2021-05-28 00:18:44

by Eric Biggers

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

From: Eric Biggers <[email protected]>

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: <[email protected]> # v5.6+
Signed-off-by: Eric Biggers <[email protected]>
---
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

2021-06-05 07:32:01

by Eric Biggers

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

On Thu, May 27, 2021 at 03:55:25PM -0700, Eric Biggers wrote:
> From: Eric Biggers <[email protected]>
>
> 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: <[email protected]> # v5.6+
> Signed-off-by: Eric Biggers <[email protected]>
> ---
> fs/crypto/keysetup.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)

I missed that fscrypt_setup_iv_ino_lblk_32_key() has the same bug too.
I'll send out a new patch which fixes both of these...

- Eric