SELinux Basics (2008-11-08 classroom)

Fedora Classroom - SELinux Basics - Clint Savage - Saturday, November 7, 2008

• PDF Slides
• Impress Slides

IRC Log of the Class

-!- nirik changed the topic of #fedora-classroom to: Fedora Classroom - Introduction - See Communicate/IRC/Classroom for more info		20:00
@nirik	A few general guidelines: Please try to keep on topic... if you have general fedora questions, #fedora is open for business as usual.	20:01
@nirik	If you want some more social chatting, #fedora-social is open for that.	20:01
@nirik	Some teachers may want you to hold questions, and some will want you to just chime in... they will say when they start their session.	20:01
Guest86715	nick brunowolff	20:02
Guest86715	\nick brunowolff	20:02
@nirik	Also, note that I will be logging the classes for posting on the wiki.	20:02
@nirik	So, our first class up today is SElinux Basics. Without further jabbering, I will hand things off to herlo...	20:02
herlo	Hi all, my name is Clint Savage, and I am North American Fedora Ambassador western USA region. I work for a small Linux training company in Utah called Guru Labs. People online call me herlo	20:02
herlo	thanks nirik	20:02
herlo	so for those of you who might have missed it, I have slides up	20:03
herlo	Classroom/SELinux_Basics	20:03
herlo	from that link you can get either pdf or odp	20:03
-!- nirik changed the topic of #fedora-classroom to: Fedora Classroom - SElinux Basics with your teacher: herlo - See Communicate/IRC/Classroom for more info		20:03
herlo	I'll be pretty much following the flow there. If you have questions, please feel free to jump in.	20:03
herlo	SELinux Basics	20:04
herlo	What is SELinux?	20:04
linuxguru	Security Enhanced Linux	20:05
herlo	Well, a few years back, the NSA designed a set of rules that would help in keeping their confidential information safe. One fo the major functionalities that came out of this was SELinux	20:05
herlo	linuxguru: right, Security Enhanced Linux	20:05
herlo	one of the things that is interesting about security in Linux is the many ways to protect your boxen	20:06
herlo	Firewalls, acls, etc.	20:06
herlo	iptables has been around for some time and does a great job on the network	20:06
herlo	thing is, it's really intended for network security	20:07
herlo	so that's one layer	20:07
herlo	but we want more layers	20:07
herlo	In Unix we've always had the rwx permissions, which has been pretty good to us	20:07
herlo	processes check the permissions of a file and make sure they have rights to access the file.	20:08
herlo	SELinux is just a layer above.	20:08
herlo	SELinux can protect local filesystems even better, providing tools to make it easy to use the applications without fear of attacks on the system	20:09
herlo	So two terms came about DAC and MAC	20:09
herlo	I have two slides describing both	20:09
herlo	DAC - Discretionary Access Control	20:09
VileGent	! when you change page say page please	20:09
thomasj	:D	20:09
herlo	This is traditional Linux/Unix type file perms	20:09
herlo	VileGent: k, that was the first change, and thank you	20:10
JamesB192_thekky	and ACLs?	20:10
herlo	the simple rwx permissions, SUID SGID, etc	20:10
herlo	JamesB192_thekky: ACLs stands for Access Control Lists and is a supplementary feature of many filesystems	20:10
herlo	as well as many other applications too	20:11
herlo	the thing about DAC is that it's really what we've been using for 20+ years	20:11
herlo	nothing has inherently changed about it	20:11
herlo	it's pretty much the same it was back then and will continue to do a good job of protecting our boxen	20:11
* nirik notes that this is page 3 on the pdf.		20:11
herlo	but here's the thing	20:12
herlo	What about processes accessing thing that while they have permissions to access, shouldn't be accessing	20:12
herlo	?	20:12
koolhead1	?	20:12
bomama	what?	20:12
domg472_	privilege escalation	20:13
