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Daftar IsiAbstract This guide assists users and administrators in managing and using Security-Enhanced Linux (SELinux). The Red Hat Enterprise Linux 6 SELinux User Guide is for people with minimal or no experience with SELinux. Although system administration experience is not necessary, content in this guide is written for system administration tasks. This guide provides an introduction to fundamental concepts and practical applications of SELinux. After reading this guide you should have an intermediate understanding of SELinux. Thank you to everyone who offered encouragement, help, and testing - it is most appreciated. Very special thanks to: Dominick Grift, Stephen Smalley, and Russell Coker for their contributions, help, and patience.
This manual uses several conventions to highlight certain words and phrases and draw attention to specific pieces of information. In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not, alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes the Liberation Fonts set by default. 1.1. Typographic ConventionsFour typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows. Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight keys and key combinations. For example: To see the contents of the file my_next_bestselling_novel in your current working directory, enter the cat my_next_bestselling_novel command at the shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a key, all presented in mono-spaced bold and all distinguishable thanks to context. Key combinations can be distinguished from an individual key by the plus sign that connects each part of a key combination. For example: Press Enter to execute the command. Press Ctrl+Alt+F2 to switch to a virtual terminal.
The first example highlights a particular key to press. The second example highlights a key combination: a set of three keys pressed simultaneously. If source code is discussed, class names, methods, functions, variable names and returned values mentioned within a paragraph will be presented as above, in mono-spaced bold. For example: File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
Proportional Bold This denotes words or phrases encountered on a system, including application names; dialog box text; labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example: Choose ⤍ ⤍ from the main menu bar to launch Mouse Preferences. In the Buttons tab, click the Left-handed mouse check box and click Close to switch the primary mouse button from the left to the right (making the mouse suitable for use in the left hand). To insert a special character into a gedit file, choose ⤍ ⤍ from the main menu bar. Next, choose ⤍ from the Character Map menu bar, type the name of the character in the Search field and click Next. The character you sought will be highlighted in the Character Table. Double-click this highlighted character to place it in the Text to copy field and then click the Copy button. Now switch back to your document and choose ⤍ from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific menu names; and buttons and text found within a GUI interface, all presented in proportional bold and all distinguishable by context. Mono-spaced Bold ItalicProportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example: To connect to a remote machine using ssh, type ssh username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh [email protected]. The mount -o remount file-system command remounts the named file system. For example, to remount the /home file system, the command is mount -o remount /home. To see the version of a currently installed package, use the rpm -q package command. It will return a result as follows: package-version-release
Note the words in bold italics above - username, domain.name, file-system, package, version and release. Each word is a placeholder, either for text you enter when issuing a command or for text displayed by the system. Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and important term. For example: Publican is a DocBook publishing system.
1.2. Pull-quote ConventionsTerminal output and source code listings are set off visually from the surrounding text. Output sent to a terminal is set in mono-spaced roman and presented thus: books Desktop documentation drafts mss photos stuff svnbooks_tests Desktop1 downloads images notes scripts svgs Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows: package org.jboss.book.jca.ex1;import javax.naming.InitialContext;public class ExClient{ public static void main(String args[]) throws Exception { InitialContext iniCtx = new InitialContext(); Object ref = iniCtx.lookup("EchoBean"); EchoHome home = (EchoHome) ref; Echo echo = home.create(); System.out.println("Created Echo"); System.out.println("Echo.echo('Hello') = " + echo.echo("Hello")); }}Finally, we use three visual styles to draw attention to information that might otherwise be overlooked. Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier. Important boxes detail things that are easily missed: configuration changes that only apply to the current session, or services that need restarting before an update will apply. Ignoring a box labeled 'Important' will not cause data loss but may cause irritation and frustration. Warnings should not be ignored. Ignoring warnings will most likely cause data loss. If you find a typographical error in this manual, or if you have thought of a way to make this manual better, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/ against the product Red Hat Enterprise Linux.When submitting a bug report, be sure to mention the manual's identifier: doc-SELinux_User_Guide and version number: 6. If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily. Chapter 1. Trademark InformationLinux is the registered trademark of Linus Torvalds in the U.S. and other countries. UNIX is a registered trademark of The Open Group. Type Enforcement is a trademark of Secure Computing, LLC, a wholly owned subsidiary of McAfee, Inc., registered in the U.S. and in other countries. Neither McAfee nor Secure Computing, LLC, has consented to the use or reference to this trademark by the author outside of this guide. Apache is a trademark of The Apache Software Foundation. MySQL is a trademark or registered trademark of