crypto: dh - introduce common code for built-in safe-prime group support

Recent work on NVME in-band authentication support ([1]) needs to invoke
the "dh" KPP with the FFDHE safe-prime group parameters as specified in
RFC 7919 and generate ephemeral keys suitable for the respective group. By
coincidence, the requirements from NIST SP800-56Arev3,
sec. 5.5.2 ("Assurance of Domain-Parameter Validity") basically boil down
to disallowing any group parameters not among the approved safe-prime
groups specified in either RFC 7919 or RFC 3526 in FIPS mode. Furthermore,
SP800-56Arev3 specifies the respective security strength for each of the
approved safe-prime groups, which has a direct impact on the minimum key
lengths.

In this light, it's desirable to introduce built-in support for the
RFC 7919 safe-prime groups to the kernel's DH implementation, provide a
SP800-56Arev3 conforming key generation primitive for those and render
non-approved group parameters unusable in FIPS mode on the way.

As suggested ([2]) in the course of discussion to previous iterations of
this patchset, the built-in support for ffdhe groups would be best made
available in the form of templates wrapping the existing "dh"
implementation, one for each group specified by RFC 7919: ffdhe2048(dh),
ffdhe3072(dh), ffdhe4096(dh), ffdhe6144(dh) and ffdhe8192(dh). As these
templates differ only in the safe-prime constants they'd configure the
inner "dh" transforms with, they can share almost all of their
"dh"-wrapping template implementation code.

Introduce this common code to dh_generic. The actual dump of the RFC 7919
safe-prime constants will be deferred to the next patch in order to
facilitate review. The ephemeral key generation primitive mentioned above
likewise deserves a patch on its own, as does the mechanism by which
unapproved groups are rendered unusable in FIPS mode.

Define a struct dh_safe_prime container for specifying the individual
templates' associated safe-prime group constants. All ffdheXYZ(dh) template
instances will store a pointer to such a dh_safe_prime in their context
areas each. Implement the common __dh_safe_prime_create() template
instantiation helper. The intention is that the individual ffdheXYZ(dh)
crypto_templates' ->create() implementations will simply forward any calls
to __dh_safe_prime_create(), passing a suitable dh_safe_prime in addition
to the received ->create() arguments. __dh_safe_prime_create() would then
create and register a kpp_instance as appropriate, storing the given
dh_safe_prime pointer alongside a crypto_kpp_spawn for the inner "dh"
kpp_alg in the context area.

As the ffdheXYZ(dh) kpp_instances are supposed to act as proxies to the
inner "dh" kpp_alg, make each of their associated crypto_kpp transforms to
in turn own an inner "dh" transform, a pointer to which gets stored in the
context area. Setup and teardown are getting handled from the outer
->init_tfm() and ->exit_tfm() respectively.

In order to achieve the overall goal and let the ffdheXYZ(dh) kpp_instances
configure the inner "dh" transforms with the respective group parameters,
make their common ->set_secret(), the new dh_safe_prime_set_secret(), fill
in the P and G values before forwarding the call to the inner "dh"'s
->set_secret(). Note that the outer ->set_secret() can obtain the P value
associated with the given ffdheXYZ(dh) kpp_instance by means of the
dh_safe_prime referenced from the latter's context. The value of G OTOH
always equals constant 2 for the safe-prime groups.

Finally, make the remaining two kpp_alg primitives both operating on
kpp_requests, i.e. ->generate_public_key() and ->compute_shared_secret(),
to merely forward any request to the inner "dh" implementation. However, a
kpp_request instance received from the outside cannot get simply passed
on as-is, because its associated transform (crypto_kpp_reqtfm()) will have
been set to the outer ffdheXYZ(dh) one. In order to handle this, reserve
some space in the outer ffdheXYZ(dh) kpp_requests' context areas for in
turn storing an inner kpp_request suitable for "dh" each. Make the outer
->generate_public_key() and ->compute_shared_secret() respectively to setup
this inner kpp_request by means of the new dh_safe_prime_prepare_dh_req()
helper before handing it over to the "dh" implementation for further
processing. dh_safe_prime_prepare_dh_req() basically copies the outer
kpp_request received from the outside over to the inner one, but installs
the inner transform and its own ->complete() proxy callback therein. This
completion callback, the new dh_safe_prime_complete_req(), doesn't do
anything beyond completing the outer request. Note that there exist some
examples in crypto/, which would simply install the completion handler
from the outer request at the inner one in similar setups, e.g. seqiv.
However, this would mean that the user-provided completion handler won't
get called with the address of the outer kpp_request initially submitted
and the handler might not be prepared for this. Users could certainly work
around this by setting the callback ->data properly, but IMO it's cleaner
this way. Furthermore, it might make sense to extend
dh_safe_prime_complete_req() in the future and move e.g. those
post-computation FIPS checks from the generic "dh" implementation to the
ffdheXYZ(dh) templates.

[1] https://lore.kernel.org/r/20211202152358.60116-1-hare@suse.de
[2] https://lore.kernel.org/r/20211217055227.GA20698@gondor.apana.org.au

Signed-off-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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