herlo	For instance, should the named (DNS daemon) be accessing files within apache?	20:13
Abd4llA	nop	20:13
linuxguru	naw.	20:13
herlo	domg472_: right, something we don't want to happen	20:13
herlo	next page	20:13
herlo	this is where MAC - Mandatory Access Control comes in...	20:13
brunowolff	I think the main point is that without selinux any process you run has all of your access rights. You don't always want to have that.	20:13
herlo	brunowolff: correct	20:13
herlo	brunowolff: many processes, not all	20:14
LinuxCode	shouldnt questions/comments be directed at the end ?	20:14
herlo	and it's possible that the process could perform an exploit on an unsecured application	20:14
LinuxCode	wont get through the class otherwise	20:14
herlo	LinuxCode: it's fine, questions are good...	20:14
herlo	LinuxCode: we will...	20:14
LinuxCode	k ;-]	20:14
herlo	we're doing fine right now	20:14
herlo	so MAC	20:14
koolhead1	+1	20:14
herlo	provides this functionality where instead of standard permissions, we have what's typically called a security context	20:15
herlo	this security context is part of a policy	20:15
herlo	and the policy defines the rules as to which processes can access which files	20:15
linuxguru	one question here. regarding unconfined processes (server stuff) running on the system	20:16
herlo	this also goes for ports, links, and many other elements in a Linux system	20:16
herlo	linuxguru: we'll get to that in a minute	20:16
linuxguru	okay	20:16
herlo	so the policy says, here's the rule for that process accessing that file, if it's allowed, then the normal permissions apply	20:16
herlo	if, however, that process is not allowed by policy it is denied	20:17
herlo	also, if there is no policy rule for that particular process/file, the action is denied	20:17
herlo	next page	20:17
herlo	in comes security contexts	20:17
herlo	this is page 5, btw	20:18
herlo	each process has a context and each file has a context	20:18
herlo	in general, we can think of each of the components as another layer where the policy can enforce rules	20:18
herlo	user:role:type:sensitivity:category	20:18
herlo	the policy can look at any one fo these component parts of the context and evaluate whether the process can access the file based upon user, a specific role, type, sensitivity or some category definition	20:19
herlo	you might note that if you run 'ls -Z' on your home directory you'd see something like this	20:20
herlo	$ ls -Z	20:20
herlo	-rw-r--r-- clints clints system_u:object_r:user_home_dir_t:s0 (2).bash_logout	20:20
herlo	-rw-rw-r-- clints clints unconfined_u:object_r:user_home_t:s0 attendees-200808200.odb	20:20
herlo	-rw-rw-r-- clints clints unconfined_u:object_r:user_home_t:s0 attendees-20080820.odb	20:20
herlo	drwxr-xr-x clints clints unconfined_u:object_r:user_home_t:s0 bin	20:20
herlo	lrwxrwxrwx clints clints system_u:object_r:user_home_t:s0 Books -> /data/books	20:20
domg472_	.bash_logout seems mislabeled	20:21
herlo	Fedora has implemented everything but the category, but older systems slowly built up from the first three components and are adding slowly	20:21
daMaestro	.bash_logout is ok	20:21
herlo	domg472_: probably not, but we'll talk about how to change that shortly...	20:21
daMaestro	;-)	20:21
herlo	processes can also be looked at similarly	20:21
herlo	ps -ef -Z | grep httpd	20:22
herlo	unconfined_u:system_r:httpd_t:s0 root 6740 1 0 09:30 ? 00:00:00 /usr/sbin/httpd	20:22
herlo	unconfined_u:system_r:httpd_t:s0 apache 6742 6740 0 09:30 ? 00:00:00 /usr/sbin/httpd	20:22
herlo	unconfined_u:system_r:httpd_t:s0 apache 6743 6740 0 09:30 ? 00:00:00 /usr/sbin/httpd	20:22
herlo	adding the -Z in either case can provide the context information.	20:22
herlo	let me do one more listing of files here	20:22
herlo	ls -Z /var/www/html/	20:23
herlo	-rw-r--r-- root root unconfined_u:object_r:httpd_sys_content_t:s0 darkice-ubuntu.cfg	20:23
herlo	-rw------- apache apache system_u:object_r:httpd_sys_content_t:s0 F8.ks	20:23
