EAP-pwd: Check element x,y coordinates explicitly

This adds an explicit check for 0 < x,y < prime based on RFC 5931,
2.8.5.2.2 requirement. The earlier checks might have covered this
implicitly, but it is safer to avoid any dependency on implicit checks
and specific crypto library behavior. (CVE-2019-9498 and CVE-2019-9499)

Furthermore, this moves the EAP-pwd element and scalar parsing and
validation steps into shared helper functions so that there is no need
to maintain two separate copies of this common functionality between the
server and peer implementations.

Signed-off-by: Jouni Malinen <jouni@codeaurora.org>

src/eap_common/eap_pwd_common.c

@@ -428,3 +428,109 @@ int compute_keys(EAP_PWD_group *grp, const struct crypto_bignum *k,
 
 	return 1;
 }
+
+
+static int eap_pwd_element_coord_ok(const struct crypto_bignum *prime,
+				    const u8 *buf, size_t len)
+{
+	struct crypto_bignum *val;
+	int ok = 1;
+
+	val = crypto_bignum_init_set(buf, len);
+	if (!val || crypto_bignum_is_zero(val) ||
+	    crypto_bignum_cmp(val, prime) >= 0)
+		ok = 0;
+	crypto_bignum_deinit(val, 0);
+	return ok;
+}
+
+
+struct crypto_ec_point * eap_pwd_get_element(EAP_PWD_group *group,
+					     const u8 *buf)
+{
+	struct crypto_ec_point *element;
+	const struct crypto_bignum *prime;
+	size_t prime_len;
+	struct crypto_bignum *cofactor = NULL;
+
+	prime = crypto_ec_get_prime(group->group);
+	prime_len = crypto_ec_prime_len(group->group);
+
+	/* RFC 5931, 2.8.5.2.2: 0 < x,y < p */
+	if (!eap_pwd_element_coord_ok(prime, buf, prime_len) ||
+	    !eap_pwd_element_coord_ok(prime, buf + prime_len, prime_len)) {
+		wpa_printf(MSG_INFO, "EAP-pwd: Invalid coordinate in element");
+		return NULL;
+	}
+
+	element = crypto_ec_point_from_bin(group->group, buf);
+	if (!element) {
+		wpa_printf(MSG_INFO, "EAP-pwd: EC point from element failed");
+		return NULL;
+	}
+
+	/* RFC 5931, 2.8.5.2.2: on curve and not the point at infinity */
+	if (!crypto_ec_point_is_on_curve(group->group, element) ||
+	    crypto_ec_point_is_at_infinity(group->group, element)) {
+		wpa_printf(MSG_INFO, "EAP-pwd: Invalid element");
+		goto fail;
+	}
+
+	cofactor = crypto_bignum_init();
+	if (!cofactor || crypto_ec_cofactor(group->group, cofactor) < 0) {
+		wpa_printf(MSG_INFO,
+			   "EAP-pwd: Unable to get cofactor for curve");
+		goto fail;
+	}
+
+	if (!crypto_bignum_is_one(cofactor)) {
+		struct crypto_ec_point *point;
+		int ok = 1;
+
+		/* check to ensure peer's element is not in a small sub-group */
+		point = crypto_ec_point_init(group->group);
+		if (!point ||
+		    crypto_ec_point_mul(group->group, element,
+					cofactor, point) != 0 ||
+		    crypto_ec_point_is_at_infinity(group->group, point))
+			ok = 0;
+		crypto_ec_point_deinit(point, 0);
+
+		if (!ok) {
+			wpa_printf(MSG_INFO,
+				   "EAP-pwd: Small sub-group check on peer element failed");
+			goto fail;
+		}
+	}
+
+out:
+	crypto_bignum_deinit(cofactor, 0);
+	return element;
+fail:
+	crypto_ec_point_deinit(element, 0);
+	element = NULL;
+	goto out;
+}
+
+
+struct crypto_bignum * eap_pwd_get_scalar(EAP_PWD_group *group, const u8 *buf)
+{
+	struct crypto_bignum *scalar;
+	const struct crypto_bignum *order;
+	size_t order_len;
+
+	order = crypto_ec_get_order(group->group);
+	order_len = crypto_ec_order_len(group->group);
+
+	/* RFC 5931, 2.8.5.2: 1 < scalar < r */
+	scalar = crypto_bignum_init_set(buf, order_len);
+	if (!scalar || crypto_bignum_is_zero(scalar) ||
+	    crypto_bignum_is_one(scalar) ||
+	    crypto_bignum_cmp(scalar, order) >= 0) {
+		wpa_printf(MSG_INFO, "EAP-pwd: received scalar is invalid");
+		crypto_bignum_deinit(scalar, 0);
+		scalar = NULL;
+	}
+
+	return scalar;
+}