MySQL AB in the U.S. and other countries. Other products mentioned may be trademarks of their respective corporations. Security-Enhanced Linux (SELinux) is an implementation of a mandatory access control mechanism in the Linux kernel, checking for allowed operations after standard discretionary access controls are checked. It was created by the National Security Agency and can enforce rules on files and processes in a Linux system, and on their actions, based on defined policies. When using SELinux, files, including directories and devices, are referred to as objects. Processes, such as a user running a command or the Mozilla Firefox application, are referred to as subjects. Most operating systems use a Discretionary Access Control (DAC) system that controls how subjects interact with objects, and how subjects interact with each other. On operating systems using DAC, users control the permissions of files (objects) that they own. For example, on Linux operating systems, users could make their home directories world-readable, giving users and processes (subjects) access to potentially sensitive information, with no further protection over this unwanted action. Relying on DAC mechanisms alone is fundamentally inadequate for strong system security. DAC access decisions are only based on user identity and ownership, ignoring other security-relevant information such as the role of the user, the function and trustworthiness of the program, and the sensitivity and integrity of the data. Each user typically has complete discretion over their files, making it difficult to enforce a system-wide security policy. Furthermore, every program run by a user inherits all of the permissions granted to the user and is free to change access to the user's files, so minimal protection is provided against malicious software. Many system services and privileged programs run with coarse-grained privileges that far exceed their requirements, so that a flaw in any one of these programs could be exploited to obtain further system access. The following is an example of permissions used on Linux operating systems that do not run Security-Enhanced Linux (SELinux). The permissions and output in these examples may differ slightly from your system. Use the ls -l command to view file permissions: ~]$ ls -l file1-rwxrw-r-- 1 user1 group1 0 2009-08-30 11:03 file1 In this example, the first three permission bits, rwx, control the access the Linux user1 user (in this case, the owner) has to file1. The next three permission bits, rw-, control the access the Linux group1 group has to file1. The last three permission bits, r--, control the access everyone else has to file1, which includes all users and processes. Security-Enhanced Linux (SELinux) adds Mandatory Access Control (MAC) to the Linux kernel, and is enabled by default in Red Hat Enterprise Linux. A general purpose MAC architecture needs the ability to enforce an administratively-set security policy over all processes and files in the system, basing decisions on labels containing a variety of security-relevant information. When properly implemented, it enables a system to adequately defend itself and offers critical support for application security by protecting against the tampering with, and bypassing of, secured applications. MAC provides strong separation of applications that permits the safe execution of untrustworthy applications. Its ability to limit the privileges associated with executing processes limits the scope of potential damage that can result from the exploitation of vulnerabilities in applications and system services. MAC enables information to be protected from legitimate users with limited authorization as well as from authorized users who have unwittingly executed malicious applications. The following is an example of the labels containing security-relevant information that are used on processes, Linux users, and files, on Linux operating systems that run SELinux. This information is called the SELinux context, and is viewed using the ls -Z command: ~]$ ls -Z file1-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1 In this example, SELinux provides a user (unconfined_u), a role (object_r), a type (user_home_t), and a level (s0). This information is used to make access control decisions. With DAC, access is controlled based only on Linux user and group IDs. It is important to remember that SELinux policy rules are checked after DAC rules. SELinux policy rules are not used if DAC rules deny access first. On Linux operating systems that run SELinux, there are Linux users as well as SELinux users. SELinux users are part of SELinux policy. Linux users are mapped to SELinux users. To avoid confusion, this guide uses "Linux user" and "SELinux user" to differentiate between the two. 2.1. Benefits of running SELinuxAll processes and files are labeled with a type. A type defines a domain for processes, and a type for files. Processes are separated from each other by running in their own domains, and SELinux policy rules define how processes interact with files, as well as how processes interact with each other. Access is only allowed if an SELinux policy rule exists that specifically allows it. Fine-grained access control. Stepping beyond traditional UNIX permissions that are controlled at user discretion and based on Linux user and group IDs, SELinux access decisions are based on all available information, such as an SELinux user, role, type, and, optionally, a level. SELinux policy is administratively-defined, enforced system-wide, and is not set at user discretion. Reduced vulnerability to privilege escalation attacks. One example: since processes run in domains, and are therefore separated from each other, and because SELinux policy rules define how processes access files and other processes, if a process is compromised, the attacker only has access to the normal functions of that process, and to files the process has been configured to have access to. For example, if the Apache HTTP Server is compromised, an attacker can not use that process to read files in user home directories, unless a specific SELinux policy rule was added or configured to allow such access. SELinux can be used to enforce data confidentiality and integrity, as well as protecting processes from untrusted inputs.
