crypto: dh - allow for passing NULL to the ffdheXYZ(dh)s' ->set_secret()

Ephemeral key generation can be requested from any of the ffdheXYZ(dh)
variants' common ->set_secret() by passing it an (encoded) struct dh
with the key parameter being unset, i.e. with ->key_size == 0. As the
whole purpose of the ffdheXYZ(dh) templates is to fill in the group
parameters as appropriate, they expect ->p and ->g to be unset in any
input struct dh as well. This means that a user would have to encode an
all-zeroes struct dh instance via crypto_dh_encode_key() when requesting
ephemeral key generation from a ffdheXYZ(dh) instance, which is kind of
pointless.

Make dh_safe_prime_set_secret() to decode a struct dh from the supplied
buffer only if the latter is non-NULL and initialize it with all zeroes
otherwise.

That is, it is now possible to call

crypto_kpp_set_secret(tfm, NULL, 0);

on any ffdheXYZ(dh) tfm for requesting ephemeral key generation.

Signed-off-by: Nicolai Stange <[e-mail]>
Reviewed-by: Hannes Reinecke <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/dh.c

···
 	struct dh_safe_prime_instance_ctx *inst_ctx =
 		dh_safe_prime_instance_ctx(tfm);
 	struct dh_safe_prime_tfm_ctx *tfm_ctx = kpp_tfm_ctx(tfm);
-	struct dh params;
+	struct dh params = {};
 	void *buf = NULL, *key = NULL;
 	unsigned int buf_size;
 	int err;
 
-	err = __crypto_dh_decode_key(buffer, len, &params);
-	if (err)
-		return err;
-
-	if (params.p_size || params.g_size)
-		return -EINVAL;
+	if (buffer) {
+		err = __crypto_dh_decode_key(buffer, len, &params);
+		if (err)
+			return err;
+		if (params.p_size || params.g_size)
+			return -EINVAL;
+	}
 
 	params.p = inst_ctx->safe_prime->p;
 	params.p_size = inst_ctx->safe_prime->p_size;