TiDB 赋权问题

作者：jiyf​

复现问题

1. 创建了一个用户 abc
2. use information_schema
3. 给新建的 abc 添加 insert 权限失败，提示权限验证失败

mysql> select @@version;
+---------------------+
| @@version           |
+---------------------+
| 5.7.25-TiDB-v4.0.11 |
+---------------------+
1 row in set (0.00 sec)

mysql> create user 'abc'@'%';
Query OK, 0 rows affected (0.01 sec)

mysql> use information_schema
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> grant insert on *.* to 'abc'@'%';
ERROR 8121 (HY000): privilege check fail

正常情况下这里不应该报错的，以下的操作就没有问题，这也是期望的结果：

Type 'help;' or '\"h' for help. Type '\"c' to clear the current input statement.

mysql> grant insert on *.* to 'abc'@'%';
Query OK, 0 rows affected (0.01 sec)

mysql> use test
Database changed
mysql> grant insert on *.* to 'abc'@'%';
Query OK, 0 rows affected (0.01 sec)

只要在 information_schema，就会报错：

mysql> use information_schema
Database changed
mysql> grant insert on *.* to 'abc'@'%';
ERROR 8121 (HY000): privilege check fail

排查问题

怀疑是下面的原因导致的问题：

1. use information_schema 使得当前会话的当前 DB 为 information_schema
2. information_schema 非用户表，所以不允许 update 权限
3. 在权限检查时候，指定的赋权范围是 *.*，但是权限检查却错误的检查了当前 DB information_schema

顺着这个思路查看 TiDB 源码，翻到下面一段：

func (p *UserPrivileges) RequestVerification(activeRoles []*auth.RoleIdentity, db, table, column string, priv mysql.PrivilegeType) bool {
    ...
    // Skip check for system databases.
    // See https://dev.mysql.com/doc/refman/5.7/en/information-schema.html
    dbLowerName := strings.ToLower(db)
    switch dbLowerName {
    // 如果 db 是 InformationSchema，且权限检查是 mysql.UpdatePri，那这里就返回验证权限失败
    case util.InformationSchemaName.L:
        switch priv {
        case mysql.CreatePriv, mysql.AlterPriv, mysql.DropPriv, mysql.IndexPriv, mysql.CreateViewPriv,
            mysql.InsertPriv, mysql.UpdatePriv, mysql.DeletePriv:
            return false
        }
        return true
    // We should be very careful of limiting privileges, so ignore `mysql` for now.
    case util.PerformanceSchemaName.L, util.MetricSchemaName.L:
        if (dbLowerName == util.PerformanceSchemaName.L && perfschema.IsPredefinedTable(table)) ||
            (dbLowerName == util.MetricSchemaName.L && infoschema.IsMetricTable(table)) {
            switch priv {
            case mysql.CreatePriv, mysql.AlterPriv, mysql.DropPriv, mysql.IndexPriv, mysql.InsertPriv, mysql.UpdatePriv, mysql.DeletePriv:
                return false
            case mysql.SelectPriv:
                return true
            }
        }
    }

    ...
}

这里对系统表 InformationSchema、PerformanceSchema 进行了权限限制。从代码上看，如果验证用户对 information_schema 执行 update 权限，那返回不通过。

mysql> grant insert on information_schema.* to 'abc'@'%';
ERROR 8121 (HY000): privilege check fail

这里尝试给用户在 information_schema 库上添加 insert 权限，返回权限不通过，符合预期。

所以接下来排查，在 use information_schema 情况下，哪里错把 information_schema 作为了 *.* 的验证参数。

打开 planbuilder.go，看下 grant 操作需要对哪些权限进行验证：

func (b *PlanBuilder) buildSimple(node ast.StmtNode) (Plan, error) {
    p := &Simple{Statement: node}
    switch raw := node.(type) {
    ...
    case *ast.GrantStmt:
        if b.ctx.GetSessionVars().CurrentDB == "" && raw.Level.DBName == "" {
            if raw.Level.Level == ast.GrantLevelTable {
                return nil, ErrNoDB
            }
        }
        b.visitInfo = collectVisitInfoFromGrantStmt(b.ctx, b.visitInfo, raw)
    ...
}

func collectVisitInfoFromGrantStmt(sctx sessionctx.Context, vi []visitInfo, stmt *ast.GrantStmt) []visitInfo {
    // To use GRANT, you must have the GRANT OPTION privilege,
    // and you must have the privileges that you are granting.
    dbName := stmt.Level.DBName
    tableName := stmt.Level.TableName
    // 这里就是出现问题的原因
    if dbName == "" {
        dbName = sctx.GetSessionVars().CurrentDB
    }
    vi = appendVisitInfo(vi, mysql.GrantPriv, dbName, tableName, "", nil)
    ...
}

