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SELinux by Example
Posted by
samzenpus
on Wed Mar 14, 2007 03:26 PM
from the read-all-about-it dept.
from the read-all-about-it dept.
Ravi writes "SELinux is a project started and actively maintained by the U.S Department of Defense to provide a Mandatory Access Controls mechanism in Linux. It had been a long standing grouse of Linux power users and system administrators over its lack of fine grained access control over various running processes as well as files in Linux. While Solaris touts its famous RBAC and Microsoft Windows has its own way of providing finer rights to its resources, Linux had to put up with the simple but crude user rights known in tech speak as discretionary access control to control user access of files. With SELinux project making great strides and now being bundled with many major Linux distributions, it is possible to effectively lock down a Linux system through judicious use of SELinux policies. SELinux implements a more flexible form of MAC called type enforcement and an optional form of multilevel security." Read the rest of Ravi's review.
The book SELinux by Example is authored by three people — Frank Mayer, Karl Macmillan and David Caplan and is published by Prentice Hall. There are a total of 14 chapters and 4 appendices spread just over 400 pages. The 14 chapters are in turn broadly divided into three parts with the first part containing chapters which provide an overview of SELinux, its background and the concepts behind it. The second part contains 7 chapters which are most useful for SELinux policy writers and contain detailed explanation of the syntax used in writing the policy files. It is the third part, "Creating and Writing SELinux Security Policies" which could be most put to use by system administrators.
In the second chapter, the authors introduce the concept of type enforcement access control, the understanding of which is imperative to ones knowledge of SELinux. They further discuss the concept of roles and multi level security. True to the title of the book, all these concepts are explained by analyzing the security controls of the ubiquitous passwd program.
In the succeeding chapter the authors explain the underlying architecture of SELinux. More specifically, how SELinux integrates with the Linux kernel via the Linux security module (LSM), the organization of the policy source file and how to build and install policies.
SELinux policies to a large extent are based on object classes. For example, you can create an object class and associate a set of permissions to that class. All objects associated with that class will share the same set of permissions. In the fourth chapter, one get to know about different types of object classes and the permissions that can be assigned to these classes. A total of 40 classes and 48 permissions are discussed in this chapter.
The next chapter titled "Types Enforcement" goes into a detailed analysis of all the types and attributes as well as the rules that could be used. The majority of SELinux policy is a set of statements and rules that collectively define the type enforcement policy. Going through the chapter, I was able to get a fair idea of the syntax used in writing TE policies.
Keeping in mind the complexity of the subject, it helps a great deal that at the end of each chapter there is a summary section where the authors have listed the important points covered. More over, one gets to answer a couple of questions and check one's knowledge about the topic being discussed.
In the 6th chapter, the authors explain in detail the concept of roles and their relationship in SELinux. What I really like about this book is the fact that each concept of SELinux has been dedicated a chapter of its own. For instance, constraints, multilevel security, type enforcement, conditional policies,... all are explained in chapters of their own.
One thing worth noting is that Fedora Core 4 and RHEL 4 and above ship with the targeted policy by default. Where as to completely lock down a Linux machine, you need to embrace the strict SELinux policy. This has the side effect of causing breakages with some of the existing Linux applications which expect looser security controls. In targeted policy, the more confining rules are focused on a subset of likely to be attacked network applications. In most cases, one can manage by using targeted policy. This book mostly deals with the strict policy of SELinux and in chapter 11, the authors dissect the strict example policy maintained and updated via the NSA and Fedora Core mailing lists.
There is another policy called the Reference Policy which is an attempt to water down the strict policy maintained by NSA. In the process making it easier to use, understand, maintain, and more modular. This is covered in the succeeding chapter titled "Reference Policy".
The next chapter titled "Managing an SELinux system" is one which the system administrators will relate to, where the authors throw light on the hierarchy of SELinux configuration files. The purpose of each file is explained in simple terms. Considering that SELinux comes bundled with a rich set of tools meant to be used by system administrators, one gets to know the usage of some of them and also learn about the common problems that are faced by administrators while administering an SELinux system.
