[PATCH 5.10 585/593] fscrypt: fix derivation of SipHash keys on big endian CPUs

From: Greg Kroah-Hartman
Date: Mon Jul 12 2021 - 03:16:04 EST

From: Eric Biggers <ebiggers@xxxxxxxxxx>

commit 2fc2b430f559fdf32d5d1dd5ceaa40e12fb77bdf upstream.

Typically, the cryptographic APIs that fscrypt uses take keys as byte
arrays, which avoids endianness issues. However, siphash_key_t is an
exception. It is defined as 'u64 key[2];', i.e. the 128-bit key is
expected to be given directly as two 64-bit words in CPU endianness.

fscrypt_derive_dirhash_key() and fscrypt_setup_iv_ino_lblk_32_key()
forgot to take this into account. Therefore, the SipHash keys used to
index encrypted+casefolded directories differ on big endian vs. little
endian platforms, as do the SipHash keys used to hash inode numbers for
IV_INO_LBLK_32-encrypted directories. This makes such directories
non-portable between these platforms.

Fix this by always using the little endian order. This is a breaking
change for big endian platforms, but this should be fine in practice
since these features (encrypt+casefold support, and the IV_INO_LBLK_32
flag) aren't known to actually be used on any big endian platforms yet.

Fixes: aa408f835d02 ("fscrypt: derive dirhash key for casefolded directories")
Fixes: e3b1078bedd3 ("fscrypt: add support for IV_INO_LBLK_32 policies")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.6+
Link: https://lore.kernel.org/r/20210605075033.54424-1-ebiggers@xxxxxxxxxx
Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/crypto/keysetup.c | 40 ++++++++++++++++++++++++++++++++--------
1 file changed, 32 insertions(+), 8 deletions(-)

--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -210,15 +210,40 @@ out_unlock:
return err;

+ * Derive a SipHash key from the given fscrypt master key and the given
+ * application-specific information string.
+ *
+ * Note that the KDF produces a byte array, but the SipHash APIs expect the key
+ * as a pair of 64-bit words. Therefore, on big endian CPUs we have to do an
+ * endianness swap in order to get the same results as on little endian CPUs.
+ */
+static int fscrypt_derive_siphash_key(const struct fscrypt_master_key *mk,
+ u8 context, const u8 *info,
+ unsigned int infolen, siphash_key_t *key)
+ int err;
+ err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf, context, info, infolen,
+ (u8 *)key, sizeof(*key));
+ if (err)
+ return err;
+ BUILD_BUG_ON(sizeof(*key) != 16);
+ BUILD_BUG_ON(ARRAY_SIZE(key->key) != 2);
+ le64_to_cpus(&key->key[0]);
+ le64_to_cpus(&key->key[1]);
+ return 0;
int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
const struct fscrypt_master_key *mk)
int err;

- err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf, HKDF_CONTEXT_DIRHASH_KEY,
- ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
- (u8 *)&ci->ci_dirhash_key,
- sizeof(ci->ci_dirhash_key));
+ err = fscrypt_derive_siphash_key(mk, HKDF_CONTEXT_DIRHASH_KEY,
+ ci->ci_nonce, FSCRYPT_FILE_NONCE_SIZE,
+ &ci->ci_dirhash_key);
if (err)
return err;
ci->ci_dirhash_key_initialized = true;
@@ -253,10 +278,9 @@ static int fscrypt_setup_iv_ino_lblk_32_
if (mk->mk_ino_hash_key_initialized)
goto unlock;

- err = fscrypt_hkdf_expand(&mk->mk_secret.hkdf,
- (u8 *)&mk->mk_ino_hash_key,
- sizeof(mk->mk_ino_hash_key));
+ err = fscrypt_derive_siphash_key(mk,
+ NULL, 0, &mk->mk_ino_hash_key);
if (err)
goto unlock;
/* pairs with smp_load_acquire() above */