fscrypt: fix derivation of SipHash keys on big endian CPUs

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+
Link: https://lore.kernel.org/r/20210605075033.54424-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index 261293fb709745e48faf858628c15cb4cfd38e7e..bca9c6658a7c5e918d36bfddb1758b4b7892d844 100644 (file)
--- 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_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 */