crypto: ecdh - Pass private key in proper byte order to check valid key

ecc_is_key_valid expects a key with the most significant digit in the last
entry of the digit array. Currently ecdh_set_secret passes a reversed key
to ecc_is_key_valid that then passes the rather simple test checking
whether the private key is in range [2, n-3]. For all current ecdh-
supported curves (NIST P192/256/384) the 'n' parameter is a rather large
number, therefore easily passing this test.

Throughout the ecdh and ecc codebase the variable 'priv' is used for a
private_key holding the bytes in proper byte order. Therefore, introduce
priv in ecdh_set_secret and copy the bytes from ctx->private_key into
priv in proper byte order by using ecc_swap_digits. Pass priv to
ecc_is_valid_key.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Salvatore Benedetto <salvatore.benedetto@intel.com>
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/ecdh.c

@@ -27,7 +27,9 @@ static int ecdh_set_secret(struct crypto_kpp *tfm, const void *buf,
 			   unsigned int len)
 {
 	struct ecdh_ctx *ctx = ecdh_get_ctx(tfm);
+	u64 priv[ECC_MAX_DIGITS];
 	struct ecdh params;
+	int ret = 0;
 
 	if (crypto_ecdh_decode_key(buf, len, &params) < 0 ||
 	    params.key_size > sizeof(u64) * ctx->ndigits)
@@ -40,13 +42,16 @@ static int ecdh_set_secret(struct crypto_kpp *tfm, const void *buf,
 				       ctx->private_key);
 
 	memcpy(ctx->private_key, params.key, params.key_size);
+	ecc_swap_digits(ctx->private_key, priv, ctx->ndigits);
 
 	if (ecc_is_key_valid(ctx->curve_id, ctx->ndigits,
-			     ctx->private_key, params.key_size) < 0) {
+			     priv, params.key_size) < 0) {
 		memzero_explicit(ctx->private_key, params.key_size);
-		return -EINVAL;
+		ret = -EINVAL;
 	}
-	return 0;
+	memzero_explicit(priv, sizeof(priv));
+
+	return ret;
 }
 
 static int ecdh_compute_value(struct kpp_request *req)