Re: [PATCH] fscrypt: add support for IV_INO_LBLK_32 policies

Eric Biggers
2020-05-25T15:33:36Z

On Fri, May 15, 2020 at 01:41:41PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> The eMMC inline crypto standard will only specify 32 DUN bits (a.k.a. IV
> bits), unlike UFS's 64. IV_INO_LBLK_64 is therefore not applicable, but
> an encryption format which uses one key per policy and permits the
> moving of encrypted file contents (as f2fs's garbage collector requires)
> is still desirable.
>
> To support such hardware, add a new encryption format IV_INO_LBLK_32
> that makes the best use of the 32 bits: the IV is set to
> 'SipHash-2-4(inode_number) + file_logical_block_number mod 2^32', where
> the SipHash key is derived from the fscrypt master key. We hash only
> the inode number and not also the block number, because we need to
> maintain contiguity of DUNs to merge bios.
>
> Unlike with IV_INO_LBLK_64, with this format IV reuse is possible; this
> is unavoidable given the size of the DUN. This means this format should
> only be used where the requirements of the first paragraph apply.
> However, the hash spreads out the IVs in the whole usable range, and the
> use of a keyed hash makes it difficult for an attacker to determine
> which files use which IVs.
>
> Besides the above differences, this flag works like IV_INO_LBLK_64 in
> that on ext4 it is only allowed if the stable_inodes feature has been
> enabled to prevent inode numbers and the filesystem UUID from changing.
>
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> Documentation/filesystems/fscrypt.rst | 33 +++++++++--
> fs/crypto/crypto.c | 6 +-
> fs/crypto/fscrypt_private.h | 20 +++++--
> fs/crypto/keyring.c | 5 +-
> fs/crypto/keysetup.c | 85 +++++++++++++++++++++------
> fs/crypto/policy.c | 51 +++++++++++-----
> include/uapi/linux/fscrypt.h | 3 +-
> 7 files changed, 157 insertions(+), 46 deletions(-)

Applied to fscrypt.git#master for 5.8.

- Eric