Re: [PATCH v4 05/15] crypto: dh - split out deserialization code from crypto_dh_decode()

[Hannes Reinecke]

On 2/21/22 13:10, Nicolai Stange wrote:
> A subsequent commit will introduce "dh" wrapping templates of the form
> "ffdhe2048(dh)", "ffdhe3072(dh)" and so on in order to provide built-in
> support for the well-known safe-prime ffdhe group parameters specified in
> RFC 7919.
>
> Those templates' ->set_secret() will wrap the inner "dh" implementation's
> ->set_secret() and set the ->p and ->g group parameters as appropriate on
> the way inwards. More specifically,
> - A ffdheXYZ(dh) user would call crypto_dh_encode() on a struct dh instance
>    having ->p == ->g == NULL as well as ->p_size == ->g_size == 0 and pass
>    the resulting buffer to the outer ->set_secret().
> - This outer ->set_secret() would then decode the struct dh via
>    crypto_dh_decode_key(), set ->p, ->g, ->p_size as well as ->g_size as
>    appropriate for the group in question and encode the struct dh again
>    before passing it further down to the inner "dh"'s ->set_secret().
>
> The problem is that crypto_dh_decode_key() implements some basic checks
> which would reject parameter sets with ->p_size == 0 and thus, the ffdheXYZ
> templates' ->set_secret() cannot use it as-is for decoding the passed
> buffer. As the inner "dh"'s ->set_secret() will eventually conduct said
> checks on the final parameter set anyway, the outer ->set_secret() really
> only needs the decoding functionality.
>
> Split out the pure struct dh decoding part from crypto_dh_decode_key() into
> the new __crypto_dh_decode_key().
>
> __crypto_dh_decode_key() gets defined in crypto/dh_helper.c, but will have
> to get called from crypto/dh.c and thus, its declaration must be somehow
> made available to the latter. Strictly speaking, __crypto_dh_decode_key()
> is internal to the dh_generic module, yet it would be a bit over the top
> to introduce a new header like e.g. include/crypto/internal/dh.h
> containing just a single prototype. Add the __crypto_dh_decode_key()
> declaration to include/crypto/dh.h instead.
>
> Provide a proper kernel-doc annotation, even though
> __crypto_dh_decode_key() is purposedly not on the function list specified
> in Documentation/crypto/api-kpp.rst.
>
> Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
> ---
>   crypto/dh_helper.c  | 27 +++++++++++++++++++--------
>   include/crypto/dh.h | 16 ++++++++++++++++
>   2 files changed, 35 insertions(+), 8 deletions(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke		           Kernel Storage Architect
hare@xxxxxxx			                  +49 911 74053 688
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