linux-ext4 - Re: [PATCH v2] fscrypt: fix derivation of SipHash keys on big endian CPUs

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Date:   Thu, 10 Jun 2021 19:25:17 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     linux-fscrypt@...r.kernel.org
Cc:     linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        Daniel Rosenberg <drosen@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] fscrypt: fix derivation of SipHash keys on big endian
 CPUs

On Sat, Jun 05, 2021 at 12:50:33AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@...gle.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@...r.kernel.org> # v5.6+
> Signed-off-by: Eric Biggers <ebiggers@...gle.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(-)
> 

Applied to fscrypt.git#master for 5.14.

- Eric