src/eap_common/eap_pwd_common.h

@@ -67,5 +67,8 @@ int compute_keys(EAP_PWD_group *grp, const struct crypto_bignum *k,
 struct crypto_hash * eap_pwd_h_init(void);
 void eap_pwd_h_update(struct crypto_hash *hash, const u8 *data, size_t len);
 void eap_pwd_h_final(struct crypto_hash *hash, u8 *digest);
+struct crypto_ec_point * eap_pwd_get_element(EAP_PWD_group *group,
+					     const u8 *buf);
+struct crypto_bignum * eap_pwd_get_scalar(EAP_PWD_group *group, const u8 *buf);
 
 #endif  /* EAP_PWD_COMMON_H */

src/eap_peer/eap_pwd.c

@@ -308,7 +308,7 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 				const struct wpabuf *reqData,
 				const u8 *payload, size_t payload_len)
 {
-	struct crypto_ec_point *K = NULL, *point = NULL;
+	struct crypto_ec_point *K = NULL;
 	struct crypto_bignum *mask = NULL, *cofactor = NULL;
 	const u8 *ptr = payload;
 	u8 *scalar = NULL, *element = NULL;
@@ -572,63 +572,27 @@ eap_pwd_perform_commit_exchange(struct eap_sm *sm, struct eap_pwd_data *data,
 	/* process the request */
 	data->k = crypto_bignum_init();
 	K = crypto_ec_point_init(data->grp->group);
-	point = crypto_ec_point_init(data->grp->group);
+	if (!data->k || !K) {
-	if (!data->k || !K || !point) {
 		wpa_printf(MSG_INFO, "EAP-PWD (peer): peer data allocation "
 			   "fail");
 		goto fin;
 	}
 
 	/* element, x then y, followed by scalar */
-	data->server_element = crypto_ec_point_from_bin(data->grp->group, ptr);
+	data->server_element = eap_pwd_get_element(data->grp, ptr);
 	if (!data->server_element) {
 		wpa_printf(MSG_INFO, "EAP-PWD (peer): setting peer element "
 			   "fail");
 		goto fin;
 	}
 	ptr += prime_len * 2;
-	data->server_scalar = crypto_bignum_init_set(ptr, order_len);
+	data->server_scalar = eap_pwd_get_scalar(data->grp, ptr);
 	if (!data->server_scalar) {
 		wpa_printf(MSG_INFO,
 			   "EAP-PWD (peer): setting peer scalar fail");
 		goto fin;
 	}
 
-	/* verify received scalar */
-	if (crypto_bignum_is_zero(data->server_scalar) ||
-	    crypto_bignum_is_one(data->server_scalar) ||
-	    crypto_bignum_cmp(data->server_scalar,
-			      crypto_ec_get_order(data->grp->group)) >= 0) {
-		wpa_printf(MSG_INFO,
-			   "EAP-PWD (peer): received scalar is invalid");
-		goto fin;
-	}
-
-	/* verify received element */
-	if (!crypto_ec_point_is_on_curve(data->grp->group,
-					 data->server_element) ||
-	    crypto_ec_point_is_at_infinity(data->grp->group,
-					   data->server_element)) {
-		wpa_printf(MSG_INFO,
-			   "EAP-PWD (peer): received element is invalid");
-		goto fin;
-	}
-
-	/* check to ensure server's element is not in a small sub-group */
-	if (!crypto_bignum_is_one(cofactor)) {
-		if (crypto_ec_point_mul(data->grp->group, data->server_element,
-					cofactor, point) < 0) {
-			wpa_printf(MSG_INFO, "EAP-PWD (peer): cannot multiply "
-				   "server element by order!\n");
-			goto fin;
-		}
-		if (crypto_ec_point_is_at_infinity(data->grp->group, point)) {
-			wpa_printf(MSG_INFO, "EAP-PWD (peer): server element "
-				   "is at infinity!\n");
-			goto fin;
-		}
-	}
-
 	/* compute the shared key, k */
 	if (crypto_ec_point_mul(data->grp->group, data->grp->pwe,
 				data->server_scalar, K) < 0 ||
@@ -702,7 +666,6 @@ fin:
 	crypto_bignum_deinit(mask, 1);
 	crypto_bignum_deinit(cofactor, 1);
 	crypto_ec_point_deinit(K, 1);
-	crypto_ec_point_deinit(point, 1);
 	if (data->outbuf == NULL)
 		eap_pwd_state(data, FAILURE);
 	else