However, SELinux is not: antivirus software, a replacement for passwords, firewalls, or other security systems, an all-in-one security solution.
SELinux is designed to enhance existing security solutions, not replace them. Even when running SELinux, it is important to continue to follow good security practices, such as keeping software up-to-date, using hard-to-guess passwords, firewalls, and so on. The following examples demonstrate how SELinux increases security: The default action is deny. If an SELinux policy rule does not exist to allow access, such as for a process opening a file, access is denied. SELinux can confine Linux users. A number of confined SELinux users exist in SELinux policy. Linux users can be mapped to confined SELinux users to take advantage of the security rules and mechanisms applied to them. For example, mapping a Linux user to the SELinux user_u user, results in a Linux user that is not able to run (unless configured otherwise) set user ID (setuid) applications, such as sudo and su, as well as preventing them from executing files and applications in their home directory - if configured, this prevents users from executing malicious files from their home directories. Process separation is used. Processes run in their own domains, preventing processes from accessing files used by other processes, as well as preventing processes from accessing other processes. For example, when running SELinux, unless otherwise configured, an attacker can not compromise a Samba server, and then use that Samba server as an attack vector to read and write to files used by other processes, such as databases used by MySQL. SELinux helps limit the damage made by configuration mistakes. Domain Name System (DNS) servers often replicate information between each other in what is known as a zone transfer. Attackers can use zone transfers to update DNS servers with false information. When running the Berkeley Internet Name Domain (BIND) as a DNS server in Red Hat Enterprise Linux, even if an administrator forgets to limit which servers can perform a zone transfer, the default SELinux policy prevents zone files from being updated via zone transfers, by the BIND named daemon itself, and by other processes.
2.3. SELinux ArchitectureSELinux is a Linux security module that is built into the Linux kernel. SELinux is driven by loadable policy rules. When security-relevant access is taking place, such as when a process attempts to open a file, the operation is intercepted in the kernel by SELinux. If an SELinux policy rule allows the operation, it continues, otherwise, the operation is blocked and the process receives an error. SELinux decisions, such as allowing or disallowing access, are cached. This cache is known as the Access Vector Cache (AVC). Caching decisions decrease how often SELinux policy rules need to be checked, which increases performance. Remember that SELinux policy rules have no effect if DAC rules deny access first. 2.4. SELinux on Other Operating SystemsRefer to the following for information about running SELinux on other Linux distributions: Chapter 3. SELinux ContextsProcesses and files are labeled with an SELinux context that contains additional information, such as an SELinux user, role, type, and, optionally, a level. When running SELinux, all of this information is used to make access control decisions. In Red Hat Enterprise Linux, SELinux provides a combination of Role-Based Access Control (RBAC), Type Enforcement (TE), and, optionally, Multi-Level Security (MLS). The following is an example showing SELinux context. SELinux contexts are used on processes, Linux users, and files, on Linux operating systems that run SELinux. Use the ls -Z command to view the SELinux context of files and directories: ~]$ ls -Z file1-rwxrw-r-- user1 group1 unconfined_u:object_r:user_home_t:s0 file1 SELinux contexts follow the SELinux user:role:type:level syntax. The fields are as follows: - SELinux user
The SELinux user identity is an identity known to the policy that is authorized for a specific set of roles, and for a specific MLS/MCS range. Each Linux user is mapped to an SELinux user via SELinux policy. This allows Linux users to inherit the restrictions placed on SELinux users. The mapped SELinux user identity is used in the SELinux context for processes in that session, in order to define what roles and levels they can enter. Run the semanage login -l command as the Linux root user to view a list of mappings between SELinux and Linux user accounts (you need to have the policycoreutils-python package installed): ~]# semanage login -lLogin Name SELinux User MLS/MCS Range__default__ unconfined_u s0-s0:c0.c1023root unconfined_u s0-s0:c0.c1023system_u system_u s0-s0:c0.c1023 Output may differ slightly from system to system. The Login Name column lists Linux users, and the SELinux User column lists which SELinux user the Linux user is mapped to. For processes, the SELinux user limits which roles and levels are accessible. The last column, MLS/MCS Range, is the level used by Multi-Level Security (MLS) and Multi-Category Security (MCS). - role
