Access ssl->hostname through abstractions in certificate verification

New abstractions to access ssl->hostname:
mbedtls_ssl_has_set_hostname_been_called(), mbedtls_ssl_free_hostname().

Use these abstractions to access the hostname with the opportunity for
extra checks in mbedtls_ssl_verify_certificate().

No behavior change except for a new log message.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/ssl_tls.c

@@ -2516,6 +2516,36 @@ void mbedtls_ssl_conf_groups(mbedtls_ssl_config *conf,
 }
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
+
+#if defined(MBEDTLS_SSL_HANDSHAKE_WITH_CERT_ENABLED)
+/** Whether mbedtls_ssl_set_hostname() has been called.
+ *
+ * \param[in]   ssl     SSL context
+ *
+ * \return \c 1 if mbedtls_ssl_set_hostname() has been called on \p ssl
+ *         (including `mbedtls_ssl_set_hostname(ssl, NULL)`),
+ *         otherwise \c 0.
+ */
+static int mbedtls_ssl_has_set_hostname_been_called(
+    const mbedtls_ssl_context *ssl)
+{
+    /* We can't tell the difference between the case where
+     * mbedtls_ssl_set_hostname() has not been called at all, and
+     * the case where it was last called with NULL. For the time
+     * being, we assume the latter, i.e. we behave as if there had
+     * been an implicit call to mbedtls_ssl_set_hostname(ssl, NULL). */
+    return ssl->hostname != NULL;
+}
+#endif
+
+static void mbedtls_ssl_free_hostname(mbedtls_ssl_context *ssl)
+{
+    if (ssl->hostname != NULL) {
+        mbedtls_zeroize_and_free(ssl->hostname, strlen(ssl->hostname));
+    }
+    ssl->hostname = NULL;
+}
+
 int mbedtls_ssl_set_hostname(mbedtls_ssl_context *ssl, const char *hostname)
 {
     /* Initialize to suppress unnecessary compiler warning */
@@ -2533,10 +2563,7 @@ int mbedtls_ssl_set_hostname(mbedtls_ssl_context *ssl, const char *hostname)
 
     /* Now it's clear that we will overwrite the old hostname,
      * so we can free it safely */
-
+    mbedtls_ssl_free_hostname(ssl);
-    if (ssl->hostname != NULL) {
-        mbedtls_zeroize_and_free(ssl->hostname, strlen(ssl->hostname));
-    }
 
     /* Passing NULL as hostname shall clear the old one */
 
@@ -5295,9 +5322,7 @@ void mbedtls_ssl_free(mbedtls_ssl_context *ssl)
     }
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
-    if (ssl->hostname != NULL) {
+    mbedtls_ssl_free_hostname(ssl);
-        mbedtls_zeroize_and_free(ssl->hostname, strlen(ssl->hostname));
-    }
 #endif
 
 #if defined(MBEDTLS_SSL_DTLS_HELLO_VERIFY) && defined(MBEDTLS_SSL_SRV_C)
@@ -8845,6 +8870,21 @@ int mbedtls_ssl_check_cert_usage(const mbedtls_x509_crt *cert,
     return ret;
 }
 
+static int get_hostname_for_verification(mbedtls_ssl_context *ssl,
+                                         const char **hostname)
+{
+    if (!mbedtls_ssl_has_set_hostname_been_called(ssl)) {
+        MBEDTLS_SSL_DEBUG_MSG(1, ("Certificate verification without having set hostname"));
+    }
+
+    *hostname = ssl->hostname;
+    if (*hostname == NULL) {
+        MBEDTLS_SSL_DEBUG_MSG(2, ("Certificate verification without CN verification"));
+    }
+
+    return 0;
+}
+
 int mbedtls_ssl_verify_certificate(mbedtls_ssl_context *ssl,
                                    int authmode,
                                    mbedtls_x509_crt *chain,
@@ -8870,7 +8910,13 @@ int mbedtls_ssl_verify_certificate(mbedtls_ssl_context *ssl,
         p_vrfy = ssl->conf->p_vrfy;
     }
 
-    int ret = 0;
+    const char *hostname = "";
+    int ret = get_hostname_for_verification(ssl, &hostname);
+    if (ret != 0) {
+        MBEDTLS_SSL_DEBUG_RET(1, "get_hostname_for_verification", ret);
+        return ret;
+    }
+
     int have_ca_chain_or_callback = 0;
 #if defined(MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK)
     if (ssl->conf->f_ca_cb != NULL) {
@@ -8883,7 +8929,7 @@ int mbedtls_ssl_verify_certificate(mbedtls_ssl_context *ssl,
             ssl->conf->f_ca_cb,
             ssl->conf->p_ca_cb,
             ssl->conf->cert_profile,
-            ssl->hostname,
+            hostname,
             &ssl->session_negotiate->verify_result,
             f_vrfy, p_vrfy);
     } else
@@ -8910,7 +8956,7 @@ int mbedtls_ssl_verify_certificate(mbedtls_ssl_context *ssl,
             chain,
             ca_chain, ca_crl,
             ssl->conf->cert_profile,
-            ssl->hostname,
+            hostname,
             &ssl->session_negotiate->verify_result,
             f_vrfy, p_vrfy, rs_ctx);
     }