从代码上看，如果 stmt.Level.DBName 是空，那么就赋值 CurrentDB。

所以如果对 *.* 处理后，认为 stmt.Level.DBName 是空，那么就赋值 CurrentDB（information_schema ），后面自然就出现验证不通过的情景。

在这里添加一条日志：

    case *ast.GrantStmt:
        if b.ctx.GetSessionVars().CurrentDB == "" && raw.Level.DBName == "" { 
            if raw.Level.Level == ast.GrantLevelTable {
                return nil, ErrNoDB
            }    
        }    
        logutil.BgLogger().Error("hello", zap.String("CurrentDB", b.ctx.GetSessionVars().CurrentDB), zap.String("rawDB", raw.Level.DBName), zap.String("rawTable", raw.Level.TableName))
        b.visitInfo = collectVisitInfoFromGrantStmt(b.ctx, b.visitInfo, raw)

启动后执行以下 mysql 操作：

mysql> use information_schema
Database changed
mysql> grant insert on *.* to 'abc'@'%';
ERROR 8121 (HY000): privilege check fail

查看日志看到下面的记录：

[2021/06/24 14:09:27.698 +08:00] [ERROR] [planbuilder.go:2019] [hello] [CurrentDB=information_schema] [rawDB=] [rawTable=]

这里果然对于 *.* 把 raw.Level.DBName、raw.Level.TableName，都作为空置对待，然后出现 dbName = sctx.GetSessionVars().CurrentDB，导致验证失败。

再添加一条日志：

        dbLowerName := strings.ToLower(db)
        logutil.BgLogger().Error("hello world", zap.String("db", dbLowerName), zap.String("table", table), zap.String("user", p.user), zap.String("host", p.host), zap.Uint64("priv", uint64(priv)))
        switch dbLowerName {
        case util.InformationSchemaName.L:
            switch priv {
            case mysql.CreatePriv, mysql.AlterPriv, mysql.DropPriv, mysql.IndexPriv, mysql.CreateViewPriv,
                mysql.InsertPriv, mysql.UpdatePriv, mysql.DeletePriv:
                return false
            }

看到以下日志：

[2021/06/24 14:05:03.419 +08:00] [ERROR] [privileges.go:60] [hello world] [db=information_schema] [table=] [user=root] [host=%] [priv=4096]
[2021/06/24 14:05:03.419 +08:00] [ERROR] [privileges.go:60] [hello world] [db=information_schema] [table=] [user=root] [host=%] [priv=8]

其中 priv=4096 是 GrantPriv，priv=8 就是 InsertPriv，到这里就验证不通过，添加权限失败。

安全权限问题

给用户赋权需要以下条件：

1. 进行赋权的用户带有 grant 权限
2. 只能对自己拥有的权限赋给其他用户，比如自己只有 select 权限，那自然没有权利去给其他用户添加 insert 权限。

进行 grant 操作时候，权限检查实际就是检查赋权人是否满足以上条件。

结合以上几个方面，可以实现一个带有 grant 权限的非超级用户达到扩展自己权限的目的。

扩大权限范围

扩大权限方法如下：

1. grant priv on *.* to ‘user’@‘host’，参考日志，*.* 会使得 [rawDB=] [rawTable=]
2. 如果当前 CurrentDB 不为空，那么验证权限的 db 就被设置为 CurrentDB
3. 然后会对当前用户对 CurrentDB 进行 priv 权限验证
4. 而当前用户拥有对 CurrentDB 的 priv 权限
5. 那么权限验证成功
6. 给 ‘user’@‘host’ 添加权限成功，而这些 priv 权限是对所有库表，超过 CurrentDB。

创建用户 ‘abc’@’%’，给 Select,Insert,Update,Delete,GRANT 权限：

mysql> create user 'abc'@'%' identified by '123';
Query OK, 0 rows affected (0.02 sec)

mysql> grant insert,update,delete,select on test.* to 'abc'@'%' with grant option;
Query OK, 0 rows affected (0.01 sec)

mysql> show grants for 'abc'@'%';
+----------------------------------------------------------------------------+
| Grants for abc@%                                                           |
+----------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%'                                            |
| GRANT Select,Insert,Update,Delete ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+----------------------------------------------------------------------------+
2 rows in set (0.00 sec)

使用 ‘abc’@’%’ 登录数据库，可以看到它拥有的权限:

mysql> show grants;
+----------------------------------------------------------------------------+
| Grants for User                                                            |
+----------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%'                                            |
| GRANT Select,Insert,Update,Delete ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+----------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| test               |
+--------------------+
2 rows in set (0.00 sec)