Part of SELinux is the Role-Based Access Control (RBAC) security model. The role is an attribute of RBAC. SELinux users are authorized for roles, and roles are authorized for domains. The role serves as an intermediary between domains and SELinux users. The roles that can be entered determine which domains can be entered - ultimately, this controls which object types can be accessed. This helps reduce vulnerability to privilege escalation attacks. - type
The type is an attribute of Type Enforcement. The type defines a domain for processes, and a type for files. SELinux policy rules define how types can access each other, whether it be a domain accessing a type, or a domain accessing another domain. Access is only allowed if a specific SELinux policy rule exists that allows it. - level
The level is an attribute of MLS and MCS. An MLS range is a pair of levels, written as lowlevel-highlevel if the levels differ, or lowlevel if the levels are identical (s0-s0 is the same as s0). Each level is a sensitivity-category pair, with categories being optional. If there are categories, the level is written as sensitivity:category-set. If there are no categories, it is written as sensitivity. If the category set is a contiguous series, it can be abbreviated. For example, c0.c3 is the same as c0,c1,c2,c3. The /etc/selinux/targeted/setrans.conf file maps levels (s0:c0) to human-readable form (ie. CompanyConfidential). Do not edit setrans.conf with a text editor: use semanage to make changes. Refer to the semanage(8) manual page for further information. In Red Hat Enterprise Linux, targeted policy enforces MCS, and in MCS, there is just one sensitivity, s0. MCS in Red Hat Enterprise Linux supports 1024 different categories: c0 through to c1023. s0-s0:c0.c1023 is sensitivity s0 and authorized for all categories. MLS enforces the Bell-La Padula Mandatory Access Model, and is used in Labeled Security Protection Profile (LSPP) environments. To use MLS restrictions, install the selinux-policy-mls package, and configure MLS to be the default SELinux policy. The MLS policy shipped with Red Hat Enterprise Linux omits many program domains that were not part of the evaluated configuration, and therefore, MLS on a desktop workstation is unusable (no support for the X Window System); however, an MLS policy from the upstream SELinux Reference Policy can be built that includes all program domains. For more information on MLS configuration, refer to Section 5.12, "Multi-Level Security (MLS)".
A process in one domain transitions to another domain by executing an application that has the entrypoint type for the new domain. The entrypoint permission is used in SELinux policy, and controls which applications can be used to enter a domain. The following example demonstrates a domain transition: A user wants to change their password. To do this, they run the passwd application. The /usr/bin/passwd executable is labeled with the passwd_exec_t type: ~]$ ls -Z /usr/bin/passwd-rwsr-xr-x root root system_u:object_r:passwd_exec_t:s0 /usr/bin/passwd The passwd application accesses /etc/shadow, which is labeled with the shadow_t type: ~]$ ls -Z /etc/shadow-r--------. root root system_u:object_r:shadow_t:s0 /etc/shadow An SELinux policy rule states that processes running in the passwd_t domain are allowed to read and write to files labeled with the shadow_t type. The shadow_t type is only applied to files that are required for a password change. This includes /etc/gshadow, /etc/shadow, and their backup files. An SELinux policy rule states that the passwd_t domain has entrypoint permission to the passwd_exec_t type. When a user runs the passwd application, the user's shell process transitions to the passwd_t domain. With SELinux, since the default action is to deny, and a rule exists that allows (among other things) applications running in the passwd_t domain to access files labeled with the shadow_t type, the passwd application is allowed to access /etc/shadow, and update the user's password.
This example is not exhaustive, and is used as a basic example to explain domain transition. Although there is an actual rule that allows subjects running in the passwd_t domain to access objects labeled with the shadow_t file type, other SELinux policy rules must be met before the subject can transition to a new domain. In this example, Type Enforcement ensures: The passwd_t domain can only be entered by executing an application labeled with the passwd_exec_t type; can only execute from authorized shared libraries, such as the lib_t type; and can not execute any other applications. Only authorized domains, such as passwd_t, can write to files labeled with the shadow_t type. Even if other processes are running with superuser privileges, those processes can not write to files labeled with the shadow_t type, as they are not running in the passwd_t domain. Only authorized domains can transition to the passwd_t domain. For example, the sendmail process running in the sendmail_t domain does not have a legitimate reason to execute passwd; therefore, it can never transition to the passwd_t domain. Processes running in the passwd_t domain can only read and write to authorized types, such as files labeled with the etc_t or shadow_t types. This prevents the passwd application from being tricked into reading or writing arbitrary files.
