fscrypt: Add HCTR2 support for filename encryption

HCTR2 is a tweakable, length-preserving encryption mode that is intended
for use on CPUs with dedicated crypto instructions.  HCTR2 has the
property that a bitflip in the plaintext changes the entire ciphertext.
This property fixes a known weakness with filename encryption: when two
filenames in the same directory share a prefix of >= 16 bytes, with
AES-CTS-CBC their encrypted filenames share a common substring, leaking
information.  HCTR2 does not have this problem.

More information on HCTR2 can be found here: "Length-preserving
encryption with HCTR2": https://eprint.iacr.org/2021/1441.pdf

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/filesystems/fscrypt.rst

@@ -337,6 +337,7 @@ Currently, the following pairs of encryption modes are supported:
 - AES-256-XTS for contents and AES-256-CTS-CBC for filenames
 - AES-128-CBC for contents and AES-128-CTS-CBC for filenames
 - Adiantum for both contents and filenames
+- AES-256-XTS for contents and AES-256-HCTR2 for filenames (v2 policies only)
 
 If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
 
@@ -357,6 +358,17 @@ To use Adiantum, CONFIG_CRYPTO_ADIANTUM must be enabled.  Also, fast
 implementations of ChaCha and NHPoly1305 should be enabled, e.g.
 CONFIG_CRYPTO_CHACHA20_NEON and CONFIG_CRYPTO_NHPOLY1305_NEON for ARM.
 
+AES-256-HCTR2 is another true wide-block encryption mode that is intended for
+use on CPUs with dedicated crypto instructions.  AES-256-HCTR2 has the property
+that a bitflip in the plaintext changes the entire ciphertext.  This property
+makes it desirable for filename encryption since initialization vectors are
+reused within a directory.  For more details on AES-256-HCTR2, see the paper
+"Length-preserving encryption with HCTR2"
+(https://eprint.iacr.org/2021/1441.pdf).  To use AES-256-HCTR2,
+CONFIG_CRYPTO_HCTR2 must be enabled.  Also, fast implementations of XCTR and
+POLYVAL should be enabled, e.g. CRYPTO_POLYVAL_ARM64_CE and
+CRYPTO_AES_ARM64_CE_BLK for ARM64.
+
 New encryption modes can be added relatively easily, without changes
 to individual filesystems.  However, authenticated encryption (AE)
 modes are not currently supported because of the difficulty of dealing
@@ -404,11 +416,11 @@ alternatively has the file's nonce (for `DIRECT_KEY policies`_) or
 inode number (for `IV_INO_LBLK_64 policies`_) included in the IVs.
 Thus, IV reuse is limited to within a single directory.
 
-With CTS-CBC, the IV reuse means that when the plaintext filenames
+With CTS-CBC, the IV reuse means that when the plaintext filenames share a
-share a common prefix at least as long as the cipher block size (16
+common prefix at least as long as the cipher block size (16 bytes for AES), the
-bytes for AES), the corresponding encrypted filenames will also share
+corresponding encrypted filenames will also share a common prefix.  This is
-a common prefix.  This is undesirable.  Adiantum does not have this
+undesirable.  Adiantum and HCTR2 do not have this weakness, as they are
-weakness, as it is a wide-block encryption mode.
+wide-block encryption modes.
 
 All supported filenames encryption modes accept any plaintext length
 >= 16 bytes; cipher block alignment is not required.  However,

fs/crypto/fscrypt_private.h

@@ -31,7 +31,7 @@
 #define FSCRYPT_CONTEXT_V2	2
 
 /* Keep this in sync with include/uapi/linux/fscrypt.h */
-#define FSCRYPT_MODE_MAX	FSCRYPT_MODE_ADIANTUM
+#define FSCRYPT_MODE_MAX	FSCRYPT_MODE_AES_256_HCTR2
 
 struct fscrypt_context_v1 {
 	u8 version; /* FSCRYPT_CONTEXT_V1 */

fs/crypto/keysetup.c

@@ -53,6 +53,13 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.ivsize = 32,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_ADIANTUM,
 	},
+	[FSCRYPT_MODE_AES_256_HCTR2] = {
+		.friendly_name = "AES-256-HCTR2",
+		.cipher_str = "hctr2(aes)",
+		.keysize = 32,
+		.security_strength = 32,
+		.ivsize = 32,
+	},
 };
 
 static DEFINE_MUTEX(fscrypt_mode_key_setup_mutex);

fs/crypto/policy.c

@@ -61,7 +61,7 @@ fscrypt_get_dummy_policy(struct super_block *sb)
 	return sb->s_cop->get_dummy_policy(sb);
 }
 
-static bool fscrypt_valid_enc_modes(u32 contents_mode, u32 filenames_mode)
+static bool fscrypt_valid_enc_modes_v1(u32 contents_mode, u32 filenames_mode)
 {
 	if (contents_mode == FSCRYPT_MODE_AES_256_XTS &&
 	    filenames_mode == FSCRYPT_MODE_AES_256_CTS)
@@ -78,6 +78,14 @@ static bool fscrypt_valid_enc_modes(u32 contents_mode, u32 filenames_mode)
 	return false;
 }
 
+static bool fscrypt_valid_enc_modes_v2(u32 contents_mode, u32 filenames_mode)
+{
+	if (contents_mode == FSCRYPT_MODE_AES_256_XTS &&
+	    filenames_mode == FSCRYPT_MODE_AES_256_HCTR2)
+		return true;
+	return fscrypt_valid_enc_modes_v1(contents_mode, filenames_mode);
+}
+
 static bool supported_direct_key_modes(const struct inode *inode,
 				       u32 contents_mode, u32 filenames_mode)
 {
@@ -151,7 +159,7 @@ static bool supported_iv_ino_lblk_policy(const struct fscrypt_policy_v2 *policy,
 static bool fscrypt_supported_v1_policy(const struct fscrypt_policy_v1 *policy,
 					const struct inode *inode)
 {
-	if (!fscrypt_valid_enc_modes(policy->contents_encryption_mode,
+	if (!fscrypt_valid_enc_modes_v1(policy->contents_encryption_mode,
 				     policy->filenames_encryption_mode)) {
 		fscrypt_warn(inode,
 			     "Unsupported encryption modes (contents %d, filenames %d)",
@@ -187,7 +195,7 @@ static bool fscrypt_supported_v2_policy(const struct fscrypt_policy_v2 *policy,
 {
 	int count = 0;
 
-	if (!fscrypt_valid_enc_modes(policy->contents_encryption_mode,
+	if (!fscrypt_valid_enc_modes_v2(policy->contents_encryption_mode,
 				     policy->filenames_encryption_mode)) {
 		fscrypt_warn(inode,
 			     "Unsupported encryption modes (contents %d, filenames %d)",

include/uapi/linux/fscrypt.h

@@ -27,7 +27,8 @@
 #define FSCRYPT_MODE_AES_128_CBC		5
 #define FSCRYPT_MODE_AES_128_CTS		6
 #define FSCRYPT_MODE_ADIANTUM			9
-/* If adding a mode number > 9, update FSCRYPT_MODE_MAX in fscrypt_private.h */
+#define FSCRYPT_MODE_AES_256_HCTR2		10
+/* If adding a mode number > 10, update FSCRYPT_MODE_MAX in fscrypt_private.h */
 
 /*
  * Legacy policy version; ad-hoc KDF and no key verification.