crypto: dh - split out deserialization code from crypto_dh_decode()

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

crypto/dh_helper.c

···
 }
 EXPORT_SYMBOL_GPL(crypto_dh_encode_key);
 
-int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
+int __crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
 {
 	const u8 *ptr = buf;
 	struct kpp_secret secret;
···
 	if (secret.len != crypto_dh_key_len(params))
 		return -EINVAL;
 
+	/* Don't allocate memory. Set pointers to data within
+	 * the given buffer
+	 */
+	params->key = (void *)ptr;
+	params->p = (void *)(ptr + params->key_size);
+	params->g = (void *)(ptr + params->key_size + params->p_size);
+
+	return 0;
+}
+
+int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params)
+{
+	int err;
+
+	err = __crypto_dh_decode_key(buf, len, params);
+	if (err)
+		return err;
+
 	/*
 	 * Don't permit the buffer for 'key' or 'g' to be larger than 'p', since
 	 * some drivers assume otherwise.
···
 	if (params->key_size > params->p_size ||
 	    params->g_size > params->p_size)
 		return -EINVAL;
-
-	/* Don't allocate memory. Set pointers to data within
-	 * the given buffer
-	 */
-	params->key = (void *)ptr;
-	params->p = (void *)(ptr + params->key_size);
-	params->g = (void *)(ptr + params->key_size + params->p_size);
 
 	/*
 	 * Don't permit 'p' to be 0.  It's not a prime number, and it's subject

include/crypto/dh.h

···
  */
 int crypto_dh_decode_key(const char *buf, unsigned int len, struct dh *params);
 
+/**
+ * __crypto_dh_decode_key() - decode a private key without parameter checks
+ * @buf:	Buffer holding a packet key that should be decoded
+ * @len:	Length of the packet private key buffer
+ * @params:	Buffer allocated by the caller that is filled with the
+ *		unpacked DH private key.
+ *
+ * Internal function providing the same services as the exported
+ * crypto_dh_decode_key(), but without any of those basic parameter
+ * checks conducted by the latter.
+ *
+ * Return:	-EINVAL if buffer has insufficient size, 0 on success
+ */
+int __crypto_dh_decode_key(const char *buf, unsigned int len,
+			   struct dh *params);
+
 #endif