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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.

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