In the last chapter of the book, one is introduced to the task of writing policy modules. Here the authors hand hold in the creation of a policy module for the IRC daemon for Fedora Core 4, from the planning stage to writing and applying the policy module, to the final testing.
The book also includes 4 appendices which contain a wealth of knowledge on SELinux. I especially liked appendix C which lists all the object classes and permissions as well as appendix D which has a list of SELinux system tools and third party utilities with explanations.
I found that I was better able to assimilate what the authors explained when I read the 13th chapter of this book first and then went back to read the 4th chapter onwards. Having said that, I find this book to be an excellent resource for people interested in developing SELinux policies and to a slightly lesser extent a resource for system administrators. At the very least, this book imparts a deep understanding of the features, structure, syntax and working of SELinux.
Ravi Kumar maintains a blog at linuxhelp.blogspot.com where he shares his thoughts and experiences on all things related to Linux.
You can purchase SELinux by Example from amazon.com. Slashdot welcomes readers' book reviews -- to see your own review here, read the book review guidelines, then visit the submission page.
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Linux: How the NSA Took Linux To the Next Level 172 comments
An anonymous reader brings us IBM Developerworks' recent analysis of how the NSA built SELinux to withstand attacks. The article shows us some of the relevant kernel architecture and compares SELinux to a few other approaches. We've discussed SELinux in the past. Quoting:
"If you have a program that responds to socket requests but doesn't need to access the file system, then that program should be able to listen on a given socket but not have access to the file system. That way, if the program is exploited in some way, its access is explicitly minimized. This type of control is called mandatory access control (MAC). Another approach to controlling access is role-based access control (RBAC). In RBAC, permissions are provided based on roles that are granted by the security system. The concept of a role differs from that of a traditional group in that a group represents one or more users. A role can represent multiple users, but it also represents the permissions that a set of users can perform. SELinux adds both MAC and RBAC to the GNU/Linux operating system."
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Monkey SE, Monkey DO (Score:3, Funny)
You can say that, sure, but I think for most people, SE'ing is believing.
Interesting (Score:2, Interesting)
But is it useful? For military and some business use, I can see it... but does anyone actually run SELinux on a home system?
If so -- why?
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Re:Interesting (Score:5, Informative)
I wrote the UnOfficial SELinux FAQ [crypt.gen.nz] and I'll tell you what the most common search query that Google sends to that page, its "disable selinux". About 80% of the hits to that FAQ are from people wanting to know how to disable it.
Lots of people like the MAC idea, and they're keen to try it out. But its causing pain - its hard to understand and it stops stuff from working. The majority of people out there, even the open source boffins, just don't have the spare time to figure it out and work with it.
Despite this, the SELinux by Example book is good. If you're developing software which you want to run on an SELinux system the book will help you a lot in showing you how to write the policy for your package. In fact, if you want to do serious work with SELinux then you pretty much need this book. Any online documentation you can find is likely to be very old and of little use.
Parent
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Yes it is, and it's on by default in enforce mode. There's even been some reports (although I have not checked them) that you cannot automatically disable it via kickstart.
No, the better question is -- is there anything you do where it would actually get in the way? In the two years I've been running
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I can pseudo-confirm those reports. I've installed RHEL 4 probably over a hundred times in various configurations, and I've often had problems getting SELinux to "stay dead" from the installer. The only problem is that I never really paid attention to the circumstances in which it wouldn't stay disabled, so I can't tell you if I was using kickstart or a regular interactive install.
For wh
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Yes I run it with FC6 - because I can (Score:5, Interesting)
Yes, I selinux it with FC6. For several reasons. Firstly because I can; It just completely doesn't get in the way. I've come across a two policy things I had to change; in both cases the built in tool warned me about them, so I knew it was an SeLinux problem and didn't spend ages serching. Secondly, in both cases it gave me reasonable (but not complete) information about what to to to fixi it and finlally, if you learn how to use audit2allow all my problems were really easy to fix (and if you report them with audit message RedHat does a fix which gets rid of them in future almost immediately anyway).
Secondly I have a few servers on my system, it's nice to know that there is a reasonable chance they won't break my desktop if they get hacked into.
