fscrypt: allow 256-bit master keys with AES-256-XTS

fscrypt currently requires a 512-bit master key when AES-256-XTS is
used, since AES-256-XTS keys are 512-bit and fscrypt requires that the
master key be at least as long any key that will be derived from it.

However, this is overly strict because AES-256-XTS doesn't actually have
a 512-bit security strength, but rather 256-bit.  The fact that XTS
takes twice the expected key size is a quirk of the XTS mode.  It is
sufficient to use 256 bits of entropy for AES-256-XTS, provided that it
is first properly expanded into a 512-bit key, which HKDF-SHA512 does.

Therefore, relax the check of the master key size to use the security
strength of the derived key rather than the size of the derived key
(except for v1 encryption policies, which don't use HKDF).

Besides making things more flexible for userspace, this is needed in
order for the use of a KDF which only takes a 256-bit key to be
introduced into the fscrypt key hierarchy.  This will happen with
hardware-wrapped keys support, as all known hardware which supports that
feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys
are wrapped 256-bit AES keys.  Moreover, there is interest in fscrypt
supporting the same type of AES-256-CMAC based KDF in software as an
alternative to HKDF-SHA512.  There is no security problem with such
features, so fix the key length check to work properly with them.

Reviewed-by: Paul Crowley <paulcrowley@google.com>
Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 45 insertions, 12 deletions

diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index bca9c6658a7c..89cd533a88bf 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -19,6 +19,7 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-256-XTS",
 		.cipher_str = "xts(aes)",
 		.keysize = 64,
+		.security_strength = 32,
 		.ivsize = 16,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_256_XTS,
 	},
@@ -26,12 +27,14 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-256-CTS-CBC",
 		.cipher_str = "cts(cbc(aes))",
 		.keysize = 32,
+		.security_strength = 32,
 		.ivsize = 16,
 	},
 	[FSCRYPT_MODE_AES_128_CBC] = {
 		.friendly_name = "AES-128-CBC-ESSIV",
 		.cipher_str = "essiv(cbc(aes),sha256)",
 		.keysize = 16,
+		.security_strength = 16,
 		.ivsize = 16,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_AES_128_CBC_ESSIV,
 	},
@@ -39,12 +42,14 @@ struct fscrypt_mode fscrypt_modes[] = {
 		.friendly_name = "AES-128-CTS-CBC",
 		.cipher_str = "cts(cbc(aes))",
 		.keysize = 16,
+		.security_strength = 16,
 		.ivsize = 16,
 	},
 	[FSCRYPT_MODE_ADIANTUM] = {
 		.friendly_name = "Adiantum",
 		.cipher_str = "adiantum(xchacha12,aes)",
 		.keysize = 32,
+		.security_strength = 32,
 		.ivsize = 32,
 		.blk_crypto_mode = BLK_ENCRYPTION_MODE_ADIANTUM,
 	},
@@ -358,6 +363,45 @@ static int fscrypt_setup_v2_file_key(struct fscrypt_info *ci,
 }
 
 /*
+ * Check whether the size of the given master key (@mk) is appropriate for the
+ * encryption settings which a particular file will use (@ci).
+ *
+ * If the file uses a v1 encryption policy, then the master key must be at least
+ * as long as the derived key, as this is a requirement of the v1 KDF.
+ *
+ * Otherwise, the KDF can accept any size key, so we enforce a slightly looser
+ * requirement: we require that the size of the master key be at least the
+ * maximum security strength of any algorithm whose key will be derived from it
+ * (but in practice we only need to consider @ci->ci_mode, since any other
+ * possible subkeys such as DIRHASH and INODE_HASH will never increase the
+ * required key size over @ci->ci_mode).  This allows AES-256-XTS keys to be
+ * derived from a 256-bit master key, which is cryptographically sufficient,
+ * rather than requiring a 512-bit master key which is unnecessarily long.  (We
+ * still allow 512-bit master keys if the user chooses to use them, though.)
+ */
+static bool fscrypt_valid_master_key_size(const struct fscrypt_master_key *mk,
+					  const struct fscrypt_info *ci)
+{
+	unsigned int min_keysize;
+
+	if (ci->ci_policy.version == FSCRYPT_POLICY_V1)
+		min_keysize = ci->ci_mode->keysize;
+	else
+		min_keysize = ci->ci_mode->security_strength;
+
+	if (mk->mk_secret.size < min_keysize) {
+		fscrypt_warn(NULL,
+			     "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
+			     master_key_spec_type(&mk->mk_spec),
+			     master_key_spec_len(&mk->mk_spec),
+			     (u8 *)&mk->mk_spec.u,
+			     mk->mk_secret.size, min_keysize);
+		return false;
+	}
+	return true;
+}
+
+/*
  * Find the master key, then set up the inode's actual encryption key.
  *
  * If the master key is found in the filesystem-level keyring, then the
@@ -422,18 +466,7 @@ static int setup_file_encryption_key(struct fscrypt_info *ci,
 		goto out_release_key;
 	}
 
-	/*
-	 * Require that the master key be at least as long as the derived key.
-	 * Otherwise, the derived key cannot possibly contain as much entropy as
-	 * that required by the encryption mode it will be used for.  For v1
-	 * policies it's also required for the KDF to work at all.
-	 */
-	if (mk->mk_secret.size < ci->ci_mode->keysize) {
-		fscrypt_warn(NULL,
-			     "key with %s %*phN is too short (got %u bytes, need %u+ bytes)",
-			     master_key_spec_type(&mk_spec),
-			     master_key_spec_len(&mk_spec), (u8 *)&mk_spec.u,
-			     mk->mk_secret.size, ci->ci_mode->keysize);
+	if (!fscrypt_valid_master_key_size(mk, ci)) {
 		err = -ENOKEY;
 		goto out_release_key;
 	}