Re: [PATCH] crypto: af_alg - Document the deprecation of AF_ALG

[Eric Biggers <ebiggers@kernel.org>]

On Mon, May 04, 2026 at 02:39:12PM +0000, Jon Kohler wrote:
> > On Apr 29, 2026, at 9:15 PM, Eric Biggers <ebiggers@kernel.org> wrote:
> > 
> > AF_ALG is almost completely unnecessary, and it exposes a massive attack
> > surface that hasn't been standing up to modern vulnerability discovery
> > tools.  The latest one even has its own website, providing a small
> > Python script that reliably roots most Linux distros: https://copy.fail/
> > 
> > This isn't sustainable, especially as LLMs have accelerated the rate the
> > vulnerabilities are coming in.  The effort that is being put into this
> > thing is vastly disproportional to the few programs that actually use
> > it, and those programs would be better served by userspace code anyway.
> > 
> > These issues have been noted in many mailing list discussions already.
> > But until now they haven't been reflected in the documentation or
> > kconfig menu itself, and the vulnerabilities are still coming in.
> > 
> > Let's go ahead and document the deprecation.
> > 
> > This isn't intended to change anything overnight.  After all, most Linux
> > distros won't be able to disable the kconfig options quite yet, mainly
> > because of iwd.  But this should create a bit more impetus for these
> > userspace programs to be fixed, and the documentation update should also
> > help prevent more users from appearing.
> > 
> > Signed-off-by: Eric Biggers <ebiggers@kernel.org>
> > ---
> 
> Quick passing observation
> I noticed that when attempting to completely disable these Crypto APIs,
> I was experiencing boot failures with fips=1 enabled systems.
> 
> Using 6.18-based kernel with an el9-based user space, I see the
> following hang in the early boot console from dracut-pre-pivot:
>   Check integrity of kernel
>   libkcapi - Error: AF_ALG: socket syscall failed (errno: -97)
>   Allocation of hmac(sha512) cipher failed (-97)
> 
> I haven't looked at every elX version, but at least in el9 and el10,
> they use libkcapi-hmaccalc to provide sha512hmac, which dracut [1]
> uses to calculate the HMAC value in do_fips().
> 
> Digging further, I was only able to disable RNG and AEAD APIs, but
> not HASH and SKCIPHER APIs when FIPS was in the picture with el9++.
> 
> I’m not sure how other distros do the same, but this could be problematic
> elsehwere if other distros went down the libkcapi route.
> 
> [1] https://github.com/dracutdevs/dracut/blob/059/modules.d/01fips/fips.sh#L167

That seems to be an implementation of FIPS 140-3's integrity self-check.
A few observations:

- It could easily use userspace SHA-512 code instead.  If including
  libcrypto.so in the "FIPS cryptographic boundary" would cause
  certification difficulties, then a sha512.c file could simply be added
  to 'libkcapi-hmaccalc' which is already in it.

- It's compatible with all of the proposed hardening.  It doesn't
  require zero-copy performance.  It runs as root, so it would be
  compatible with a capability check.  "hmac(sha512)" will need to be on
  the algorithm allowlist anyway for iwd.

- FIPS 140-3 might also allow it to be simplified to use a plain hash
  instead of pointlessly using HMAC with a fixed key.

Anyway, just another one of the long tail of odd users that could have
solved their problem in a better way.  This one is at least compatible
with the hardening that's being considered.

By the way, also on the topic of FIPS 140-3, some people do use AF_ALG
for ACVP (even though it's not all that great for that purpose, either).
But ACVP is a testing thing, not something that is needed on production
systems.  ACVP can just be run as root on a testing build; there's no
need to enable support for it in the actual production build.

- Eric