OpenSSL: Add support for TPM2-wrapped keys

If the header of a PEM-formatted certificate or key in private_key file
indicates that it is wrapped with a TPM2 key, try to autoload the
appropriate OpenSSL engine that can transparently unwrap the key. This
enables systems to use TPM2-wrapped keys as drop-in replacements to
ordinary SSL keys.

This functionality needs
https://git.kernel.org/pub/scm/linux/kernel/git/jejb/openssl_tpm2_engine.git
to be installed as an OpenSSL engine.

Signed-off-by: Daniel Kobras <kobras@puzzle-itc.de>

src/crypto/tls_openssl.c

@@ -4908,6 +4908,76 @@ static int ocsp_status_cb(SSL *s, void *arg)
 #endif /* HAVE_OCSP */
 
 
+static size_t max_str_len(const char **lines)
+{
+	const char **p;
+	size_t max_len = 0;
+
+	for (p = lines; *p; p++) {
+		size_t len = os_strlen(*p);
+
+		if (len > max_len)
+			max_len = len;
+	}
+
+	return max_len;
+}
+
+
+static int match_lines_in_file(const char *path, const char **lines)
+{
+	FILE *f;
+	char *buf;
+	size_t bufsize;
+	int found = 0, is_linestart = 1;
+
+	bufsize = max_str_len(lines) + sizeof("\r\n");
+	buf = os_malloc(bufsize);
+	if (!buf)
+		return 0;
+
+	f = fopen(path, "r");
+	if (!f) {
+		os_free(buf);
+		return 0;
+	}
+
+	while (!found && fgets(buf, bufsize, f)) {
+		int is_lineend;
+		size_t len;
+		const char **p;
+
+		len = strcspn(buf, "\r\n");
+		is_lineend = buf[len] != '\0';
+		buf[len] = '\0';
+
+		if (is_linestart && is_lineend) {
+			for (p = lines; !found && *p; p++)
+				found = os_strcmp(buf, *p) == 0;
+		}
+		is_linestart = is_lineend;
+	}
+
+	fclose(f);
+	bin_clear_free(buf, bufsize);
+
+	return found;
+}
+
+
+static int is_tpm2_key(const char *path)
+{
+	/* Check both new and old format of TPM2 PEM guard tag */
+	static const char *tpm2_tags[] = {
+		"-----BEGIN TSS2 PRIVATE KEY-----",
+		"-----BEGIN TSS2 KEY BLOB-----",
+		NULL
+	};
+
+	return match_lines_in_file(path, tpm2_tags);
+}
+
+
 int tls_connection_set_params(void *tls_ctx, struct tls_connection *conn,
 			      const struct tls_connection_params *params)
 {
@@ -4960,6 +5030,17 @@ int tls_connection_set_params(void *tls_ctx, struct tls_connection *conn,
 	if (can_pkcs11 == 2 && !engine_id)
 		engine_id = "pkcs11";
 
+	/* If private_key points to a TPM2-wrapped key, automatically enable
+	 * tpm2 engine and use it to unwrap the key. */
+	if (params->private_key &&
+	    (!engine_id || os_strcmp(engine_id, "tpm2") == 0) &&
+	    is_tpm2_key(params->private_key)) {
+		wpa_printf(MSG_DEBUG, "OpenSSL: Found TPM2 wrapped key %s",
+			   params->private_key);
+		key_id = key_id ? key_id : params->private_key;
+		engine_id = engine_id ? engine_id : "tpm2";
+	}
+
 #if defined(EAP_FAST) || defined(EAP_FAST_DYNAMIC) || defined(EAP_SERVER_FAST)
 #if OPENSSL_VERSION_NUMBER < 0x10100000L || defined(LIBRESSL_VERSION_NUMBER)
 	if (params->flags & TLS_CONN_EAP_FAST) {
@@ -4991,7 +5072,8 @@ int tls_connection_set_params(void *tls_ctx, struct tls_connection *conn,
 	}
 
 	if (engine_id) {
-		wpa_printf(MSG_DEBUG, "SSL: Initializing TLS engine");
+		wpa_printf(MSG_DEBUG, "SSL: Initializing TLS engine %s",
+			   engine_id);
 		ret = tls_engine_init(conn, engine_id, params->pin,
 				      key_id, cert_id, ca_cert_id);
 		if (ret)