3.2. SELinux Contexts for ProcessesUse the ps -eZ command to view the SELinux context for processes. For example: Open a terminal, such as ⤍ ⤍ . Run the passwd command. Do not enter a new password. Open a new tab, or another terminal, and run the ps -eZ | grep passwd command. The output is similar to the following: unconfined_u:unconfined_r:passwd_t:s0-s0:c0.c1023 13212 pts/1 00:00:00 passwd In the first tab/terminal, press Ctrl+C to cancel the passwd application.
In this example, when the passwd application (labeled with the passwd_exec_t type) is executed, the user's shell process transitions to the passwd_t domain. Remember: the type defines a domain for processes, and a type for files. Use the ps -eZ command to view the SELinux contexts for running processes. The following is a truncated example of the output, and may differ on your system: system_u:system_r:dhcpc_t:s0 1869 ? 00:00:00 dhclientsystem_u:system_r:sshd_t:s0-s0:c0.c1023 1882 ? 00:00:00 sshdsystem_u:system_r:gpm_t:s0 1964 ? 00:00:00 gpmsystem_u:system_r:crond_t:s0-s0:c0.c1023 1973 ? 00:00:00 crondsystem_u:system_r:kerneloops_t:s0 1983 ? 00:00:05 kerneloopssystem_u:system_r:crond_t:s0-s0:c0.c1023 1991 ? 00:00:00 atd The system_r role is used for system processes, such as daemons. Type Enforcement then separates each domain. 3.3. SELinux Contexts for UsersUse the id -Z command to view the SELinux context associated with your Linux user: unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 In Red Hat Enterprise Linux, Linux users run unconfined by default. This SELinux context shows that the Linux user is mapped to the SELinux unconfined_u user, running as the unconfined_r role, and is running in the unconfined_t domain. s0-s0 is an MLS range, which in this case, is the same as just s0. The categories the user has access to is defined by c0.c1023, which is all categories (c0 through to c1023). Chapter 4. Targeted PolicyTargeted policy is the default SELinux policy used in Red Hat Enterprise Linux. When using targeted policy, processes that are targeted run in a confined domain, and processes that are not targeted run in an unconfined domain. For example, by default, logged in users run in the unconfined_t domain, and system processes started by init run in the initrc_t domain - both of these domains are unconfined. Unconfined domains (as well as confined domains) are subject to executable and writeable memory checks. By default, subjects running in an unconfined domain can not allocate writeable memory and execute it. This reduces vulnerability to buffer overflow attacks. These memory checks are disabled by setting Booleans, which allow the SELinux policy to be modified at runtime. Boolean configuration is discussed later. Almost every service that listens on a network, such as sshd or httpd, is confined in Red Hat Enterprise Linux. Also, most processes that run as the Linux root user and perform tasks for users, such as the passwd application, are confined. When a process is confined, it runs in its own domain, such as the httpd process running in the httpd_t domain. If a confined process is compromised by an attacker, depending on SELinux policy configuration, an attacker's access to resources and the possible damage they can do is limited. The following example demonstrates how SELinux prevents the Apache HTTP Server (httpd) from reading files that are not correctly labeled, such as files intended for use by Samba. This is an example, and should not be used in production. It assumes that the httpd and wget packages are installed, the SELinux targeted policy is used, and that SELinux is running in enforcing mode: Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode, and that targeted policy is being used: ~]$ sestatusSELinux status: enabledSELinuxfs mount: /selinuxCurrent mode: enforcingMode from config file: enforcingPolicy version: 24Policy from config file: targeted SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing is returned when SELinux is running in enforcing mode. Policy from config file: targeted is returned when the SELinux targeted policy is used.
