sepgsql

PostgreSQL 9.1.13 Documentation
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# F.38. sepgsql

`sepgsql` is a loadable module that supports label-based mandatory access control (MAC) based on SELinux security policy.

Warning
The current implementation has significant limitations, and does not enforce mandatory access control for all actions. See Section F.38.6.

## F.38.1. Overview

This module integrates with SELinux to provide an additional layer of security checking above and beyond what is normally provided by PostgreSQL. From the perspective of SELinux, this module allows PostgreSQL to function as a user-space object manager. Each table or function access initiated by a DML query will be checked against the system security policy. This check is in addition to the usual SQL permissions checking performed by PostgreSQL.

SELinux access control decisions are made using security labels, which are represented by strings such as `system_u:object_r:sepgsql_table_t:s0`. Each access control decision involves two labels: the label of the subject attempting to perform the action, and the label of the object on which the operation is to be performed. Since these labels can be applied to any sort of object, access control decisions for objects stored within the database can be (and, with this module, are) subjected to the same general criteria used for objects of any other type, such as files. This design is intended to allow a centralized security policy to protect information assets independent of the particulars of how those assets are stored.

The SECURITY LABEL statement allows assignment of a security label to a database object.

## F.38.2. Installation

`sepgsql` can only be used on Linux 2.6.28 or higher with SELinux enabled. It is not available on any other platform. You will also need libselinux 2.0.93 or higher and selinux-policy 3.9.13 or higher (although some distributions may backport the necessary rules into older policy versions).

The `sestatus` command allows you to check the status of SELinux. A typical display is:

    $ sestatus
    SELinux status:                 enabled
    SELinuxfs mount:                /selinux
    Current mode:                   enforcing
    Mode from config file:          enforcing
    Policy version:                 24
    Policy from config file:        targeted

If SELinux is disabled or not installed, you must set that product up first before installing this module.

To build this module, include the option `--with-selinux` in your PostgreSQL `configure` command. Be sure that the `libselinux-devel` RPM is installed at build time.

To use this module, you must include `sepgsql` in the shared_preload_libraries parameter in `postgresql.conf`. The module will not function correctly if loaded in any other manner. Once the module is loaded, you should execute `sepgsql.sql` in each database. This will install functions needed for security label management, and assign initial security labels.

Here is an example showing how to initialize a fresh database cluster with `sepgsql` functions and security labels installed. Adjust the paths shown as appropriate for your installation:

    $ export PGDATA=/path/to/data/directory
    $ initdb
    $ vi $PGDATA/postgresql.conf
      change
        #shared_preload_libraries = ''                # (change requires restart)
      to
        shared_preload_libraries = 'sepgsql'          # (change requires restart)
    $ for DBNAME in template0 template1 postgres; do
        postgres --single -F -c exit_on_error=true $DBNAME \
          </usr/local/pgsql/share/contrib/sepgsql.sql >/dev/null
      done

Please note that you may see some or all of the following notifications depending on the particular versions you have of libselinux and selinux-policy:

    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 33 has invalid object type db_blobs
    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 36 has invalid object type db_language
    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 37 has invalid object type db_language
    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 38 has invalid object type db_language
    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 39 has invalid object type db_language
    /etc/selinux/targeted/contexts/sepgsql_contexts:  line 40 has invalid object type db_language

These messages are harmless and should be ignored.

If the installation process completes without error, you can now start the server normally.

## F.38.3. Regression Tests

Due to the nature of SELinux, running the regression tests for `sepgsql` requires several extra configuration steps, some of which must be done as root. The regression tests will not be run by an ordinary `make check` or `make installcheck` command; you must set up the configuration and then invoke the test script manually. The tests must be run in the `contrib/sepgsql` directory of a configured PostgreSQL build tree. Although they require a build tree, the tests are designed to be executed against an installed server, that is they are comparable to `make installcheck` not `make check`.

First, set up `sepgsql` in a working database according to the instructions in Section F.38.2. Note that the current operating system user must be able to connect to the database as superuser without password authentication.