herlo	-rw-r--r-- apache apache system_u:object_r:httpd_sys_content_t:s0 F8VM.ks	20:23
herlo	the ubuntu file is there because I record my local user groups... :)	20:23
herlo	anyway,	20:23
herlo	one thing you'll notice is the similarity in a couple areas between the processes adn the files...	20:23
Abd4llA	will we get to the meanings for the differents tags ?	20:23
herlo	Abd4llA: which tags?	20:23
Abd4llA	object_r , system_r ..etc	20:24
Abd4llA	in the context	20:24
daMaestro	unconfined_u:system_r:httpd_t:s0 -> chcon [OPTION]... [-u USER] [-r ROLE] [-l RANGE] [-t TYPE] FILE	20:24
herlo	Abd4llA: yes, we will	20:24
herlo	daMaestro: we'll get to that soon	20:24
daMaestro	k	20:24
herlo	Abd4llA: essentially, the _r stuff implies it's a role component	20:25
herlo	the _u is for users	20:25
herlo	_t is for type, etc...	20:25
herlo	and it has to do with which policy is affecting what parts of the SELinux context	20:25
herlo	next slide - Default Policy: Targeted	20:26
Abd4llA	k	20:26
herlo	when policies are in force, you can look through them in the /selinux virtual filesystem	20:26
herlo	as I recall, these are read-only	20:26
herlo	sorry, having a bit of network issues, bear with me	20:27
herlo	but the /selinux dir is fun to peruse and can teach you a lot about the policy	20:27
herlo	the policy Fedora installs by default is called the 'Targeted' policy	20:27
herlo	and primarily uses _t (or type) enforcement	20:28
domg472_	and rbac	20:28
herlo	going back a bit to my example of the apache DocumentRoot /var/ww/html and the httpd process, one might now notice that the processes and files have very similar types	20:28
herlo	unconfined_u:system_r:httpd_t:s0 root 6740 1 0 09:30 ? 00:00:00 /usr/sbin/httpd	20:29
herlo	-rw------- apache apache system_u:object_r:httpd_sys_content_t:s0 F8.ks	20:29
herlo	in the policy it says, httpd_t processes can access httpd_sys_content_t type files...	20:29
herlo	next slide: Manipulating Contexts	20:30
herlo	but sometimes, the contexts are incorrect in the files	20:30
herlo	and thus the proper process cannot access the file even though its permissions are correct and its in the correct directory	20:31
herlo	this is where chcon and restorecon come in	20:31
domg472_	run it on .bash_logout for example	20:31
herlo	chcon can modify user, role, type, sensitivity, category on a particular file. Kind of think of it as the chown/chmod for SELinux	20:31
herlo	domg472_: right	20:32
herlo	domg472_: in that case, what I'd want to do is run restorecon, because it would follow the policy rules	20:32
domg472_	chcon is for unprivileged users, prifileged user should use semanage	20:32
domg472_	chcon is not persistent	20:32
jds2001	domg472_: it is.	20:32
domg472_	restorecon will override	20:32
jds2001	oh yes.	20:32
jds2001	or a filesystem relabel	20:33
herlo	# restorecon .bash_logout	20:33
herlo	[root@herlo-lap clints]# ls -Z .bash_logout	20:33
herlo	-rw-r--r-- clints clints unconfined_u:object_r:user_home_t:s0 .bash_logout	20:33
herlo	doplease don't get ahead of where wwe are	20:33
domg472_	ok	20:33
herlo	semanage will work too, but we're not there yet	20:33
herlo	this is SELinux Basics.. semanage can be used for much more advanced stuffs	20:34
herlo	domg472_: however, you are right about chcon vs restorecon for who can use it...	20:34
herlo	next slide: Manage/Modify the Policy	20:35
nuonguy	herlo: question: how does restorecon know what the context for .bash_logout should be?	20:36
herlo	essentially, the policy can be in one of three states	20:36
herlo	nuonguy: the policy knows	20:36
herlo	nuonguy: I'll show where you can get that information in a short bit	20:36
nuonguy	k, thanks	20:36
herlo	nuonguy: but to be honest, you don't actually need to know the policy to be effective with SELinux	20:36
herlo	which sounds strange, but it's true	20:37
herlo	however, we show it here shortly	20:37
nuonguy	even if I need to install an app that provides no selinux/conext info?	20:37