As the Linux root user, run the touch /var/www/html/testfile command to create a file. Run the ls -Z /var/www/html/testfile command to view the SELinux context: -rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/testfile By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the testfile file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles do not have a meaning for files - the object_r role is a generic role used for files (on persistent storage and network file systems). Under the /proc/ directory, files related to processes may use the system_r role. The httpd_sys_content_t type allows the httpd process to access this file. As the Linux root user, run the service httpd start command to start the httpd process. The output is as follows if httpd starts successfully: ~]# service httpd startStarting httpd: [ OK ] Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile command. Unless there are changes to the default configuration, this command succeeds: ~]$ wget http://localhost/testfile--2009-11-06 17:43:01-- http://localhost/testfileResolving localhost... 127.0.0.1Connecting to localhost|127.0.0.1|:80... connected.HTTP request sent, awaiting response... 200 OKLength: 0 [text/plain]Saving to: `testfile'[ <=> ] 0 --.-K/s in 0s2009-11-06 17:43:01 (0.00 B/s) - `testfile' saved [0/0] The chcon command relabels files; however, such label changes do not survive when the file system is relabeled. For permanent changes that survive a file system relabel, use the semanage command, which is discussed later. As the Linux root user, run the following command to change the type to a type used by Samba: ~]# chcon -t samba_share_t /var/www/html/testfile Run the ls -Z /var/www/html/testfile command to view the changes: -rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile Note: the current DAC permissions allow the httpd process access to testfile. Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile command. Unless there are changes to the default configuration, this command fails: ~]$ wget http://localhost/testfile--2009-11-06 14:11:23-- http://localhost/testfileResolving localhost... 127.0.0.1Connecting to localhost|127.0.0.1|:80... connected.HTTP request sent, awaiting response... 403 Forbidden2009-11-06 14:11:23 ERROR 403: Forbidden. As the Linux root user, run the rm -i /var/www/html/testfile command to remove testfile. If you do not require httpd to be running, as the Linux root user, run the service httpd stop command to stop httpd: ~]# service httpd stopStopping httpd: [ OK ]
This example demonstrates the additional security added by SELinux. Although DAC rules allowed the httpd process access to testfile in step 7, because the file was labeled with a type that the httpd process does not have access to, SELinux denied access. An error similar to the following is logged to /var/log/audit/audit.log: type=AVC msg=audit(1220706212.937:70): avc: denied { getattr } for pid=1904 comm="httpd" path="/var/www/html/testfile" dev=sda5 ino=247576 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=unconfined_u:object_r:samba_share_t:s0 tclass=filetype=SYSCALL msg=audit(1220706212.937:70): arch=40000003 syscall=196 success=no exit=-13 a0=b9e21da0 a1=bf9581dc a2=555ff4 a3=2008171 items=0 ppid=1902 pid=1904 auid=500 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=1 comm="httpd" exe="/usr/sbin/httpd" subj=unconfined_u:system_r:httpd_t:s0 key=(null)Also, an error similar to the following is logged to /var/log/httpd/error_log: [Wed May 06 23:00:54 2009] [error] [client 127.0.0.1] (13)Permission denied: access to /testfile denied 4.2. Unconfined ProcessesUnconfined processes run in unconfined domains, for example, init programs run in the unconfined initrc_t domain, unconfined kernel processes run in the kernel_t domain, and unconfined Linux users run in the unconfined_t domain. For unconfined processes, SELinux policy rules are applied, but policy rules exist that allow processes running in unconfined domains almost all access. Processes running in unconfined domains fall back to using DAC rules exclusively. If an unconfined process is compromised, SELinux does not prevent an attacker from gaining access to system resources and data, but of course, DAC rules are still used. SELinux is a security enhancement on top of DAC rules - it does not replace them. The following example demonstrates how the Apache HTTP Server (httpd) can access data intended for use by Samba, when running unconfined. Note: in Red Hat Enterprise Linux, the httpd process runs in the confined httpd_t domain by default. This is an example, and should not be used in production. It assumes that the httpd, wget, dbus and audit packages are installed, that the SELinux targeted policy is used, and that SELinux is running in enforcing mode: Run the sestatus command to confirm that SELinux is enabled, is running in enforcing mode, and that targeted policy is being used: ~]$ sestatusSELinux status: enabledSELinuxfs mount: /selinuxCurrent mode: enforcingMode from config file: enforcingPolicy version: 24Policy from config file: targeted SELinux status: enabled is returned when SELinux is enabled. Current mode: enforcing is returned when SELinux is running in enforcing mode. Policy from config file: targeted is returned when the SELinux targeted policy is used.