Second, build and install the policy package for the regression test. The `sepgsql-regtest` policy is a special purpose policy package which provides a set of rules to be allowed during the regression tests. It should be built from the policy source file `sepgsql-regtest.te`, which is done using `make` with a Makefile supplied by SELinux. You will need to locate the appropriate Makefile on your system; the path shown below is only an example. Once built, install this policy package using the `semodule` command, which loads supplied policy packages into the kernel. If the package is correctly installed, ``semodule` -l` should list sepgsql-regtest as an available policy package:

    $ cd .../contrib/sepgsql
    $ make -f /usr/share/selinux/devel/Makefile
    $ sudo semodule -u sepgsql-regtest.pp
    $ sudo semodule -l | grep sepgsql
    sepgsql-regtest 1.03

Third, turn on `sepgsql_regression_test_mode`. We don't enable all the rules in `sepgsql-regtest` by default, for your system's safety. The `sepgsql_regression_test_mode` parameter enables the rules needed to launch the regression tests. It can be turned on using the `setsebool` command:

    $ sudo setsebool sepgsql_regression_test_mode on
    $ getsebool sepgsql_regression_test_mode
    sepgsql_regression_test_mode --> on

Fourth, verify your shell is operating in the `unconfined_t` domain:

    $ id -Z
    unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

See Section F.38.7 for details on adjusting your working domain, if necessary.

Finally, run the regression test script:

    $ ./test_sepgsql

This script will attempt to verify that you have done all the configuration steps correctly, and then it will run the regression tests for the `sepgsql` module.

After completing the tests, it's recommended you disable the `sepgsql_regression_test_mode` parameter:

    $ sudo setsebool sepgsql_regression_test_mode off

You might prefer to remove the `sepgsql-regtest` policy entirely:

    $ sudo semodule -r sepgsql-regtest

## F.38.4. GUC Parameters

`sepgsql.permissive` (`boolean`)

  This parameter enables `sepgsql` to function in permissive mode, regardless of the system setting. The default is off. This parameter can only be set in the `postgresql.conf` file or on the server command line.

  When this parameter is on, `sepgsql` functions in permissive mode, even if SELinux in general is working in enforcing mode. This parameter is primarily useful for testing purposes.

`sepgsql.debug_audit` (`boolean`)

  This parameter enables the printing of audit messages regardless of the system policy settings. The default is off, which means that messages will be printed according to the system settings.

  The security policy of SELinux also has rules to control whether or not particular accesses are logged. By default, access violations are logged, but allowed accesses are not.

  This parameter forces all possible logging to be turned on, regardless of the system policy.

## F.38.5. Features

### F.38.5.1. Controlled Object Classes

The security model of SELinux describes all the access control rules as relationships between a subject entity (typically, a client of the database) and an object entity (such as a database object), each of which is identified by a security label. If access to an unlabelled object is attempted, the object is treated as if it were assigned the label `unlabeled_t`.

Currently, `sepgsql` allows security labels to be assigned to schemas, tables, columns, sequences, views, and functions. When `sepgsql` is in use, security labels are automatically assigned to supported database objects at creation time. This label is called a default security label, and is decided according to the system security policy, which takes as input the creator's label and the label assigned to the new object's parent object.

A new database object basically inherits the security label of the parent object, except when the security policy has special rules known as type-transition rules, in which case a different label may be applied. For schemas, the parent object is the current database; for tables, sequences, views, and functions, it is the containing schema; for columns, it is the containing table.

### F.38.5.2. DML Permissions

For tables, `db_table:select`, `db_table:insert`, `db_table:update` or `db_table:delete` is checked for all the referenced target tables depending on the kind of statement; in addition, `db_table:select` is also checked for all the tables that contain the columns referenced in the `WHERE` or `RETURNING` clause, as a data source of `UPDATE`, and so on. For example, consider:

    UPDATE t1 SET x = 2, y = md5sum(y) WHERE z = 100;

In this case we must have `db_table:select` in addition to `db_table:update`, because `t1.a` is referenced within the `WHERE` clause. Column-level permissions will also be checked for each referenced column.

For columns, `db_column:select` is checked on not only the columns being read using `SELECT`, but those being referenced in other DML statements. Of course, it also checks `db_column:update` or `db_column:insert` on columns being modified by `UPDATE` or `INSERT`.

    UPDATE t1 SET x = 2, y = md5sum(y) WHERE z = 100;

In this case, it checks `db_column:update` on the column `t1.x` being updated, `db_column:{select update}` on the column `t1.y` being updated and referenced, and `db_column:select` on the column `t1.z`, since that is only referenced in the `WHERE` clause. `db_table:{select update}` will also be checked at the table level.

