DPP2: Validate CSR on Configurator before forwarding to CA/RA

Parse the received CSR, verify that it has been signed correctly, and
verify that the challengePassword is present and matches the derived cp.

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

src/common/dpp.c

@@ -1694,6 +1694,11 @@ dpp_build_conf_resp(struct dpp_authentication *auth, const u8 *e_nonce,
 	size_t len[1];
 	enum dpp_status_error status;
 
+	if (auth->force_conf_resp_status != DPP_STATUS_OK) {
+		status = auth->force_conf_resp_status;
+		goto forced_status;
+	}
+
 	if (netrole == DPP_NETROLE_CONFIGURATOR) {
 #ifdef CONFIG_DPP2
 		env_data = dpp_build_enveloped_data(auth);
@@ -1715,6 +1720,7 @@ dpp_build_conf_resp(struct dpp_authentication *auth, const u8 *e_nonce,
 		status = DPP_STATUS_CSR_NEEDED;
 	else
 		status = DPP_STATUS_CONFIGURE_FAILURE;
+forced_status:
 	auth->conf_resp_status = status;
 
 	/* { E-nonce, configurationObject[, sendConnStatus]}ke */
@@ -2040,6 +2046,12 @@ dpp_conf_req_rx(struct dpp_authentication *auth, const u8 *attr_start,
 		char *txt;
 
 		wpa_hexdump_buf(MSG_DEBUG, "DPP: CertificateRequest", cert_req);
+		if (dpp_validate_csr(auth, cert_req) < 0) {
+			wpa_printf(MSG_DEBUG, "DPP: CSR is not valid");
+			auth->force_conf_resp_status = DPP_STATUS_CSR_BAD;
+			goto cont;
+		}
+		wpa_printf(MSG_DEBUG, "DPP: CSR is valid - forward to CA/RA");
 		txt = base64_encode_no_lf(wpabuf_head(cert_req),
 					  wpabuf_len(cert_req), NULL);
 		if (!txt)
@@ -2051,6 +2063,7 @@ dpp_conf_req_rx(struct dpp_authentication *auth, const u8 *attr_start,
 		auth->waiting_cert = true;
 		goto fail;
 	}
+cont:
 #endif /* CONFIG_DPP2 */
 
 	resp = dpp_build_conf_resp(auth, e_nonce, e_nonce_len, netrole,

src/common/dpp.h

@@ -250,6 +250,7 @@ struct dpp_authentication {
 	enum dpp_connector_key reconfig_connector_key;
 	enum dpp_status_error auth_resp_status;
 	enum dpp_status_error conf_resp_status;
+	enum dpp_status_error force_conf_resp_status;
 	u8 peer_mac_addr[ETH_ALEN];
 	u8 i_nonce[DPP_MAX_NONCE_LEN];
 	u8 r_nonce[DPP_MAX_NONCE_LEN];
@@ -617,6 +618,7 @@ void dpp_pfs_free(struct dpp_pfs *pfs);
 
 struct wpabuf * dpp_build_csr(struct dpp_authentication *auth);
 struct wpabuf * dpp_pkcs7_certs(const struct wpabuf *pkcs7);
+int dpp_validate_csr(struct dpp_authentication *auth, const struct wpabuf *csr);
 
 struct dpp_bootstrap_info * dpp_add_qr_code(struct dpp_global *dpp,
 					    const char *uri);

src/common/dpp_crypto.c

@@ -2844,6 +2844,125 @@ fail:
 	return pem;
 }
 
+
+int dpp_validate_csr(struct dpp_authentication *auth, const struct wpabuf *csr)
+{
+	X509_REQ *req;
+	const unsigned char *pos;
+	EVP_PKEY *pkey;
+	int res, loc, ret = -1;
+	X509_ATTRIBUTE *attr;
+	ASN1_TYPE *type;
+	ASN1_STRING *str;
+	unsigned char *utf8 = NULL;
+	unsigned char *cp = NULL;
+	size_t cp_len;
+	u8 exp_cp[DPP_CP_LEN];
+	unsigned int hash_len = auth->curve->hash_len;
+
+	pos = wpabuf_head(csr);
+	req = d2i_X509_REQ(NULL, &pos, wpabuf_len(csr));
+	if (!req) {
+		wpa_printf(MSG_DEBUG, "DPP: Failed to parse CSR");
+		return -1;
+	}
+
+	pkey = X509_REQ_get_pubkey(req);
+	if (!pkey) {
+		wpa_printf(MSG_DEBUG, "DPP: Failed to get public key from CSR");
+		goto fail;
+	}
+
+	res = X509_REQ_verify(req, pkey);
+	EVP_PKEY_free(pkey);
+	if (res != 1) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: CSR does not have a valid signature");
+		goto fail;
+	}
+
+	loc = X509_REQ_get_attr_by_NID(req, NID_pkcs9_challengePassword, -1);
+	if (loc < 0) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: CSR does not include challengePassword");
+		goto fail;
+	}
+
+	attr = X509_REQ_get_attr(req, loc);
+	if (!attr) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Could not get challengePassword attribute");
+		goto fail;
+	}
+
+	type = X509_ATTRIBUTE_get0_type(attr, 0);
+	if (!type) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Could not get challengePassword attribute type");
+		goto fail;
+	}
+
+	res = ASN1_TYPE_get(type);
+	/* This is supposed to be UTF8String, but allow other strings as well
+	 * since challengePassword is using ASCII (base64 encoded). */
+	if (res != V_ASN1_UTF8STRING && res != V_ASN1_PRINTABLESTRING &&
+	    res != V_ASN1_IA5STRING) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Unexpected challengePassword attribute type %d",
+			   res);
+		goto fail;
+	}
+
+	str = X509_ATTRIBUTE_get0_data(attr, 0, res, NULL);
+	if (!str) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Could not get ASN.1 string for challengePassword");
+		goto fail;
+	}
+
+	res = ASN1_STRING_to_UTF8(&utf8, str);
+	if (res < 0) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Could not get UTF8 version of challengePassword");
+		goto fail;
+	}
+
+	cp = base64_decode((const char *) utf8, res, &cp_len);
+	OPENSSL_free(utf8);
+	if (!cp) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Could not base64 decode challengePassword");
+		goto fail;
+	}
+	if (cp_len != DPP_CP_LEN) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: Unexpected cp length (%zu) in CSR challengePassword",
+			   cp_len);
+		goto fail;
+	}
+	wpa_hexdump_key(MSG_DEBUG, "DPP: cp from CSR challengePassword",
+			cp, cp_len);
+
+	/* cp = HKDF-Expand(bk, "CSR challengePassword", 64) */
+	if (dpp_hkdf_expand(hash_len, auth->bk, hash_len,
+			    "CSR challengePassword", exp_cp, DPP_CP_LEN) < 0)
+		goto fail;
+	wpa_hexdump_key(MSG_DEBUG,
+			"DPP: cp = HKDF-Expand(bk, \"CSR challengePassword\", 64)",
+			exp_cp, DPP_CP_LEN);
+	if (os_memcmp_const(cp, exp_cp, DPP_CP_LEN) != 0) {
+		wpa_printf(MSG_DEBUG,
+			   "DPP: CSR challengePassword does not match calculated cp");
+		goto fail;
+	}
+
+	ret = 0;
+fail:
+	os_free(cp);
+	X509_REQ_free(req);
+	return ret;
+}
+
 #endif /* CONFIG_DPP2 */