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Date:   Tue, 12 Apr 2022 17:28:16 +0000
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Subject: [PATCH v4 8/8] fscrypt: Add HCTR2 support for filename encryption
From:   Nathan Huckleberry <nhuck@google.com>
To:     linux-crypto@vger.kernel.org
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        linux-arm-kernel@lists.infradead.org,
        Paul Crowley <paulcrowley@google.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Nathan Huckleberry <nhuck@google.com>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

HCTR2 is a tweakable, length-preserving encryption mode.  It has the
same security guarantees as Adiantum, but is intended for use on CPUs
with dedicated crypto instructions.  It fixes a known weakness with
filename encryption: when two filenames in the same directory share a
prefix of >= 16 bytes, with 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>
---
 Documentation/filesystems/fscrypt.rst | 19 ++++++++++++++-----
 fs/crypto/fscrypt_private.h           |  2 +-
 fs/crypto/keysetup.c                  |  7 +++++++
 fs/crypto/policy.c                    |  4 ++++
 include/uapi/linux/fscrypt.h          |  3 ++-
 tools/include/uapi/linux/fscrypt.h    |  3 ++-
 6 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 4d5d50dca65c..09915086abd8 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/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
 
 If unsure, you should use the (AES-256-XTS, AES-256-CTS-CBC) pair.
 
@@ -357,6 +358,14 @@ 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.  It has the same
+security guarantees as Adiantum, but is intended for use on CPUs with dedicated
+crypto instructions. See the paper "Length-preserving encryption with HCTR2"
+(https://eprint.iacr.org/2021/1441.pdf) for more details. To use 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 +413,11 @@ 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
-share a common prefix at least as long as the cipher block size (16
-bytes for AES), the corresponding encrypted filenames will also share
-a common prefix.  This is undesirable.  Adiantum does not have this
-weakness, as it is a wide-block encryption mode.
+With CTS-CBC, the IV reuse means that when the plaintext filenames share a
+common prefix at least as long as the cipher block size (16 bytes for AES), the
+corresponding encrypted filenames will also share a common prefix.  This is
+undesirable.  Adiantum and HCTR2 do not have this weakness, as they are
+wide-block encryption modes.
 
 All supported filenames encryption modes accept any plaintext length
 >= 16 bytes; cipher block alignment is not required.  However,
diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h
index 5b0a9e6478b5..d8617d01f7bd 100644
--- a/fs/crypto/fscrypt_private.h
+++ b/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 */
diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index eede186b04ce..ae24b581d3d7 100644
--- a/fs/crypto/keysetup.c
+++ b/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 = "HCTR2",
+		.cipher_str = "hctr2(aes)",
+		.keysize = 32,
+		.security_strength = 32,
+		.ivsize = 32,
+	},
 };
 
 static DEFINE_MUTEX(fscrypt_mode_key_setup_mutex);
diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index ed3d623724cd..fa8bdb8c76b7 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -54,6 +54,10 @@ static bool fscrypt_valid_enc_modes(u32 contents_mode, u32 filenames_mode)
 	    filenames_mode == FSCRYPT_MODE_ADIANTUM)
 		return true;
 
+	if (contents_mode == FSCRYPT_MODE_AES_256_XTS &&
+	    filenames_mode == FSCRYPT_MODE_AES_256_HCTR2)
+		return true;
+
 	return false;
 }
 
diff --git a/include/uapi/linux/fscrypt.h b/include/uapi/linux/fscrypt.h
index 9f4428be3e36..a756b29afcc2 100644
--- a/include/uapi/linux/fscrypt.h
+++ b/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.
diff --git a/tools/include/uapi/linux/fscrypt.h b/tools/include/uapi/linux/fscrypt.h
index 9f4428be3e36..a756b29afcc2 100644
--- a/tools/include/uapi/linux/fscrypt.h
+++ b/tools/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.
-- 
2.35.1.1178.g4f1659d476-goog