From: Eric Biggers ebiggers@google.com

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@vger.kernel.org # v5.6+ Signed-off-by: Eric Biggers ebiggers@google.com ---

v2: Fixed fscrypt_setup_iv_ino_lblk_32_key() too, not just fscrypt_derive_dirhash_key().

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

diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c index 261293fb70974..bca9c6658a7c5 100644 --- a/fs/crypto/keysetup.c +++ b/fs/crypto/keysetup.c @@ -210,15 +210,40 @@ static int setup_per_mode_enc_key(struct fscrypt_info *ci, 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_key(struct fscrypt_info *ci, if (mk->mk_ino_hash_key_initialized) goto unlock;

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

base-commit: 77f30bfcfcf484da7208affd6a9e63406420bf91