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Introduction to Linux Capabilities and ACL's

Created: 27 Aug 2000 • Updated: 03 Nov 2010
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by Jeremy Rauch

  1. Introduction

    Unix systems have always utilized a security system that gives normal users a minimal amount of privilege, while creating a single account, known as the 'root' account, that has full privileges. The root account is used to administer the machine, install software, create new users, and run certain services. Many common activities that require root privileges are often run as the root user, via the concept of setuid.

    This dependence upon a single account for performing all actions requiring privilege has proven to be somewhat dangerous. Programs often need root privileges for a single activity, such as binding to a privileged port, or opening a file only root can access. Vulnerabilities are often found that could perhaps be eliminated if these programs didn't run as root.

    In version 2.1 of the Linux kernel, work was started to add what are known as capabilities. The goal of this work was to eliminate the dependence on the root account for certain actions. As of the 2.2 kernels, this code is beginning to become useful. While problems still exist with it, it is a step in the right direction.

    Another interesting project being worked on, also to compensate for some short comings in the file access control realm, is the Linux ACL project. These extend the ext2fs to allow for a finer degree of access control than is normally allowed on a Unix filesystem.

    We'll briefly discuss each of these features, talk about where they may be useful, and give short examples on using them and their associated utilities. Some portions of this article will require installing kernel patches, and patching and replacing system utilities. Make sure you take extreme care when doing so, don't attempt to do so on production or other critical systems.

  2. Linux Capabilities

    As was briefly touched upon in the introduction, the Linux capabilities system was designed to help remove the problems associated with the need for root privileges. Before we go any further, we need to touch upon why certain privileges are reserved for root, and why certain programs need to run as root.

    According to Unix security conventions, there are certain privileges reserved only for the root account. Privileged actions include things like binding a process to a privileged port, loading kernel modules, mounting and unmounting file systems, and a variety of other system activities. root is also the only account capable of adding/altering account information, installing system utilities. The privileges of root are extensive.

    So what kinds of things would an average user need root access for? A simple example in the ping program. Ping needs to be able to open a raw socket in order to send packets to the network. This is an operation that only root may perform, so the ping program is installed setuid root. This allows any user to use the program, and still be able to open a raw socket. Unfortunately, this means the program has the full privileges of root. Should a flaw be discovered in the program, it could be possible for the attacker to perform any activity root can perform.

    Capabilities work by breaking the actions normally reserved for root down in to smaller portions. Originally broken down in a POSIX specification, Linux has implemented 7 of the capabilities outlined in the Posix document 1003.1e, and another 20 or so Linux specific ones. Some of the more useful ones are described in the table below.

    Capability Name Meaning
    CAP_CHOWN Allow for the changing of file ownership
    CAP_DAC_OVERRIDE Override all DAC access restrictions
    CAP_DAC_READ_SEARCH Override all DAC restrictions regarding read and search
    CAP_KILL Allow the sending of signals to processes belonging to others
    CAP_SETGID Allow changing of the GID
    CAP_SETUID Allow changing of the UID
    CAP_SETPCAP Allow the transfering and removal of current set to any PID
    CAP_LINUX_IMMUTABLE Allow the modification of immutable and append-only files
    CAP_NET_BIND_SERVICE Allow binding to ports below 1024
    CAP_NET_RAW Allow use of raw sockets

    There are a number of other capabilities implemented; if you're interested in looking at them, and you have a 2.2.x kernel installed, you should be able to view them in the /usr/include/linux/capability file.

    For the most part, capabilities are most useful to programmers. The use of capabilities is only beginning to trickle in to userland applications; most system utilities do not shed their root privileges. In addition, the mechanism to set capabilities on binaries on the filesystem does not exist. We should expect to actually see capabilities become useful in the 2.4 kernels. There are, however, possible uses of capabilities for system administrators.

    In Linux kernels 2.2.11 and later, the concept of capability bounding sets exists. This is a table of the capabilities allowed on the system. By removing capabilities from the set, it is possible to eliminate capabilities outright. Once a capability has been removed, it can not be added again. The /proc/sys/kernel/cap-bound proc file allows the controlling of this set. The lcap program provides a clean, simple interface for removing capabilities. It is available at http://home.netcom.com/~spoon/lcap/. One of the nicer things which can now be performed is the disabling of module loading.

