tls13: Add function to search for a supported_versions extension

Move in a dedicated function the search for the
supported_versions extension in a list of
extensions, to be able to use it on server side
as well.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

library/ssl_misc.h

@@ -2048,6 +2048,32 @@ int mbedtls_ssl_tls13_fetch_handshake_msg(mbedtls_ssl_context *ssl,
                                           unsigned char **buf,
                                           size_t *buf_len);
 
+/**
+ * \brief Detect if a list of extensions contains a supported_versions
+ *        extension or not.
+ *
+ * \param[in] ssl  SSL context
+ * \param[in] buf  Address of the first byte of the extensions vector.
+ * \param[in] end  End of the buffer containing the list of extensions.
+ * \param[out] extension_data  If the extension is present, address of its first
+ *                             byte of data, NULL otherwise.
+ * \param[out] extension_data_end  If the extension is present, address of the
+ *                                 first byte immediately following the extension
+ *                                 data, NULL otherwise.
+ * \return 0  if the list of extensions does not contain a supported_versions
+ *            extension.
+ * \return 1  if the list of extensions contains a supported_versions
+ *            extension.
+ * \return    A negative value if an error occurred while parsing the
+ *            extensions.
+ */
+MBEDTLS_CHECK_RETURN_CRITICAL
+int mbedtls_ssl_tls13_is_supported_versions_ext_present_in_exts(
+    mbedtls_ssl_context *ssl,
+    const unsigned char *buf, const unsigned char *end,
+    const unsigned char **extension_data,
+    const unsigned char **extension_data_end);
+
 /*
  * Handler of TLS 1.3 server certificate message
  */

library/ssl_tls13_client.c

@@ -1324,8 +1324,8 @@ static int ssl_tls13_is_supported_versions_ext_present(
 {
     const unsigned char *p = buf;
     size_t legacy_session_id_echo_len;
-    size_t extensions_len;
+    const unsigned char *supported_versions_ext;
-    const unsigned char *extensions_end;
+    const unsigned char *supported_versions_ext_end;
 
     /*
      * Check there is enough data to access the legacy_session_id_echo vector
@@ -1347,45 +1347,9 @@ static int ssl_tls13_is_supported_versions_ext_present(
     MBEDTLS_SSL_CHK_BUF_READ_PTR(p, end, legacy_session_id_echo_len + 4);
     p += legacy_session_id_echo_len + 4;
 
-    /* Case of no extension */
+    return mbedtls_ssl_tls13_is_supported_versions_ext_present_in_exts(
-    if (p == end) {
+        ssl, p, end,
-        return 0;
+        &supported_versions_ext, &supported_versions_ext_end);
-    }
-
-    /* ...
-     * Extension extensions<6..2^16-1>;
-     * ...
-     * struct {
-     *      ExtensionType extension_type; (2 bytes)
-     *      opaque extension_data<0..2^16-1>;
-     * } Extension;
-     */
-    MBEDTLS_SSL_CHK_BUF_READ_PTR(p, end, 2);
-    extensions_len = MBEDTLS_GET_UINT16_BE(p, 0);
-    p += 2;
-
-    /* Check extensions do not go beyond the buffer of data. */
-    MBEDTLS_SSL_CHK_BUF_READ_PTR(p, end, extensions_len);
-    extensions_end = p + extensions_len;
-
-    while (p < extensions_end) {
-        unsigned int extension_type;
-        size_t extension_data_len;
-
-        MBEDTLS_SSL_CHK_BUF_READ_PTR(p, extensions_end, 4);
-        extension_type = MBEDTLS_GET_UINT16_BE(p, 0);
-        extension_data_len = MBEDTLS_GET_UINT16_BE(p, 2);
-        p += 4;
-
-        if (extension_type == MBEDTLS_TLS_EXT_SUPPORTED_VERSIONS) {
-            return 1;
-        }
-
-        MBEDTLS_SSL_CHK_BUF_READ_PTR(p, extensions_end, extension_data_len);
-        p += extension_data_len;
-    }
-
-    return 0;
 }
 
 /* Returns a negative value on failure, and otherwise

library/ssl_tls13_generic.c

@@ -86,6 +86,61 @@ cleanup:
     return ret;
 }
 
+int mbedtls_ssl_tls13_is_supported_versions_ext_present_in_exts(
+    mbedtls_ssl_context *ssl,
+    const unsigned char *buf, const unsigned char *end,
+    const unsigned char **extension_data,
+    const unsigned char **extension_data_end)
+{
+    const unsigned char *p = buf;
+    size_t extensions_len;
+    const unsigned char *extensions_end;
+
+    *extension_data = NULL;
+    *extension_data_end = NULL;
+
+    /* Case of no extension */
+    if (p == end) {
+        return 0;
+    }
+
+    /* ...
+     * Extension extensions<x..2^16-1>;
+     * ...
+     * struct {
+     *      ExtensionType extension_type; (2 bytes)
+     *      opaque extension_data<0..2^16-1>;
+     * } Extension;
+     */
+    MBEDTLS_SSL_CHK_BUF_READ_PTR(p, end, 2);
+    extensions_len = MBEDTLS_GET_UINT16_BE(p, 0);
+    p += 2;
+
+    /* Check extensions do not go beyond the buffer of data. */
+    MBEDTLS_SSL_CHK_BUF_READ_PTR(p, end, extensions_len);
+    extensions_end = p + extensions_len;
+
+    while (p < extensions_end) {
+        unsigned int extension_type;
+        size_t extension_data_len;
+
+        MBEDTLS_SSL_CHK_BUF_READ_PTR(p, extensions_end, 4);
+        extension_type = MBEDTLS_GET_UINT16_BE(p, 0);
+        extension_data_len = MBEDTLS_GET_UINT16_BE(p, 2);
+        p += 4;
+        MBEDTLS_SSL_CHK_BUF_READ_PTR(p, extensions_end, extension_data_len);
+
+        if (extension_type == MBEDTLS_TLS_EXT_SUPPORTED_VERSIONS) {
+            *extension_data = p;
+            *extension_data_end = p + extension_data_len;
+            return 1;
+        }
+        p += extension_data_len;
+    }
+
+    return 0;
+}
+
 #if defined(MBEDTLS_SSL_TLS1_3_KEY_EXCHANGE_MODE_EPHEMERAL_ENABLED)
 /*
  * STATE HANDLING: Read CertificateVerify