[+Cc [email protected]]
On Tue, Mar 03, 2026 at 07:09:26AM -0800, Christoph Hellwig wrote:
> On Tue, Mar 03, 2026 at 12:05:09AM -0600, Joachim Vandersmissen wrote:
> > xxhash64 is not a cryptographic hash algorithm, but is offered in the
> > same API (shash) as actual cryptographic hash algorithms such as
> > SHA-256. The Cryptographic Module Validation Program (CMVP), managing
> > FIPS certification, believes that this could cause confusion. xxhash64
> > must therefore be blocked in FIPS mode.
> >
> > The only usage of xxhash64 in the kernel is btrfs. Commit fe11ac191ce0
> > ("btrfs: switch to library APIs for checksums") recently modified the
> > btrfs code to use the lib/crypto API, avoiding the Kernel Cryptographic
> > API. Consequently, the removal of xxhash64 from the Crypto API in FIPS
> > mode should now have no impact on btrfs usage.
>
> It sounds like xxhash should be removed the crypto API entirely.
> There's no user of it, it's not crypto, and doing xxhash through
> the userspace crypto API socket is so stupid that I doubt anyone
> attempted it.
dm-integrity, which uses crypto_shash and accepts arbitrary hash
algorithm strings from userspace, might be relying on "xxhash64" being
supported in crypto_shash. The integritysetup man page specifically
mentions xxhash64:
--integrity, -I algorithm
Use internal integrity calculation (standalone mode). The integrity
algorithm can be CRC (crc32c/crc32), a non-cryptographic hash function
(xxhash64) or a hash function (sha1, sha256).
For HMAC (hmac-sha256), you must specify an integrity key and its
size.
Maybe the device-mapper maintainers have some insight into whether
anyone is actually using xxhash64 with dm-integrity.
If yes, then dm-integrity could still switch to using the library API
for it. dm-integrity would just need to gain some helper functions that
call either the xxhash64 library or crypto_shash depending on the
configured algorithm. If the full set of algorithms being used can be
determined, then dm-integrity could even switch to the library APIs
entirely, like many other kernel subsystems such as btrfs have.
- Eric
