MDEV-3915 COM_CHANGE_USER allows fast password brute-forcing

  
allow only three failed change_user per connection.
successful change_user do NOT reset the counter

tests/mysql_client_test.c:
  make --error to work for --change_user errors

3 files changed, 18 insertions, 1 deletions

diff --git a/sql/sql_class.cc b/sql/sql_class.cc
index b7a37ae3f69..d44d28eaae2 100644
--- a/sql/sql_class.cc
+++ b/sql/sql_class.cc
@@ -675,6 +675,7 @@ THD::THD()
    stmt_depends_on_first_successful_insert_id_in_prev_stmt(FALSE),
    examined_row_count(0),
    global_read_lock(0),
+   failed_com_change_user(0),
    is_fatal_error(0),
    transaction_rollback_request(0),
    is_fatal_sub_stmt_error(0),
diff --git a/sql/sql_class.h b/sql/sql_class.h
index 6b277add4a5..d55200efea4 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -1865,6 +1865,7 @@ public:
   bool	     no_errors, password;
   bool       extra_port;                        /* If extra connection */
 
+  uint8      failed_com_change_user;
   /**
     Set to TRUE if execution of the current compound statement
     can not continue. In particular, disables activation of
diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index be0e2db43c6..0c47b7a8bb3 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -1144,6 +1144,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
 
     uint save_db_length= thd->db_length;
     char *save_db= thd->db;
+    int rc;
     USER_CONN *save_user_connect= thd->user_connect;
     Security_context save_security_ctx= *thd->security_ctx;
     CHARSET_INFO *save_character_set_client=
@@ -1157,7 +1158,19 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
     thd->security_ctx->user= 0;
     thd->user_connect= 0;
 
-    if (acl_authenticate(thd, 0, packet_length))
+    /*
+      to limit COM_CHANGE_USER ability to brute-force passwords,
+      we only allow three unsuccessful COM_CHANGE_USER per connection.
+    */
+    if (thd->failed_com_change_user >= 3)
+    {
+      my_message(ER_UNKNOWN_COM_ERROR, ER(ER_UNKNOWN_COM_ERROR), MYF(0));
+      rc= 1;
+    }
+    else
+      rc= acl_authenticate(thd, 0, packet_length);
+
+    if (rc)
     {
       /* Free user if allocated by acl_authenticate */
       x_free(thd->security_ctx->user);
@@ -1170,6 +1183,8 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
       thd->variables.collation_connection= save_collation_connection;
       thd->variables.character_set_results= save_character_set_results;
       thd->update_charset();
+      thd->failed_com_change_user++;
+      my_sleep(1000000);
     }
     else
     {