herlo	the first command 'getenforce' will show you the state of enforcement SELinux is currently in	20:37
herlo	nuonguy: sure, but we're not going to broach that today	20:37
herlo	nuonguy: however, I will show you where you can set those rules	20:38
nuonguy	awesome, thanks	20:38
herlo	# getenforce	20:38
herlo	Permissive	20:38
herlo	to toggle between Permissive and Enforcing, one could use setenforce	20:39
herlo	# setenforce 1	20:39
herlo	[root@herlo-lap ~]# getenforce	20:39
herlo	Enforcing	20:39
herlo	However, Disabled can also appear	20:39
herlo	when getenforce is run	20:40
herlo	but it must be set and then a reboot will remove the labels (security contexts) from the system...	20:40
herlo	next slide: making the policy persist	20:40
herlo	this is where everyone has been jumping to	20:41
herlo	system-config-selinux is a very nice gui that can manage much of what you'd like to see in an SELinux policy	20:41
herlo	it can set enforcing, Permissive, Disabled for boot,	20:41
herlo	it can modify booleans, or small parts of the policy	20:42
herlo	it can also show you what contexts files/ports/links/etc will have when restorecon is run	20:42
herlo	as well as allow you to modify the policy rules right there	20:43
herlo	that's under File Labeling / User Mapping / Network Ports and probably a few others	20:43
jMCg	I suppose it already has some sensible templates for often used services.	20:44
herlo	in addition, you can modify /etc/sysconfig/selinux and set the policy and/or Enforcement	20:44
herlo	jMCg: it does, for type enforcement only	20:44
herlo	but there are other policies, including strict (which most others are based upon) and Multi-Layer Security (MLS)	20:44
brunowolff	nuonguy, part of the policy is a set of patterns used be restorecon to decide which is the correct label. The patterns aren't used when creating files normally.	20:44
herlo	which you can import and install. These use more of the tags of the context..	20:44
herlo	brunowolff: yes, correct. Thanks	20:45
herlo	you can also relabel the system according to the changes made here	20:45
herlo	another tool listed on this page is semanage	20:45
herlo	semanage can do many things including many of hte things that system-config-selinux does	20:46
herlo	it's the command line tool to make policy components stick, including context changes	20:46
herlo	the best thing I can say about semanage is that it has an excellent man page and shows examples of many things you can do to your system...	20:46
herlo	getsebool/setsebool are also useful tools if you know the boolean you'd like to change	20:47
herlo	next slide: Troubleshooting	20:47
herlo	this is my favorite part	20:47
herlo	as it says in the slide, many people turn SELinux off because they can't understand the avc messages in the logs	20:48
herlo	/var/log/audit/audit.log shows many of these messages and an experienced SELinux user can learn what these things mean	20:49
fengshaun	excuse me, where can we get the slides?	20:49
herlo	but most people have a hard time reading them...	20:49
herlo	fengshaun: Classroom/SELinux_Basics	20:49
fengshaun	thank you	20:49
herlo	np	20:49
herlo	so here's the tool that will make it easier than ever to read those messages	20:49
herlo	setroubleshoot	20:49
herlo	the daemon /usr/sbin/setroubleshootd, available in the setroubleshoot-server rpm	20:50
herlo	is my favorite friend	20:50
herlo	it's a sysV service that provides clear text solutions for allowing access when something doesn't work right	20:50
herlo	because odds are, the user's permissions are correct, but SELinux is causing some sort of issue	20:51
herlo	so I install setroubleshoot-server	20:51
herlo	next slide: troubleshooting cont'd	20:51
herlo	page 11	20:51
herlo	and then run	20:52
herlo	/etc/init.d/setroubleshoot start	20:52
herlo	all of the sudden, I get clear messages in /var/log/messages	20:52
domg472_	setroubleshoot is enabled by default	20:52
