F.40. sepgsql

F.40. sepgsql
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F.40.1. Overview
F.40.2. Installation
F.40.3. Regression Tests
F.40.4. GUC Parameters
F.40.5. Features
F.40.6. Sepgsql Functions
F.40.7. Limitations
F.40.8. External Resources
F.40.9. Author

+ 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.40.7. +

F.40.1. 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.40.2. 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 2.1.10 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.40.3. 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.40.2. + 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. + (This Makefile is usually supplied by the + selinux-policy-devel or + selinux-policy RPM.) + 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 1.07
+

+ Third, turn on sepgsql_regression_test_mode. + For security reasons, the rules in sepgsql-regtest + are not enabled by default; + 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.40.8 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.40.4. 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.40.5. Features

F.40.5.1. 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 unlabeled 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, + the label assigned to the new object's parent object and optionally name + of the constructed 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.40.5.2. DML Permissions

+ For tables, db_table:select, db_table:insert, + db_table:update or db_table:delete are + 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 columns referenced in the + WHERE or RETURNING clause, as a data source + for UPDATE, and so on. +

+ Column-level permissions will also be checked for each referenced column. + db_column:select is checked on not only the columns being + read using SELECT, but those being referenced in other DML + statements; db_column:update or db_column:insert + will also be checked for columns being modified by UPDATE or + INSERT. +

+ For example, consider: +

+UPDATE t1 SET x = 2, y = func1(y) WHERE z = 100;
+

+ + Here, db_column:update will be checked for + t1.x, since it is being updated, + db_column:{select update} will be checked for + t1.y, since it is both updated and referenced, and + db_column:select will be checked for t1.z, since + it is only referenced. + 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} will be checked when + user tries to execute a function as a part of query, or using fast-path + invocation. If this function is a trusted procedure, it also checks + db_procedure:{entrypoint} permission to check whether it + can perform as entry point of trusted procedure. +

+ In order to access any schema object, db_schema:search + permission is required on the containing schema. When an object is + referenced without schema qualification, schemas on which this + permission is not present will not be searched (just as if the user did + not have USAGE privilege on the schema). If an explicit schema + qualification is present, an error will occur if the user does not have + the requisite permission on the named schema. +

+ 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.40.5.3. DDL Permissions

+ SELinux defines several permissions to control common + operations for each object type; such as creation, alter, drop and + relabel of security label. In addition, several object types have + special permissions to control their characteristic operations; such as + addition or deletion of name entries within a particular schema. +

+ Creating a new database object requires create permission. + SELinux will grant or deny this permission based on the + client's security label and the proposed security label for the new + object. In some cases, additional privileges are required: +

+ CREATE DATABASE additionally requires + getattr permission for the source or template database. +
+ Creating a schema object additionally requires add_name + permission on the parent schema. +
+ Creating a table additionally requires permission to create each + individual table column, just as if each table column were a + separate top-level object. +
+ Creating a function marked as LEAKPROOF additionally + requires install permission. (This permission is also + checked when LEAKPROOF is set for an existing function.) +

+ When DROP command is executed, drop will be + checked on the object being removed. Permissions will be also checked for + objects dropped indirectly via CASCADE. Deletion of objects + contained within a particular schema (tables, views, sequences and + procedures) additionally requires remove_name on the schema. +

+ When ALTER command is executed, setattr will be + checked on the object being modified for each object types, except for + subsidiary objects such as the indexes or triggers of a table, where + permissions are instead checked on the parent object. In some cases, + additional permissions are required: +

+ Moving an object to a new schema additionally requires + remove_name permission on the old schema and + add_name permission on the new one. +
+ Setting the LEAKPROOF attribute on a function requires + install permission. +
+ Using SECURITY LABEL on an object additionally + requires relabelfrom permission for the object in + conjunction with its old security label and relabelto + permission for the object in conjunction with its new security label. + (In cases 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.40.5.4. Trusted Procedures

+ Trusted procedures are similar to security definer functions or setuid + 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 the user + to print the credit card numbers of customers with some of the digits + masked out. +

F.40.5.5. Dynamic Domain Transitions

+ It is possible to use SELinux's dynamic domain transition feature + to switch the security label of the client process, the client domain, + to a new context, if that is allowed by the security policy. + The client domain needs the setcurrent permission and also + dyntransition from the old to the new domain. +