As the Linux root user, run the touch /var/www/html/testfile command to create a file. Run the ls -Z /var/www/html/testfile command to view the SELinux context: ~]$ ls -Z /var/www/html/testfile-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 /var/www/html/testfile By default, Linux users run unconfined in Red Hat Enterprise Linux, which is why the testfile file is labeled with the SELinux unconfined_u user. RBAC is used for processes, not files. Roles do not have a meaning for files - the object_r role is a generic role used for files (on persistent storage and network file systems). Under the /proc/ directory, files related to processes may use the system_r role. The httpd_sys_content_t type allows the httpd process to access this file. The chcon command relabels files; however, such label changes do not survive when the file system is relabeled. For permanent changes that survive a file system relabel, use the semanage command, which is discussed later. As the Linux root user, run the following command to change the type to a type used by Samba: ~]# chcon -t samba_share_t /var/www/html/testfile Run the ls -Z /var/www/html/testfile command to view the changes: ~]$ ls -Z /var/www/html/testfile-rw-r--r-- root root unconfined_u:object_r:samba_share_t:s0 /var/www/html/testfile Run the service httpd status command to confirm that the httpd process is not running: ~]$ service httpd statushttpd is stopped If the output differs, run the service httpd stop command as the Linux root user to stop the httpd process: ~]# service httpd stopStopping httpd: [ OK ] To make the httpd process run unconfined, run the following command as the Linux root user to change the type of /usr/sbin/httpd, to a type that does not transition to a confined domain: ~]# chcon -t unconfined_exec_t /usr/sbin/httpd Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with the unconfined_exec_t type: ~]$ ls -Z /usr/sbin/httpd-rwxr-xr-x root root system_u:object_r:unconfined_exec_t:s0 /usr/sbin/httpd As the Linux root user, run the service httpd start command to start the httpd process. The output is as follows if httpd starts successfully: ~]# service httpd startStarting httpd: [ OK ] Run the ps -eZ | grep httpd command to view the httpd running in the unconfined_t domain: ~]$ ps -eZ | grep httpdunconfined_u:unconfined_r:unconfined_t:s0 7721 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7723 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7724 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7725 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7726 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7727 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7728 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7729 ? 00:00:00 httpdunconfined_u:unconfined_r:unconfined_t:s0 7730 ? 00:00:00 httpd Change into a directory where your Linux user has write access to, and run the wget http://localhost/testfile command. Unless there are changes to the default configuration, this command succeeds: ~]$ wget http://localhost/testfile--2009-05-07 01:41:10-- http://localhost/testfileResolving localhost... 127.0.0.1Connecting to localhost|127.0.0.1|:80... connected.HTTP request sent, awaiting response... 200 OKLength: 0 [text/plain]Saving to: `testfile.1'[ <=> ]--.-K/s in 0s 2009-05-07 01:41:10 (0.00 B/s) - `testfile.1' saved [0/0] Although the httpd process does not have access to files labeled with the samba_share_t type, httpd is running in the unconfined unconfined_t domain, and falls back to using DAC rules, and as such, the wget command succeeds. Had httpd been running in the confined httpd_t domain, the wget command would have failed. The restorecon command restores the default SELinux context for files. As the Linux root user, run the restorecon -v /usr/sbin/httpd command to restore the default SELinux context for /usr/sbin/httpd: ~]# restorecon -v /usr/sbin/httpdrestorecon reset /usr/sbin/httpd context system_u:object_r:unconfined_exec_t:s0->system_u:object_r:httpd_exec_t:s0 Run the ls -Z /usr/sbin/httpd command to confirm that /usr/sbin/httpd is labeled with the httpd_exec_t type: ~]$ ls -Z /usr/sbin/httpd-rwxr-xr-x root root system_u:object_r:httpd_exec_t:s0 /usr/sbin/httpd As the Linux root user, run the service httpd restart command to restart httpd. After restarting, run the ps -eZ | grep httpd to confirm that httpd is running in the confined httpd_t domain: ~]# service httpd restartStopping httpd: [ OK ]Starting httpd: [ OK ]~]# ps -eZ | grep httpdunconfined_u:system_r:httpd_t:s0 8880 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8882 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8883 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8884 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8885 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8886 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8887 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8888 ? 00:00:00 httpdunconfined_u:system_r:httpd_t:s0 8889 ? 00:00:00 httpd As the Linux root user, run the rm -i /var/www/html/testfile command to remove testfile: ~]# rm -i /var/www/html/testfilerm: remove regular empty file `/var/www/html/testfile'? y If you do not require httpd to be running, as the Linux root user, run the service httpd stop command to stop httpd: ~]# service httpd stopStopping httpd: [ OK ]
