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

Theodore Y. Ts'o
2020-05-19T11:13:45Z

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>

Reviewed-by: Theodore Ts'o <tytso@mit.edu>

I kind of wish we had Kunit tests with test vectors, but that's for
another commit I think.

- Ted