src/eap_server/eap_server_pwd.c

@@ -669,7 +669,7 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 {
 	const u8 *ptr;
 	struct crypto_bignum *cofactor = NULL;
-	struct crypto_ec_point *K = NULL, *point = NULL;
+	struct crypto_ec_point *K = NULL;
 	int res = 0;
 	size_t prime_len, order_len;
 
@@ -688,9 +688,8 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 
 	data->k = crypto_bignum_init();
 	cofactor = crypto_bignum_init();
-	point = crypto_ec_point_init(data->grp->group);
 	K = crypto_ec_point_init(data->grp->group);
-	if (!data->k || !cofactor || !point || !K) {
+	if (!data->k || !cofactor || !K) {
 		wpa_printf(MSG_INFO, "EAP-PWD (server): peer data allocation "
 			   "fail");
 		goto fin;
@@ -704,55 +703,20 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 
 	/* element, x then y, followed by scalar */
 	ptr = payload;
-	data->peer_element = crypto_ec_point_from_bin(data->grp->group, ptr);
+	data->peer_element = eap_pwd_get_element(data->grp, ptr);
 	if (!data->peer_element) {
 		wpa_printf(MSG_INFO, "EAP-PWD (server): setting peer element "
 			   "fail");
 		goto fin;
 	}
 	ptr += prime_len * 2;
-	data->peer_scalar = crypto_bignum_init_set(ptr, order_len);
+	data->peer_scalar = eap_pwd_get_scalar(data->grp, ptr);
 	if (!data->peer_scalar) {
 		wpa_printf(MSG_INFO, "EAP-PWD (server): peer data allocation "
 			   "fail");
 		goto fin;
 	}
 
-	/* verify received scalar */
-	if (crypto_bignum_is_zero(data->peer_scalar) ||
-	    crypto_bignum_is_one(data->peer_scalar) ||
-	    crypto_bignum_cmp(data->peer_scalar,
-			      crypto_ec_get_order(data->grp->group)) >= 0) {
-		wpa_printf(MSG_INFO,
-			   "EAP-PWD (server): received scalar is invalid");
-		goto fin;
-	}
-
-	/* verify received element */
-	if (!crypto_ec_point_is_on_curve(data->grp->group,
-					 data->peer_element) ||
-	    crypto_ec_point_is_at_infinity(data->grp->group,
-					   data->peer_element)) {
-		wpa_printf(MSG_INFO,
-			   "EAP-PWD (server): received element is invalid");
-		goto fin;
-	}
-
-	/* check to ensure peer's element is not in a small sub-group */
-	if (!crypto_bignum_is_one(cofactor)) {
-		if (crypto_ec_point_mul(data->grp->group, data->peer_element,
-					cofactor, point) != 0) {
-			wpa_printf(MSG_INFO, "EAP-PWD (server): cannot "
-				   "multiply peer element by order");
-			goto fin;
-		}
-		if (crypto_ec_point_is_at_infinity(data->grp->group, point)) {
-			wpa_printf(MSG_INFO, "EAP-PWD (server): peer element "
-				   "is at infinity!\n");
-			goto fin;
-		}
-	}
-
 	/* detect reflection attacks */
 	if (crypto_bignum_cmp(data->my_scalar, data->peer_scalar) == 0 ||
 	    crypto_ec_point_cmp(data->grp->group, data->my_element,
@@ -804,7 +768,6 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
 
 fin:
 	crypto_ec_point_deinit(K, 1);
-	crypto_ec_point_deinit(point, 1);
 	crypto_bignum_deinit(cofactor, 1);
 
 	if (res)