For sequences, `db_sequence:get_value` is checked when we reference a sequence object using `SELECT`; however, note that we do not currently check permissions on execution of corresponding functions such as `lastval()`.

For views, `db_view:expand` will be checked, then any other required permissions will be checked on the objects being expanded from the view, individually.

For functions, `db_procedure:{execute}` is defined, but is not checked in this version.

The client must be allowed to access all referenced tables and columns, even if they originated from views which were then expanded, so that we apply consistent access control rules independent of the manner in which the table contents are referenced.

The default database privilege system allows database superusers to modify system catalogs using DML commands, and reference or modify toast tables. These operations are prohibited when `sepgsql` is enabled.

### F.38.5.3. DDL Permissions

When SECURITY LABEL is executed, `setattr` and `relabelfrom` will be checked on the object being relabeled with its old security label, then `relabelto` with the supplied new security label.

In the case where multiple label providers are installed and the user tries to set a security label, but it is not managed by SELinux, only `setattr` should be checked here. This is currently not done due to implementation restrictions.

### F.38.5.4. Trusted Procedures

Trusted procedures are similar to security definer functions or set-uid commands. SELinux provides a feature to allow trusted code to run using a security label different from that of the client, generally for the purpose of providing highly controlled access to sensitive data (e.g. rows might be omitted, or the precision of stored values might be reduced). Whether or not a function acts as a trusted procedure is controlled by its security label and the operating system security policy. For example:

    postgres=# CREATE TABLE customer (
                   cid     int primary key,
                   cname   text,
                   credit  text
               );
    CREATE TABLE
    postgres=# SECURITY LABEL ON COLUMN customer.credit
                   IS 'system_u:object_r:sepgsql_secret_table_t:s0';
    SECURITY LABEL
    postgres=# CREATE FUNCTION show_credit(int) RETURNS text
                 AS 'SELECT regexp_replace(credit, ''-[0-9]+$'', ''-xxxx'', ''g'')
                            FROM customer WHERE cid = $1'
               LANGUAGE sql;
    CREATE FUNCTION
    postgres=# SECURITY LABEL ON FUNCTION show_credit(int)
                   IS 'system_u:object_r:sepgsql_trusted_proc_exec_t:s0';
    SECURITY LABEL

The above operations should be performed by an administrative user.

    postgres=# SELECT * FROM customer;
    ERROR:  SELinux: security policy violation
    postgres=# SELECT cid, cname, show_credit(cid) FROM customer;
     cid | cname  |     show_credit
    -----+--------+---------------------
       1 | taro   | 1111-2222-3333-xxxx
       2 | hanako | 5555-6666-7777-xxxx
    (2 rows)

In this case, a regular user cannot reference `customer.credit` directly, but a trusted procedure `show_credit` allows him to print the credit card numbers of customers with some of the digits masked out.

### F.38.5.5. Miscellaneous

We reject the LOAD command across the board, because any module loaded could easily circumvent security policy enforcement.

## F.38.6. Limitations

Data Definition Language (DDL) Permissions

  Due to implementation restrictions, DDL permissions are not checked.

Data Control Language (DCL) Permissions

  Due to implementation restrictions, DCL permissions are not checked.

Row-level access control

  PostgreSQL does not support row-level access; therefore, `sepgsql` does not support it either.

Covert channels

  `sepgsql` does not try to hide the existence of a certain object, even if the user is not allowed to reference it. For example, we can infer the existence of an invisible object as a result of primary key conflicts, foreign key violations, and so on, even if we cannot obtain the contents of the object. The existence of a top secret table cannot be hidden; we only hope to conceal its contents.

## F.38.7. External Resources

SE-PostgreSQL Introduction

  This wiki page provides a brief overview, security design, architecture, administration and upcoming features.

Fedora SELinux User Guide

  This document provides a wide spectrum of knowledge to administer SELinux on your systems. It focuses primarily on Fedora, but is not limited to Fedora.

Fedora SELinux FAQ

  This document answers frequently asked questions about SELinux. It focuses primarily on Fedora, but is not limited to Fedora.

## F.38.8. Author

KaiGai Kohei `<kaigai@ak.jp.nec.com>`

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