[PATCH 1/3] crypto: ECDH - check validity of Z before export

["Stephan Müller" <smueller@chronox.de>]

From 5385865b3f44d331f91c6786a2e7f4e2fb4d8cb2 Mon Sep 17 00:00:00 2001
From: Stephan Mueller <stephan.mueller@atsec.com>
Date: Thu, 11 Jun 2020 08:12:54 +0200
Subject: 

SP800-56A rev3 section 5.7.1.2 step 2 mandates that the validity of the
calculated shared secret is verified before the data is returned to the
caller. Thus, the export function and the validity check functions are
reversed. In addition, the sensitive variables of priv and rand_z are
zeroized.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 crypto/ecc.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/crypto/ecc.c b/crypto/ecc.c
index 02d35be7702b..52e2d49262f2 100644
--- a/crypto/ecc.c
+++ b/crypto/ecc.c
@@ -1495,11 +1495,16 @@ int crypto_ecdh_shared_secret(unsigned int curve_id, unsigned int ndigits,
 
 	ecc_point_mult(product, pk, priv, rand_z, curve, ndigits);
 
-	ecc_swap_digits(product->x, secret, ndigits);
-
-	if (ecc_point_is_zero(product))
+	if (ecc_point_is_zero(product)) {
 		ret = -EFAULT;
+		goto err_validity;
+	}
+
+	ecc_swap_digits(product->x, secret, ndigits);
 
+err_validity:
+	memzero_explicit(priv, sizeof(priv));
+	memzero_explicit(rand_z, sizeof(rand_z));
 	ecc_free_point(product);
 err_alloc_product:
 	ecc_free_point(pk);
-- 
2.26.2