok);
+ }
+
encrypted_datalen = roundup(decrypted_datalen, blksize);

datablob_len = format_len + 1 + strlen(master_desc) + 1
@@ -683,8 +728,14 @@ static void __ekey_init(struct encrypted_key_payload *epayload,

if (!format)
memcpy(epayload->format, key_format_default, format_len);
- else
+ else {
+ if (!strcmp(format, key_format_ecryptfs))
+ epayload->decrypted_data =
+ ecryptfs_get_auth_tok_key((struct ecryptfs_auth_tok *)epayload->payload_data);
+
memcpy(epayload->format, format, format_len);
+ }
+
memcpy(epayload->master_desc, master_desc, strlen(master_desc));
memcpy(epayload->datalen, datalen, strlen(datalen));
}
@@ -696,11 +747,21 @@ static void __ekey_init(struct encrypted_key_payload *epayload,
* itself. For an old key, decrypt the hex encoded data.
*/
static int encrypted_init(struct encrypted_key_payload *epayload,
- const char *format, const char *master_desc,
- const char *datalen, const char *hex_encoded_iv)
+ const char *key_desc, const char *format,
+ const char *master_desc, const char *datalen,
+ const char *hex_encoded_iv)
{
int ret = 0;

+ if (format && !strcmp(format, key_format_ecryptfs)) {
+ ret = valid_ecryptfs_desc(key_desc);
+ if (ret < 0)
+ return ret;
+
+ ecryptfs_fill_auth_tok((struct ecryptfs_auth_tok *)epayload->payload_data,
+ key_desc);
+ }
+
__ekey_init(epayload, format, master_desc, datalen);
if (!hex_encoded_iv) {
get_random_bytes(epayload->iv, ivsize);
@@ -750,8 +811,8 @@ static int encrypted_instantiate(struct key *key, const void *data,
ret = PTR_ERR(epayload);
goto out;
}
- ret = encrypted_init(epayload, format, master_desc, decrypted_datalen,
- hex_encoded_iv);
+ ret = encrypted_init(epayload, key->description, format, master_desc,
+ decrypted_datalen, hex_encoded_iv);
if (ret < 0) {
kfree(epayload);
goto out;
diff --git a/security/keys/keys_ecryptfs.c b/security/keys/keys_ecryptfs.c
new file mode 100644
index 0000000..6144b91
--- /dev/null
+++ b/security/keys/keys_ecryptfs.c
@@ -0,0 +1,81 @@
+/*
+ * keys_ecryptfs.c: helper functions for the encrypted key type
+ *
+ * Copyright (C) 2006 International Business Machines Corp.
+ * Copyright (C) 2010 Politecnico di Torino, Italy
+ * TORSEC group -- http://security.polito.it
+ *
+ * Authors:
+ * Michael A. Halcrow <mahalcro@us.ibm.com>
+ * Tyler Hicks <tyhicks@ou.edu>
+ * Roberto Sassu <roberto.sassu@polito.it>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 2 of the License.
+ */
+
+#include <linux/module.h>
+#include "keys_ecryptfs.h"
+
+u8 *ecryptfs_get_auth_tok_key(struct ecryptfs_auth_tok *auth_tok)
+{
+ return auth_tok->token.password.session_key_encryption_key;
+}
+EXPORT_SYMBOL(ecryptfs_get_auth_tok_key);
+
+/*
+ * ecryptfs_get_versions()
+ *
+ * Source code taken from the software 'ecryptfs-utils' version 83.
+ *
+ */
+void ecryptfs_get_versions(int *major, int *minor, int *file_version)
+{
+ *major = ECRYPTFS_VERSION_MAJOR;
+ *minor = ECRYPTFS_VERSION_MINOR;
+ if (file_version)
+ *file_version = ECRYPTFS_SUPPORTED_FILE_VERSION;
+}
+EXPORT_SYMBOL(ecryptfs_get_versions);
+
+/*
+ * ecryptfs_fill_auth_tok - fill the ecryptfs_auth_tok structure
+ *
+ * Fill the ecryptfs_auth_tok structure with required ecryptfs data.
+ * The source code is inspired to the original function generate_payload()
+ * shipped with the software 'ecryptfs-utils' version 83.
+ *
+ */
+int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
+ const char *key_desc)
+{
+ int major, minor;
+
+ ecryptfs_get_versions(&major, &minor, NULL);
+ auth_tok->version = (((uint16_t)(major << 8) & 0xFF00)
+ | ((uint16_t)minor & 0x00FF));
+ auth_tok->token_type = ECRYPTFS_PASSWORD;
+ strncpy((char *)auth_tok->token.password.signature, key_desc,
+ ECRYPTFS_PASSWORD_SIG_SIZE);
+ auth_tok->token.password.session_key_encryption_key_bytes =
+ ECRYPTFS_MAX_KEY_BYTES;
+ /*
+ * Removed auth_tok->token.password.salt and
+ * auth_tok->token.password.session_key_encryption_key
+ * initialization from the original code
+ */
+ /* TODO: Make the hash parameterizable via policy */
+ auth_tok->token.password.flags |=
+ ECRYPTFS_SESSION_KEY_ENCRYPTION_KEY_SET;
+ /* The kernel code will encrypt the session key. */
+ auth_tok->session_key.encrypted_key[0] = 0;
+ auth_tok->session_key.encrypted_key_size = 0;
+ /* Default; subject to change by kernel eCryptfs */
+ auth_tok->token.password.hash_algo = PGP_DIGEST_ALGO_SHA512;
+ auth_tok->token.password.flags &= ~(ECRYPTFS_PERSISTENT_PASSWORD);
+ return 0;
+}
+EXPORT_SYMBOL(ecryptfs_fill_auth_tok);
+
+MODULE_LICENSE("GPL");
diff --git a/security/keys/keys_ecryptfs.h b/security/keys/keys_ecryptfs.h
new file mode 100644
index 0000000..87f77ca
--- /dev/null
+++ b/security/keys/keys_ecryptfs.h
@@ -0,0 +1,30 @@
+/*
+ * keys_ecryptfs.h: helper functions for the encrypted key type
+ *
+ * Copyright (C) 2006 International Business Machines Corp.
+ * Copyright (C) 2010 Politecnico di Torino, Italy
+ * TORSEC group -- http://security.polito.it
+ *
+ * Authors:
+ * Michael A. Halcrow <mahalcro@us.ibm.com>
+ * Tyler Hicks <tyhicks@ou.edu>
+ * Roberto Sassu <roberto.sassu@polito.it>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 2 of the License.
+ */
+
+#ifndef __KEYS_ECRYPTFS_H
+#define __KEYS_ECRYPTFS_H
+
+#include <linux/ecryptfs.h>
+
+#define PGP_DIGEST_ALGO_SHA512 10
+
+u8 *ecryptfs_get_auth_tok_key(struct ecryptfs_auth_tok *auth_tok);
+void ecryptfs_get_versions(int *major, int *minor, int *file_version);
+int ecryptfs_fill_auth_tok(struct ecryptfs_auth_tok *auth_tok,
+ const char *key_desc);
+
+#endif /* __KEYS_ECRYPTFS_H */
--
1.7.3.4

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