The examples in these sections demonstrate how data can be protected from a compromised confined-process (protected by SELinux), as well as how data is more accessible to an attacker from a compromised unconfined-process (not protected by SELinux). 4.3. Confined and Unconfined UsersEach Linux user is mapped to an SELinux user via SELinux policy. This allows Linux users to inherit the restrictions on SELinux users. This Linux user mapping is seen by running the semanage login -l command as the Linux root user: ~]# semanage login -lLogin Name SELinux User MLS/MCS Range__default__ unconfined_u s0-s0:c0.c1023root unconfined_u s0-s0:c0.c1023system_u system_u s0-s0:c0.c1023 In Red Hat Enterprise Linux 6, Linux users are mapped to the SELinux __default__ login by default, which is mapped to the SELinux unconfined_u user. The following line defines the default mapping: __default__ unconfined_u s0-s0:c0.c1023 The following procedure demonstrates how to add a new Linux user to the system and how to map that user to the SELinux unconfined_u user. It assumes that the Linux root user is running unconfined, as it does by default in Red Hat Enterprise Linux 6: As the Linux root user, run the useradd newuser command to create a new Linux user named newuser. As the Linux root user, run the passwd newuser command to assign a password to the Linux newuser user: ~]# passwd newuserChanging password for user newuser.New UNIX password: Enter a password Retype new UNIX password: Enter the same password again passwd: all authentication tokens updated successfully. Log out of your current session, and log in as the Linux newuser user. When you log in, the pam_selinux PAM module automatically maps the Linux user to an SELinux user (in this case, unconfined_u), and sets up the resulting SELinux context. The Linux user's shell is then launched with this context. Run the id -Z command to view the context of a Linux user: [newuser@localhost ~]$ id -Z unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 If you no longer need the newuser user on your system, log out of the Linux newuser's session, log in with your account, and run the userdel -r newuser command as the Linux root user. It will remove the newuser user along with his home directory.
Confined and unconfined Linux users are subject to executable and writeable memory checks, and are also restricted by MCS or MLS. If an unconfined Linux user executes an application that SELinux policy defines as one that can transition from the unconfined_t domain to its own confined domain, the unconfined Linux user is still subject to the restrictions of that confined domain. The security benefit of this is that, even though a Linux user is running unconfined, the application remains confined. Therefore, the exploitation of a flaw in the application can be limited by the policy. Similarly, we can apply these checks to confined users. However, each confined Linux user is restricted by a confined user domain against the unconfined_t domain. The SELinux policy can also define a transition from a confined user domain to its own target confined domain. In such a case, confined Linux users are subject to the restrictions of that target confined domain. The main point is that special privileges are associated with the confined users according to their role. In the table below, you can see examples of basic confined domains for Linux users in Red Hat Enterprise Linux 6: Table 4.1. SELinux User Capabilities | User | Domain | X Window System | su or sudo | Execute in home directory and /tmp/ (default) | Networking |
|---|
| sysadm_u | sysadm_t | yes | su and sudo | yes | yes | | staff_u | staff_t | yes | only sudo | yes | yes | | user_u | user_t | yes | no | yes | yes | | guest_u | guest_t | no | no | no | yes | | xguest_u | xguest_t | yes | no | no | Firefox only |
Linux users in the user_t, guest_t, xguest_t, and git_shell_t domains can only run set user ID (setuid) applications if SELinux policy permits it (for example, passwd). These users cannot run the su and sudo setuid applications, and therefore cannot use these applications to become the Linux root user. Linux users in the sysadm_t, staff_t, user_t, and xguest_t domains can log in via the X Window System and a terminal. By default, Linux users in the guest_t and xguest_t domains cannot execute applications in their home directories or /tmp/, preventing them from executing applications, which inherit users' permissions, in directories they have write access to. This helps prevent flawed or malicious applications from modifying users' files. By default, Linux users in the staff_t and user_t domains can execute applications in their home directories and /tmp/. Refer to Section 6.6, "Booleans for Users Executing Applications" for information about allowing and preventing users from executing applications in their home directories and /tmp/. The only network access Linux users in the xguest_t domain have is Firefox connecting to web pages.
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