fscrypt: v2 encryption policy support

Add a new fscrypt policy version, "v2".  It has the following changes
from the original policy version, which we call "v1" (*):

- Master keys (the user-provided encryption keys) are only ever used as
  input to HKDF-SHA512.  This is more flexible and less error-prone, and
  it avoids the quirks and limitations of the AES-128-ECB based KDF.
  Three classes of cryptographically isolated subkeys are defined:

    - Per-file keys, like used in v1 policies except for the new KDF.

    - Per-mode keys.  These implement the semantics of the DIRECT_KEY
      flag, which for v1 policies made the master key be used directly.
      These are also planned to be used for inline encryption when
      support for it is added.

    - Key identifiers (see below).

- Each master key is identified by a 16-byte master_key_identifier,
  which is derived from the key itself using HKDF-SHA512.  This prevents
  users from associating the wrong key with an encrypted file or
  directory.  This was easily possible with v1 policies, which
  identified the key by an arbitrary 8-byte master_key_descriptor.

- The key must be provided in the filesystem-level keyring, not in a
  process-subscribed keyring.

The following UAPI additions are made:

- The existing ioctl FS_IOC_SET_ENCRYPTION_POLICY can now be passed a
  fscrypt_policy_v2 to set a v2 encryption policy.  It's disambiguated
  from fscrypt_policy/fscrypt_policy_v1 by the version code prefix.

- A new ioctl FS_IOC_GET_ENCRYPTION_POLICY_EX is added.  It allows
  getting the v1 or v2 encryption policy of an encrypted file or
  directory.  The existing FS_IOC_GET_ENCRYPTION_POLICY ioctl could not
  be used because it did not have a way for userspace to indicate which
  policy structure is expected.  The new ioctl includes a size field, so
  it is extensible to future fscrypt policy versions.

- The ioctls FS_IOC_ADD_ENCRYPTION_KEY, FS_IOC_REMOVE_ENCRYPTION_KEY,
  and FS_IOC_GET_ENCRYPTION_KEY_STATUS now support managing keys for v2
  encryption policies.  Such keys are kept logically separate from keys
  for v1 encryption policies, and are identified by 'identifier' rather
  than by 'descriptor'.  The 'identifier' need not be provided when
  adding a key, since the kernel will calculate it anyway.

This patch temporarily keeps adding/removing v2 policy keys behind the
same permission check done for adding/removing v1 policy keys:
capable(CAP_SYS_ADMIN).  However, the next patch will carefully take
advantage of the cryptographically secure master_key_identifier to allow
non-root users to add/remove v2 policy keys, thus providing a full
replacement for v1 policies.

(*) Actually, in the API fscrypt_policy::version is 0 while on-disk
    fscrypt_context::format is 1.  But I believe it makes the most sense
    to advance both to '2' to have them be in sync, and to consider the
    numbering to start at 1 except for the API quirk.

Reviewed-by: Paul Crowley <paulcrowley@google.com>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 49 insertions, 8 deletions

diff --git a/include/uapi/linux/fscrypt.h b/include/uapi/linux/fscrypt.h
index ed5995b15016..f961ebf83e98 100644
--- a/include/uapi/linux/fscrypt.h
+++ b/include/uapi/linux/fscrypt.h
@@ -10,15 +10,13 @@
 
 #include <linux/types.h>
 
-#define FSCRYPT_KEY_DESCRIPTOR_SIZE	8
-
 /* Encryption policy flags */
 #define FSCRYPT_POLICY_FLAGS_PAD_4		0x00
 #define FSCRYPT_POLICY_FLAGS_PAD_8		0x01
 #define FSCRYPT_POLICY_FLAGS_PAD_16		0x02
 #define FSCRYPT_POLICY_FLAGS_PAD_32		0x03
 #define FSCRYPT_POLICY_FLAGS_PAD_MASK		0x03
-#define FSCRYPT_POLICY_FLAG_DIRECT_KEY		0x04	/* use master key directly */
+#define FSCRYPT_POLICY_FLAG_DIRECT_KEY		0x04
 #define FSCRYPT_POLICY_FLAGS_VALID		0x07
 
 /* Encryption algorithms */
@@ -27,14 +25,24 @@
 #define FSCRYPT_MODE_AES_128_CBC		5
 #define FSCRYPT_MODE_AES_128_CTS		6
 #define FSCRYPT_MODE_ADIANTUM			9
+#define __FSCRYPT_MODE_MAX			9
 