     		[root@hamachi lcap-0.0.3]# ./lcap 		Current capabilities: 0xFFFEFFFF 		0) *CAP_CHOWN                   1) *CAP_DAC_OVERRIDE          		2) *CAP_DAC_READ_SEARCH         3) *CAP_FOWNER                		4) *CAP_FSETID                  5) *CAP_KILL                  		6) *CAP_SETGID                  7) *CAP_SETUID                		8) *CAP_SETPCAP                 9) *CAP_LINUX_IMMUTABLE       		10) *CAP_NET_BIND_SERVICE       11) *CAP_NET_BROADCAST         		12) *CAP_NET_ADMIN              13) *CAP_NET_RAW               		14) *CAP_IPC_LOCK               15) *CAP_IPC_OWNER             		16) *CAP_SYS_MODULE             17) *CAP_SYS_RAWIO             		18) *CAP_SYS_CHROOT             19) *CAP_SYS_PTRACE            		20) *CAP_SYS_PACCT              21) *CAP_SYS_ADMIN             		22) *CAP_SYS_BOOT               23) *CAP_SYS_NICE              		24) *CAP_SYS_RESOURCE           25) *CAP_SYS_TIME              		26) *CAP_SYS_TTY_CONFIG        		* = Capabilities currently allowed 		[root@hamachi lcap-0.0.3]# ./lcap CAP_SYS_MODULE 		[root@hamachi lcap-0.0.3]# ./lcap 		Current capabilities: 0xFFFEFFFF 		0) *CAP_CHOWN                   1) *CAP_DAC_OVERRIDE          		2) *CAP_DAC_READ_SEARCH         3) *CAP_FOWNER                		4) *CAP_FSETID                  5) *CAP_KILL                  		6) *CAP_SETGID                  7) *CAP_SETUID                		8) *CAP_SETPCAP                 9) *CAP_LINUX_IMMUTABLE       		10) *CAP_NET_BIND_SERVICE       11) *CAP_NET_BROADCAST         		12) *CAP_NET_ADMIN              13) *CAP_NET_RAW               		14) *CAP_IPC_LOCK               15) *CAP_IPC_OWNER             		16)  CAP_SYS_MODULE             17) *CAP_SYS_RAWIO             		18) *CAP_SYS_CHROOT             19) *CAP_SYS_PTRACE            		20) *CAP_SYS_PACCT              21) *CAP_SYS_ADMIN             		22) *CAP_SYS_BOOT               23) *CAP_SYS_NICE              		24) *CAP_SYS_RESOURCE           25) *CAP_SYS_TIME              		26) *CAP_SYS_TTY_CONFIG        		* = Capabilities currently allowed 		

    As capabilities begin to enjoy more widespread use, vulnerabilities that lead to complete root access should become far less common.

  3. Posix ACL's

    Another interesting, although seemingly less mainstream kernel project than capabilities, is the work being done to introduce Posix file access control list (ACL) support in Linux. Those familiar with commercial Unix OS's may have used, or at least seen Posix ACL's in action.

    ACL's present a way to add finer file-level access control. Whereas default Unix permissions allow for permissions to be associated with a single owner, group and the rest of the world, ACL's allow permissions to be set for multiple groups or individuals. Rather than rehash, its worthwhile to read an article on the use of ACL's under Solaris. It is available here. It covers basic use of the getfacl and setfacl utilities, both of which are implemented with an almost identical interface under Linux.

    ACL's are not included by default in the Linux kernel, nor are the associated changes that need to be made to certain system utilities to support ACL's correctly. The userland ACL utilities, kernel patches, and patches to e2fsck and GNU fileutils are all available at http://acl.bestbits.at/download.html, which is the home site of the project. The e2fsprogs tarfile is available at ftp://download.sourceforge.net/pub/sourceforge/e2fsprogs/, and fileutils at ftp://ftp.gnu.org/pub/gnu/fileutils. Directions for installing the patches and building the userland utilities can be found at http://acl.bestbits.at/.

    The ACL implementation is still very much a work in progress, and is not for the faint of heart; there is a decent chance you will have compilation problems. However, ACL's can go a long way in reducing the need to give people root access. Instead of having to rely on an application like sudo to allow limited root access, ACL's can instead be used to allow specific users to run a setuid application, without needing to create large quantities of groups.

  4. Conclusion

    A number of the projects being developed in the Linux kernel will go very far towards improving security on Linux machines. By introducing methods for reducing the dependence upon root for specific privileged actions, the possible exposure from having large quantities of programs running as root will be eliminated. If applications can only conduct the actions they are required to, the likelihood of a vulnerability allowing a user to conduct arbitrary actions as root will be much smaller. Using finer grained access control mechanisms, like those presented by ACL's will even further reduce this exposure.

Relevant Links

Capabilities FAQ

kernel.org

lcap
spoon@ix.netcom.com

Focus On Sun: ACL's
Jeremy Rauch

Linux ACL's
Andreas Grunbacher

 

This article originally appeared on SecurityFocus.com -- reproduction in whole or in part is not allowed without expressed written consent.