fscrypt: add Adiantum support

Add support for the Adiantum encryption mode to fscrypt.  Adiantum is a
tweakable, length-preserving encryption mode with security provably
reducible to that of XChaCha12 and AES-256, subject to a security bound.
It's also a true wide-block mode, unlike XTS.  See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details.  Also see
commit 059c2a4d8e16 ("crypto: adiantum - add Adiantum support").

On sufficiently long messages, Adiantum's bottlenecks are XChaCha12 and
the NH hash function.  These algorithms are fast even on processors
without dedicated crypto instructions.  Adiantum makes it feasible to
enable storage encryption on low-end mobile devices that lack AES
instructions; currently such devices are unencrypted.  On ARM Cortex-A7,
on 4096-byte messages Adiantum encryption is about 4 times faster than
AES-256-XTS encryption; decryption is about 5 times faster.

In fscrypt, Adiantum is suitable for encrypting both file contents and
names.  With filenames, it fixes a known weakness: when two filenames in
a directory share a common prefix of >= 16 bytes, with CTS-CBC their
encrypted filenames share a common prefix too, leaking information.
Adiantum does not have this problem.

Since Adiantum also accepts long tweaks (IVs), it's also safe to use the
master key directly for Adiantum encryption rather than deriving
per-file keys, provided that the per-file nonce is included in the IVs
and the master key isn't used for any other encryption mode.  This
configuration saves memory and improves performance.  A new fscrypt
policy flag is added to allow users to opt-in to this configuration.

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

1 files changed, 61 insertions, 6 deletions

diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 79debfc9cef9..7424f851eb5c 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/fs/crypto/fscrypt_private.h
@@ -17,7 +17,6 @@
 #include <crypto/hash.h>
 
 /* Encryption parameters */
-#define FS_IV_SIZE			16
 #define FS_KEY_DERIVATION_NONCE_SIZE	16
 
 /**
@@ -52,16 +51,42 @@ struct fscrypt_symlink_data {
 } __packed;
 
 /*
- * A pointer to this structure is stored in the file system's in-core
- * representation of an inode.
+ * fscrypt_info - the "encryption key" for an inode
+ *
+ * When an encrypted file's key is made available, an instance of this struct is
+ * allocated and stored in ->i_crypt_info.  Once created, it remains until the
+ * inode is evicted.
  */
 struct fscrypt_info {
+
+	/* The actual crypto transform used for encryption and decryption */
+	struct crypto_skcipher *ci_ctfm;
+
+	/*
+	 * Cipher for ESSIV IV generation.  Only set for CBC contents
+	 * encryption, otherwise is NULL.
+	 */
+	struct crypto_cipher *ci_essiv_tfm;
+
+	/*
+	 * Encryption mode used for this inode.  It corresponds to either
+	 * ci_data_mode or ci_filename_mode, depending on the inode type.
+	 */
+	struct fscrypt_mode *ci_mode;
+
+	/*
+	 * If non-NULL, then this inode uses a master key directly rather than a
+	 * derived key, and ci_ctfm will equal ci_master_key->mk_ctfm.
+	 * Otherwise, this inode uses a derived key.
+	 */
+	struct fscrypt_master_key *ci_master_key;
+
+	/* fields from the fscrypt_context */
 	u8 ci_data_mode;
 	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];
+	u8 ci_master_key_descriptor[FS_KEY_DESCRIPTOR_SIZE];
+	u8 ci_nonce[FS_KEY_DERIVATION_NONCE_SIZE];
 };
 
 typedef enum {
@@ -83,6 +108,10 @@ static inline bool fscrypt_valid_enc_modes(u32 contents_mode,
 	    filenames_mode == FS_ENCRYPTION_MODE_AES_256_CTS)
 		return true;
 
+	if (contents_mode == FS_ENCRYPTION_MODE_ADIANTUM &&
+	    filenames_mode == FS_ENCRYPTION_MODE_ADIANTUM)
+		return true;
+
 	return false;
 }
 
@@ -107,6 +136,22 @@ fscrypt_msg(struct super_block *sb, const char *level, const char *fmt, ...);
 #define fscrypt_err(sb, fmt, ...)		\
 	fscrypt_msg(sb, KERN_ERR, fmt, ##__VA_ARGS__)
 
+#define FSCRYPT_MAX_IV_SIZE	32
+
+union fscrypt_iv {
+	struct {
+		/* logical block number within the file */
+		__le64 lblk_num;
+
+		/* per-file nonce; only set in DIRECT_KEY mode */
+		u8 nonce[FS_KEY_DERIVATION_NONCE_SIZE];
+	};
+	u8 raw[FSCRYPT_MAX_IV_SIZE];
+};
+
+void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
+			 const struct fscrypt_info *ci);
+
 /* fname.c */
 extern int fname_encrypt(struct inode *inode, const struct qstr *iname,
 			 u8 *out, unsigned int olen);
@@ -115,6 +160,16 @@ extern bool fscrypt_fname_encrypted_size(const struct inode *inode,
 					 u32 *encrypted_len_ret);
 
 /* keyinfo.c */
+
+struct fscrypt_mode {
+	const char *friendly_name;
+	const char *cipher_str;
+	int keysize;
+	int ivsize;
+	bool logged_impl_name;
+	bool needs_essiv;
+};
+
 extern void __exit fscrypt_essiv_cleanup(void);
 
 #endif /* _FSCRYPT_PRIVATE_H */