对它的权限进行扩充是被禁止的，这符合预期：

mysql> GRANT Select,Insert,Update,Delete ON *.* TO 'abc'@'%';     
ERROR 8121 (HY000): privilege check fail
mysql> show grants;
+----------------------------------------------------------------------------+
| Grants for User                                                            |
+----------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%'                                            |
| GRANT Select,Insert,Update,Delete ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+----------------------------------------------------------------------------+
2 rows in set (0.00 sec)

执行以下方法就可以顺利扩充它的权限：

mysql> use test
Database changed
mysql> GRANT Select,Insert,Update,Delete ON *.* TO 'abc'@'%';
Query OK, 0 rows affected (0.02 sec)

mysql> show grants;
+----------------------------------------------------------------------------+
| Grants for User                                                            |
+----------------------------------------------------------------------------+
| GRANT Select,Insert,Update,Delete ON *.* TO 'abc'@'%'                      |
| GRANT Select,Insert,Update,Delete ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+----------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| METRICS_SCHEMA     |
| PERFORMANCE_SCHEMA |
| mysql              |
| sbtest             
| test               |
| tpcc               |
+--------------------+
10 rows in set (0.00 sec)

可以看到 ’abc’@’%’ 拥有了全部库表 Select,Insert,Update,Delete 权限，实现权限扩充。

新增权限种类

再回头看下这段代码：

dbLowerName := strings.ToLower(db)
switch dbLowerName {
case util.InformationSchemaName.L:
  switch priv {
  case mysql.CreatePriv, mysql.AlterPriv, mysql.DropPriv, mysql.IndexPriv, mysql.CreateViewPriv,
    mysql.InsertPriv, mysql.UpdatePriv, mysql.DeletePriv:
    return false
  }
  return true

当检查 InformationSchema 库的 ​​mysql.CreatePriv, mysql.AlterPriv, mysql.DropPriv, mysql.IndexPriv, mysql.CreateViewPriv,mysql.InsertPriv, mysql.UpdatePriv, mysql.DeletePriv​​ 这些权限时候，返回失败；而其他权限直接返回成功，比如 Super、Select、Create User 等等。

通过以下方法实现用户授权自己没有的 Super、Select、Create User 等权限：

1. grant Super、Select、Create User on *.* to ‘user’@‘host’，参考日志，*.* 会使得 [rawDB=] [rawTable=]
2. 如果当前 CurrentDB 不为空，那么验证权限的 db 就被设置为 CurrentDB
3. 然后会对当前用户对 CurrentDB 进行 priv 权限验证
4. 而 CurrentDB 为 InformationSchema，priv 为 Super、Select、Create User，上面验证直接返回通过
5. 那么权限验证成功
6. 给 ‘user’@‘host’ 添加 Super、Select、Create User 权限成功，而这些 priv 权限赋权用户本来是没有的，它实现了对未拥有权限进行授权。

创建用户 ‘abc’@’%’，给 Select，GRANT 权限：

mysql> create user 'abc'@'%' identified by '123';
Query OK, 0 rows affected (0.01 sec)

mysql> grant select on test.* to 'abc'@'%' with grant option;                     
Query OK, 0 rows affected (0.01 sec)

mysql>  show grants for 'abc'@'%';
+-------------------------------------------------------+
| Grants for abc@%                                      |
+-------------------------------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%'                       |
| GRANT Select ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+-------------------------------------------------------+
2 rows in set (0.00 sec)

使用 ‘abc’@’%’ 登录数据库，可以看到它拥有的权限:

mysql> show grants;
+-------------------------------------------------------+
| Grants for User                                       |
+-------------------------------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%'                       |
| GRANT Select ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+-------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| INFORMATION_SCHEMA |
| test               |
+--------------------+
2 rows in set (0.00 sec)

尝试创建新用户，但是被禁止，因为 Create User 权限，所以失败符合预期；给自己添加 Create User 权限失败也符合预期：

mysql> create user 'efg'@'%';
ERROR 1227 (42000): Access denied; you need (at least one of) the CREATE User privilege(s) for this operation
mysql> grant CREATE User on test.* to 'abc'@'%'; 
ERROR 8121 (HY000): privilege check fail

执行以下方法就可以顺利添加 Create User，并能创建新用户：

mysql> use information_schema
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

Database changed
mysql> grant CREATE User on *.* to 'abc'@'%';
Query OK, 0 rows affected (0.01 sec)

mysql> show grants;
+-------------------------------------------------------+
| Grants for User                                       |
+-------------------------------------------------------+
| GRANT Create User ON *.* TO 'abc'@'%'                 |
| GRANT Select ON test.* TO 'abc'@'%' WITH GRANT OPTION |
+-------------------------------------------------------+
2 rows in set (0.00 sec)
mysql> create user 'efg'@'%';
Query OK, 0 rows affected (0.01 sec)