herlo	domg472_: setroubleshoot is the client tools	20:52
herlo	setroubleshoot-server might be enabled by default, I hadn't checked	20:52
* thomasj reminds domg472_ that this is herlo's class. So please let him teach, he's doing a great job.		20:53
domg472_	ok	20:53
herlo	but it's going to tell you to look at a specific sealert message	20:53
herlo	copying the sealert command along with ath ugly long string...	20:53
herlo	and voila, you have a solution as to how to allow access.	20:53
herlo	for those of you who like gui's try sealert -b	20:54
herlo	that's the sealert browser and it can also be launched from the Notification Area (the little star) in GNOME	20:54
herlo	NOW	20:54
herlo	here's the onlyt thing I want to warn you on	20:54
herlo	don't by any circumstances take 'Allowing Access' to mean that you *should* perform the task listed there	20:55
herlo	instead, you should use your critical minds and make a smart decision regarding whether allowing access is the right thing to do	20:55
herlo	so I'm out of material and it looks like out of time	20:56
herlo	any questions about this process?	20:56
jMCg	Ad troubleshooting.	20:56
kdn	Great job; thanks.	20:56
jMCg	What troubles me most, is to remember that there's SELinux, and it could be responsible.	20:56
* VileGent gives herlo a hand and thanks		20:56
thomasj	+1	20:56
* erinlea80 applauds!		20:57
kdn	+1	20:57
poti	+1	20:57
djohngo	herlo: Thanks!	20:57
domg472_	thanks	20:57
* fengshaun applauds too!		20:57
jds2001	great job herlo :)	20:57
brunowolff	The resources didn't include Dan Walsh's journal (http://danwalsh.livejournal.com/) which has up to date info about selinux.	20:57
thomasj	herlo, awesome, thank you very much	20:57
SSlater	+1	20:57
herlo	jMCg: yes, I understand that, but it will become much more normal as you get used to it	20:57
* JMakey thanks herlo		20:57
herlo	brunowolff: oh, yes, I should add that	20:57
jMCg	When you get an EACCESS, you think of permissions, it'd be great if there was some different class of error to be used...	20:57
Bugz	herlo: Very good, thanks	20:57
linuxguru	yeah i had this doubt about unconfined processes running on my system. if a attacker is able to compromise my system using those processes (which have ports opened) will he be able to access stuff which is managed by selinux like confined processes such apache/samba etc.	20:57
herlo	I'm glad you all liked it	20:57
zless	thanks herlo.	20:57
domg472_	if anyone has questions about selinux join #fedora-selinux and/or #selinux	20:57
fengshaun	but date -u gives me 19:58! is my clock wrong?	20:57
Ineluctable	thank you	20:57
jds2001	fengshaun: an hour off.	20:58
fengshaun	jds2001, oh god!	20:58
herlo	linuxguru: right, so I'd consider a tighter policy or modify the policy to adjust the unconfined processes	20:58
@nirik	thanks herlo !	20:58
herlo	linuxguru: so they aren't unconfined	20:58
SSlater	?	20:58
herlo	SSlater: go	20:58
zless	i'd just like to say that selinux has be "in the background" in f9 (for the desktop) much more than previously. the #1 thing i need to tweak is allowing firefox to use notstandard ports.	20:58
SSlater	Why does sealert sometimes give a proposed solution and othertimes Not?	20:59
zless	e.g.: semanage port -a -t http_port -p tcp 8880	20:59
domg472_	that not a valid type zless	20:59
stmg_	thanks so mucj	20:59
zless	domg472_, _t	20:59
* nirik notes the next class up is Jon Stanley ( jds2001 ) - An introduction to Bugzilla		20:59
jds2001	httpd_t would be in that case.	20:59
stmg_	*much	20:59
zless	domg472_, synergyc isn't up here atm, so had to go from memory	21:00
domg472_	join #fedora-selinux for details	21:00
herlo	SSlater: I believe that if the policy knows how to solve it the solution can be given. If the policy writer didn't anticipate that sort of thing, it's kind of hard to give a solution	21:00
Dufflepod	Thanks herlo	21:00
herlo	ciao all	21:00
herlo	on to jds2001	21:00

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