Finally, I have several proprietary applications I use (e.g. Skype) given past experience, I don't trust these not to do bad things like sending of my private data. Making an SeLinux policy lets me control which data these applications have access to.
Generally, running SeLinux just gives more of a feel of having control over what your programs are doing on your computer. Without it, you can limit programs from one UserID to the next, but there's no easy way to limit access within a UserID (well; chroot, but that's not really easy).
Parent
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SELinux as mighty as it may be for tracking skype and alike, it will probably not track down your perl script include tags (lets say mailfilters that run somewhere, or from another app, the "safe" javascript in mozilla ?). Yep you can tell your mozilla that it shouldn't read a or b or c nor should it write d, but how much can you restrict perl on your system ? or bash for this sake ? bash scripts are not always harmless
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If, on
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I would like details of the logging tool you use. Mine only records details which have been logged, so far silent changes elude it.
Gentoo has a good kernel hardening guide (Score:1)
Hardening [gentoo.org]
And keep in mind: Even if you are not paranoid, they still could be out to get you.
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AppArmor? (Score:4, Informative)
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It seems to me that AppArmor is still a much more suitable tool for MAC under Linux for 99% of the systems that need it.
The truth is, the vast majority of systems don't need either, but the concept is a nice security architecture to have in place for those rare instances where it is needed and as a built in part of security going forward.
Having used both SELinux and AppArmor I can say there's no comparison in terms of effectiveness. If a security tool it too complex to use it will be used incorrectly and can lead to even worse security problems. I would rather stick with a much simpler approach that still provides all the confinement of MAC but only where I need it.
If you're trying to secure a system today, you might be better of with AppArmor from what I understand. If you're trying to decide upon a MAC architecture that will be part of Linux going forward, SELinux looks like a much better bet. Ubiquitous application of MAC is a big win in the l
SE....x (Score:1)
Sounds like an ad (Score:3, Interesting)
Hell, it is an ad. Read the last line of the article.
SELinux is a great idea, but almost nobody gets it. NSA wrote it so that commercial and open source application developers could get accustomed to writing programs that would work on a system that enforced mandatory security. The hope was that, for example, Firefox and Apache would be modified to work well under very restrictive security models, so that if some app misbehaved, its damage would be limited. This was the first step in getting out of the mess we're in now with patch-based insecurity.
Not too much of that has happened.
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You haven't run modern linux distros for a while, don't you? Linux distros have been shipping SELinux for years, and not just "for fun" - they wouldn't go through the pain of including it if they didn't use it.
Red Hat 4, which was released on February 2005 already used SELinux at least for: apache, dhcpd, mysqld, named, nscd, ntpd, portmap, postgres, snmpd, squid, syslogdm winbind. RHEL 5 (released today) probably adds more.
No, people still hasn't wrote SELinux rules for fi
Why I don't use SELinux (Score:1)
Just installed FC5 with SELinux turned on. (Score:2)
So, before i can turn SELinux back on, i have to go through the SELinux learni
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So, before i can turn SELinux back on, i have to go through the SELinux learning curve. A book like this could help. I've not yet looked for on-line docs.
You might not need the book any more. The configuration has been simplified a lot in FC6, it has a daemon that monitors the log files, and a gui tool that pops up a notification whenever SELinux blocks something, and in common cases tells you what do to tweak the specific setting.
For example, I tried temporarily turning on the "don't allow apache to read home dirs", and get this if I try to access them: http://www.cc.puv.fi/~e0600613/sealert.png [cc.puv.fi]
The nice about user/group permissions (Score:2)
SELinux - Useable? (Score:2, Interesting)
One thing you DO NOT need if you are trying to run SELINUX is 400 pages of abstract security theory and discussions on the 'flask' model etc. etc. There is way too much info of that sort out there and not nearly enough
I want to get rid of all of SeLinux on Debian (Score:1)
Yes, but... (Score:1, Funny)
oh, wait...
Solaris' Role Based Access Control Proves Unix (Score:2)
As of 2 years ago, there was little, if nothing that RBAC did that wasn't available to a well-tooled sysadmin on a normal UNI*X box (without SELinux capabilities).