问题原因

出现问题的根本原因应该还是在这一步：

func collectVisitInfoFromGrantStmt(sctx sessionctx.Context, vi []visitInfo, stmt *ast.GrantStmt) []visitInfo {
  // To use GRANT, you must have the GRANT OPTION privilege,
  // and you must have the privileges that you are granting.
  dbName := stmt.Level.DBName
  tableName := stmt.Level.TableName
  // 这里就是出现问题的原因
  if dbName == "" {
    dbName = sctx.GetSessionVars().CurrentDB
  }
  vi = appendVisitInfo(vi, mysql.GrantPriv, dbName, tableName, "", nil)
  ...
}

TiDB 授权分为三种类型：

类型	说明
GrantLevelGlobal	*.*
grantDBLevel	test.*
GrantLevelTable	test.t1

对于 GrantLevelGlobal 形式，dbName 为空，就会出现前面的问题。所以这里就要加判断，对于 GrantLevelGlobal 情形保持 dbName 为空即可。

但是这还不能解决所有问题，以 MySQL 5.7 做对比，执行以下语句：

Server version: 5.7.36-log MySQL Community Server (GPL)

Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\"h' for help. Type '\"c' to clear the current input statement.

mysql> grant select on * to 'bcd'@'%';
ERROR 1046 (3D000): No database select

在 tidb 上，用户却赋权成功，不过 GRANT Select ON .* TO ‘abc’@’%’ 这是一个无效的权限。

Type 'help;' or '\"h' for help. Type '\"c' to clear the current input statement.

mysql> grant select on * to 'abc'@'%';   
Query OK, 0 rows affected (0.01 sec)

mysql> show grants for 'abc'@'%';
+---------------------------------+
| Grants for abc@%                |
+---------------------------------+
| GRANT USAGE ON *.* TO 'abc'@'%' |
| GRANT Select ON .* TO 'abc'@'%' |
+---------------------------------+
2 rows in set (0.00 sec)

mysql> select @@version;
+---------------------+
| @@version           |
+---------------------+
| 5.7.25-TiDB-v4.0.11 |
+---------------------+
1 row in set (0.00 sec)

TIDB 行为在于对 grant select on * to ‘abc’@’%’ 语句，解析为类型 grantDBLevel，而 [rawDB=] [rawTable=]，* 被改写为 CurrentDB.* ，所以 CurrentDB 为空时候出现上面 GRANT Select ON .* TO ‘abc’@’%’ 权限情况：

case *ast.GrantStmt:
  if b.ctx.GetSessionVars().CurrentDB == "" && raw.Level.DBName == "" {
    // if raw.Level.Level == ast.GrantLevelTable {
    if raw.Level.Level == ast.GrantLevelTable || raw.Level.Level == ast.GrantLevelDB {
      return nil, ErrNoDB
    }
  }
  b.visitInfo = collectVisitInfoFromGrantStmt(b.ctx, b.visitInfo, raw)

在这里添加检查 GrantLevelDB 情况，直接返回报错，跟 MySQL 行为保持一致。

总结

1. 修改buildSimple 函数，在 GrantLevelDB 情况下，db 为空时候保持与 MySQL 一致直接报错。
   func (b *PlanBuilder) buildSimple(node ast.StmtNode) (Plan, error) { p := &Simple{Statement: node} switch raw := node.(type) { … case *ast.GrantStmt: if b.ctx.GetSessionVars().CurrentDB == “” && raw.Level.DBName == “” { // if raw.Level.Level == ast.GrantLevelTable { if raw.Level.Level == ast.GrantLevelTable || raw.Level.Level == ast.GrantLevelDB { return nil, ErrNoDB } } b.visitInfo = collectVisitInfoFromGrantStmt(b.ctx, b.visitInfo, raw) …}
2. 修改 collectVisitInfoFromGrantStmt 函数，在 GrantLevelGlobal 情况下，不设置 dbName = sctx.GetSessionVars().CurrentDB。

func collectVisitInfoFromGrantStmt(sctx sessionctx.Context, vi []visitInfo, stmt *ast.GrantStmt) []visitInfo {  // To use GRANT, you must have the GRANT OPTION privilege,  // and you must have the privileges that you are granting.  dbName := stmt.Level.DBName tableName := stmt.Level.TableName if dbName == "" && (stmt.Level.Level == ast.GrantLevelTable || stmt.Level.Level == ast.GrantLevelDB){   dbName = sctx.GetSessionVars().CurrentDB  } vi = appendVisitInfo(vi, mysql.GrantPriv, dbName, tableName, "", nil) ...}

其他

revoke 可能存在类型问题。