[PATCH] fscrypt: fix derivation of SipHash keys on big endian CPUs

[Eric Biggers <ebiggers@kernel.org>]

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() forgot to take this into account.
Therefore, the SipHash keys used to index encrypted+casefolded
directories differ on big endian vs. little endian platforms.
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 the encrypt+casefold support isn't known to actually be used on
any big endian platforms yet.

Fixes: aa408f835d02 ("fscrypt: derive dirhash key for casefolded directories")
Cc: <stable@vger.kernel.org> # v5.6+
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 fs/crypto/keysetup.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/crypto/keysetup.c b/fs/crypto/keysetup.c
index 261293fb7097..4d98377c07a7 100644
--- a/fs/crypto/keysetup.c
+++ b/fs/crypto/keysetup.c
@@ -221,6 +221,16 @@ int fscrypt_derive_dirhash_key(struct fscrypt_info *ci,
 				  sizeof(ci->ci_dirhash_key));
 	if (err)
 		return err;
+
+	/*
+	 * The SipHash APIs expect the key as a pair of 64-bit words, not as a
+	 * byte array.  Make sure to use a consistent endianness.
+	 */
+	BUILD_BUG_ON(sizeof(ci->ci_dirhash_key) != 16);
+	BUILD_BUG_ON(ARRAY_SIZE(ci->ci_dirhash_key.key) != 2);
+	le64_to_cpus(&ci->ci_dirhash_key.key[0]);
+	le64_to_cpus(&ci->ci_dirhash_key.key[1]);
+
 	ci->ci_dirhash_key_initialized = true;
 	return 0;
 }
-- 
2.32.0.rc0.204.g9fa02ecfa5-goog