Book is out of print? (Score:2)
can't I use PAM (Score:1)
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SELinux is a type of MAC architecture for Linux. It enforces the actual security on objects based on their policy, defined separately from
mandatory pedantry (Score:2)
and Microsoft Windows has its own way of providing finer rights to its resources, Linux had to put up with the simple but crude user rights known in tech speak as discretionary access control to control user access of files.
A small point, but the access control in Windows is also called "discretionary". They are different models, but they are both discretionary.
One way of thinking about this is that mandatory means "access controlled by a mandatory policy" and discretionary means "access controlled at t
Acronym overload (Score:1)
Two bad experiences with SELinux (Score:2)
First problem. I had a shiny new install of FC3. I try to get apache to start serving webpages. It only works in one directory. The folks at fedoraforum.org were useless as usual. A couple of posts on an apache email list had me remove the php, apache, and mysql rpms and reinstall from source. After a week of nothing working, I finally stumbled upon some vague reference about S
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TOMOYO Linux (Score:1)
Hi,
If you think SELinux is too much/heavy for you, you might be interested in TOMOYO Linux. I'm so sure that most of you never heard of "TOMOYO Linux", so I'll explain briefly. "TOMOYO Linux is a project started and actively maintained by the Japanese SI company, NTT DATA CORPORATION to provide a Mandatory Access Controls mechanism in Linux."
In short, TOMOYO Linux is quite similar to AppArmor and has been available at SourceForge.jp under GPL license since Nov. 2005.
TOMOYO Linux Project [sourceforge.jp]
The project
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In that case actually, I'm selling SELinux for half off what the government charges. Interested in a purchase of some ISO's? I have some mirrors I can point you to if you send me a paypal payment of $500. Technical support will be handled over IRC on freenode.
Re:huh? (Score:4, Informative)
They don't; they give away the source code and it's been migrated into other distributions.
SELinux was started by the NSA, and they have a page about it here:
http://www.nsa.gov/selinux/ [nsa.gov]
They are pretty clear in their FAQ that SELinux was produced essentially as an internal product / demo, and they just thought other people might find it a useful starting place for securing Linux. They're not actively marketing it as a product, or even evangelizing it.
Parent
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The devs at Secure Computing, who wrote much of the code and who hold several patents covered by SELinux Type Enforcement, would beg to differ on this point. While they (grudgingly) accepted the release of SELinux, probably due to business concerns associated with suing a major and prestigious customer such as the NSA, they have never been all that happy about the open availability of the core concepts of their firewall product.
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Unclear who's to credit or blame. (Score:4, Interesting)
From SELinux FAQ #11 [nsa.gov]:Not sure I have a lot of sympathy for the SCC people; they got paid for what they delivered, and then the client decided to open it up.
It's not really clear what happened afterwards; it sounds like SCC might have threatened users of SELinux with their patents, or prepared to [lwn.net], but later on decided this was a Bad Move [linuxsecurity.com] --- it's not clear whether the NSA had a hand in convincing them of this, or it was a result of negative publicity from the Linux community, or what, but they eventually put out a statement [securecomputing.com] (PDF) to the effect that they wouldn't use their patents against users of the GPLed code.
Hard to unravel what the real story was at this point, or how much credit should go to SCC versus the NSA for cracking heads and getting the patent threat removed, but the ultimate outcome was certainly a positive one. But at any rate, since the NSA folks were the ones who ported it to Linux from the research OS, and turned it from an academic curiosity into something with practical applications, I'd say they deserve the lion's share.
Parent
Re:huh? (Score:4, Interesting)
The NSA wanted to do research into making a more secure Operating System. This is part of their mission. So instead of starting from scratch or trying to get access to the source of a proprietary OS they looked around and found an Open Source operating system called Linux. They had the source and an access active development community. When their research was done they released it back to the community just as the GPL says one should.
So now everyone that uses Linux can benefit from their research.
Just like NASA, NOAA, or any number of government agencies.
Parent
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umm, ok, so why is there a government linux project?
2) Hacking has gone from the script kidd13z messing with n00bs to a huge business of DOS and industrial espionage. Its really hard to mandate security, but if you make it simple and easy to use, you might make things more secure.