-struct fscrypt_policy {
+/*
+ * Legacy policy version; ad-hoc KDF and no key verification.
+ * For new encrypted directories, use fscrypt_policy_v2 instead.
+ *
+ * Careful: the .version field for this is actually 0, not 1.
+ */
+#define FSCRYPT_POLICY_V1		0
+#define FSCRYPT_KEY_DESCRIPTOR_SIZE	8
+struct fscrypt_policy_v1 {
 	__u8 version;
 	__u8 contents_encryption_mode;
 	__u8 filenames_encryption_mode;
 	__u8 flags;
 	__u8 master_key_descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
 };
+#define fscrypt_policy	fscrypt_policy_v1
 
 /*
  * Process-subscribed "logon" key description prefix and payload format.
@@ -50,14 +58,45 @@ struct fscrypt_key {
 };
 
 /*
- * Keys are specified by an arbitrary 8-byte key "descriptor",
- * matching fscrypt_policy::master_key_descriptor.
+ * New policy version with HKDF and key verification (recommended).
+ */
+#define FSCRYPT_POLICY_V2		2
+#define FSCRYPT_KEY_IDENTIFIER_SIZE	16
+struct fscrypt_policy_v2 {
+	__u8 version;
+	__u8 contents_encryption_mode;
+	__u8 filenames_encryption_mode;
+	__u8 flags;
+	__u8 __reserved[4];
+	__u8 master_key_identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
+};
+
+/* Struct passed to FS_IOC_GET_ENCRYPTION_POLICY_EX */
+struct fscrypt_get_policy_ex_arg {
+	__u64 policy_size; /* input/output */
+	union {
+		__u8 version;
+		struct fscrypt_policy_v1 v1;
+		struct fscrypt_policy_v2 v2;
+	} policy; /* output */
+};
+
+/*
+ * v1 policy keys are specified by an arbitrary 8-byte key "descriptor",
+ * matching fscrypt_policy_v1::master_key_descriptor.
  */
 #define FSCRYPT_KEY_SPEC_TYPE_DESCRIPTOR	1
 
 /*
- * Specifies a key.  This doesn't contain the actual key itself; this is just
- * the "name" of the key.
+ * v2 policy keys are specified by a 16-byte key "identifier" which the kernel
+ * calculates as a cryptographic hash of the key itself,
+ * matching fscrypt_policy_v2::master_key_identifier.
+ */
+#define FSCRYPT_KEY_SPEC_TYPE_IDENTIFIER	2
+
+/*
+ * Specifies a key, either for v1 or v2 policies.  This doesn't contain the
+ * actual key itself; this is just the "name" of the key.
  */
 struct fscrypt_key_specifier {
 	__u32 type;	/* one of FSCRYPT_KEY_SPEC_TYPE_* */
@@ -65,6 +104,7 @@ struct fscrypt_key_specifier {
 	union {
 		__u8 __reserved[32]; /* reserve some extra space */
 		__u8 descriptor[FSCRYPT_KEY_DESCRIPTOR_SIZE];
+		__u8 identifier[FSCRYPT_KEY_IDENTIFIER_SIZE];
 	} u;
 };
 
@@ -101,6 +141,7 @@ struct fscrypt_get_key_status_arg {
 #define FS_IOC_SET_ENCRYPTION_POLICY		_IOR('f', 19, struct fscrypt_policy)
 #define FS_IOC_GET_ENCRYPTION_PWSALT		_IOW('f', 20, __u8[16])
 #define FS_IOC_GET_ENCRYPTION_POLICY		_IOW('f', 21, struct fscrypt_policy)
+#define FS_IOC_GET_ENCRYPTION_POLICY_EX		_IOWR('f', 22, __u8[9]) /* size + version */
 #define FS_IOC_ADD_ENCRYPTION_KEY		_IOWR('f', 23, struct fscrypt_add_key_arg)
 #define FS_IOC_REMOVE_ENCRYPTION_KEY		_IOWR('f', 24, struct fscrypt_remove_key_arg)
 #define FS_IOC_GET_ENCRYPTION_KEY_STATUS	_IOWR('f', 26, struct fscrypt_get_key_status_arg)