+ Dynamic domain transitions should be considered carefully, because they + allow users to switch their label, and therefore their privileges, + at their option, rather than (as in the case of a trusted procedure) + as mandated by the system. + Thus, the dyntransition permission is only considered + safe when used to switch to a domain with a smaller set of privileges than + the original one. For example: +

+regression=# select sepgsql_getcon();
+                    sepgsql_getcon
+-------------------------------------------------------
+ unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
+(1 row)
+
+regression=# SELECT sepgsql_setcon('unconfined_u:unconfined_r:unconfined_t:s0-s0:c1.c4');
+ sepgsql_setcon
+----------------
+ t
+(1 row)
+
+regression=# SELECT sepgsql_setcon('unconfined_u:unconfined_r:unconfined_t:s0-s0:c1.c1023');
+ERROR:  SELinux: security policy violation
+

+ In this example above we were allowed to switch from the larger MCS + range c1.c1023 to the smaller range c1.c4, but + switching back was denied. +

+ A combination of dynamic domain transition and trusted procedure + enables an interesting use case that fits the typical process life-cycle + of connection pooling software. + Even if your connection pooling software is not allowed to run most + of SQL commands, you can allow it to switch the security label + of the client using the sepgsql_setcon() function + from within a trusted procedure; that should take some + credential to authorize the request to switch the client label. + After that, this session will have the privileges of the target user, + rather than the connection pooler. + The connection pooler can later revert the security label change by + again using sepgsql_setcon() with + NULL argument, again invoked from within a trusted + procedure with appropriate permissions checks. + The point here is that only the trusted procedure actually has permission + to change the effective security label, and only does so when given proper + credentials. Of course, for secure operation, the credential store + (table, procedure definition, or whatever) must be protected from + unauthorized access. +

F.40.5.6. Miscellaneous

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

F.40.6. Sepgsql Functions

+ Table F.29 shows the available functions. +

Table F.29. Sepgsql Functions

+ Function + + + Description +
+ `sepgsql_getcon` () + → `text` + + + Returns the client domain, the current security label of the client. +
+ `sepgsql_setcon` ( `text` ) + → `boolean` + + + Switches the client domain of the current session to the new domain, + if allowed by the security policy. + It also accepts `NULL` input as a request to transition + to the client's original domain. +
+ `sepgsql_mcstrans_in` ( `text` ) + → `text` + + + Translates the given qualified MLS/MCS range into raw format if + the mcstrans daemon is running. +
+ `sepgsql_mcstrans_out` ( `text` ) + → `text` + + + Translates the given raw MLS/MCS range into qualified format if + the mcstrans daemon is running. +
+ `sepgsql_restorecon` ( `text` ) + → `boolean` + + + Sets up initial security labels for all objects within the + current database. The argument may be `NULL`, or the + name of a specfile to be used as alternative of the system default. +

+ Function +

+ Description +

+ sepgsql_getcon () + → text +

+ Returns the client domain, the current security label of the client. +

+ sepgsql_setcon ( text ) + → boolean +

+ Switches the client domain of the current session to the new domain, + if allowed by the security policy. + It also accepts NULL input as a request to transition + to the client's original domain. +

+ sepgsql_mcstrans_in ( text ) + → text +

+ Translates the given qualified MLS/MCS range into raw format if + the mcstrans daemon is running. +

+ sepgsql_mcstrans_out ( text ) + → text +

+ Translates the given raw MLS/MCS range into qualified format if + the mcstrans daemon is running. +

+ sepgsql_restorecon ( text ) + → boolean +

+ Sets up initial security labels for all objects within the + current database. The argument may be NULL, or the + name of a specfile to be used as alternative of the system default. +

F.40.7. Limitations

Data Definition Language (DDL) Permissions: + Due to implementation restrictions, some DDL operations do not + check permissions. +
Data Control Language (DCL) Permissions: + Due to implementation restrictions, DCL operations do not check + permissions. +
Row-level access control: + PostgreSQL supports row-level access, but + sepgsql does not. +
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.40.8. External Resources

SE-PostgreSQL Introduction: + This wiki page provides a brief overview, security design, architecture, + administration and upcoming features. +
SELinux User's and Administrator's Guide: + This document provides a wide spectrum of knowledge to administer + SELinux on your systems. + It focuses primarily on Red Hat operating systems, but is not limited to them. +
Fedora SELinux FAQ: + This document answers frequently asked questions about + SELinux. + It focuses primarily on Fedora, but is not limited to Fedora. +

F.40.9. Author

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

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