[PATCH v7 9/9] fscrypt: Add HCTR2 support for filename encryption

Nathan Huckleberry
2022-05-09T19:21:07Z

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>
---
Documentation/filesystems/fscrypt.rst | 22 +++++++++++++++++-----
fs/crypto/fscrypt_private.h | 2 +-
fs/crypto/keysetup.c | 7 +++++++
fs/crypto/policy.c | 14 +++++++++++---
include/uapi/linux/fscrypt.h | 3 ++-
5 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 4d5d50dca65c..324149c58bf3 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 (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
-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..ba463eb931de 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 = "AES-256-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..5dea7b655a64 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -40,7 +40,7 @@ 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)
@@ -57,6 +57,14 @@ 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)
{
@@ -130,7 +138,7 @@ 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)",
@@ -166,7 +174,7 @@ 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)",
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.
--
2.36.0.512.ge40c2bad7a-goog