About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / Smack.txt




Custom Search

Based on kernel version 2.6.39.1. Page generated on 2011-06-03 13:47 EST.

1	
2	
3	    "Good for you, you've decided to clean the elevator!"
4	    - The Elevator, from Dark Star
5	
6	Smack is the the Simplified Mandatory Access Control Kernel.
7	Smack is a kernel based implementation of mandatory access
8	control that includes simplicity in its primary design goals.
9	
10	Smack is not the only Mandatory Access Control scheme
11	available for Linux. Those new to Mandatory Access Control
12	are encouraged to compare Smack with the other mechanisms
13	available to determine which is best suited to the problem
14	at hand.
15	
16	Smack consists of three major components:
17	    - The kernel
18	    - A start-up script and a few modified applications
19	    - Configuration data
20	
21	The kernel component of Smack is implemented as a Linux
22	Security Modules (LSM) module. It requires netlabel and
23	works best with file systems that support extended attributes,
24	although xattr support is not strictly required.
25	It is safe to run a Smack kernel under a "vanilla" distribution.
26	Smack kernels use the CIPSO IP option. Some network
27	configurations are intolerant of IP options and can impede
28	access to systems that use them as Smack does.
29	
30	The startup script etc-init.d-smack should be installed
31	in /etc/init.d/smack and should be invoked early in the
32	start-up process. On Fedora rc5.d/S02smack is recommended.
33	This script ensures that certain devices have the correct
34	Smack attributes and loads the Smack configuration if
35	any is defined. This script invokes two programs that
36	ensure configuration data is properly formatted. These
37	programs are /usr/sbin/smackload and /usr/sin/smackcipso.
38	The system will run just fine without these programs,
39	but it will be difficult to set access rules properly.
40	
41	A version of "ls" that provides a "-M" option to display
42	Smack labels on long listing is available.
43	
44	A hacked version of sshd that allows network logins by users
45	with specific Smack labels is available. This version does
46	not work for scp. You must set the /etc/ssh/sshd_config
47	line:
48	   UsePrivilegeSeparation no
49	
50	The format of /etc/smack/usr is:
51	
52	   username smack
53	
54	In keeping with the intent of Smack, configuration data is
55	minimal and not strictly required. The most important
56	configuration step is mounting the smackfs pseudo filesystem.
57	
58	Add this line to /etc/fstab:
59	
60	    smackfs /smack smackfs smackfsdef=* 0 0
61	
62	and create the /smack directory for mounting.
63	
64	Smack uses extended attributes (xattrs) to store file labels.
65	The command to set a Smack label on a file is:
66	
67	    # attr -S -s SMACK64 -V "value" path
68	
69	NOTE: Smack labels are limited to 23 characters. The attr command
70	      does not enforce this restriction and can be used to set
71	      invalid Smack labels on files.
72	
73	If you don't do anything special all users will get the floor ("_")
74	label when they log in. If you do want to log in via the hacked ssh
75	at other labels use the attr command to set the smack value on the
76	home directory and its contents.
77	
78	You can add access rules in /etc/smack/accesses. They take the form:
79	
80	    subjectlabel objectlabel access
81	
82	access is a combination of the letters rwxa which specify the
83	kind of access permitted a subject with subjectlabel on an
84	object with objectlabel. If there is no rule no access is allowed.
85	
86	A process can see the smack label it is running with by
87	reading /proc/self/attr/current. A privileged process can
88	set the process smack by writing there.
89	
90	Look for additional programs on http://schaufler-ca.com
91	
92	From the Smack Whitepaper:
93	
94	The Simplified Mandatory Access Control Kernel
95	
96	Casey Schaufler
97	casey@schaufler-ca.com
98	
99	Mandatory Access Control
100	
101	Computer systems employ a variety of schemes to constrain how information is
102	shared among the people and services using the machine. Some of these schemes
103	allow the program or user to decide what other programs or users are allowed
104	access to pieces of data. These schemes are called discretionary access
105	control mechanisms because the access control is specified at the discretion
106	of the user. Other schemes do not leave the decision regarding what a user or
107	program can access up to users or programs. These schemes are called mandatory
108	access control mechanisms because you don't have a choice regarding the users
109	or programs that have access to pieces of data.
110	
111	Bell & LaPadula
112	
113	From the middle of the 1980's until the turn of the century Mandatory Access
114	Control (MAC) was very closely associated with the Bell & LaPadula security
115	model, a mathematical description of the United States Department of Defense
116	policy for marking paper documents. MAC in this form enjoyed a following
117	within the Capital Beltway and Scandinavian supercomputer centers but was
118	often sited as failing to address general needs.
119	
120	Domain Type Enforcement
121	
122	Around the turn of the century Domain Type Enforcement (DTE) became popular.
123	This scheme organizes users, programs, and data into domains that are
124	protected from each other. This scheme has been widely deployed as a component
125	of popular Linux distributions. The administrative overhead required to
126	maintain this scheme and the detailed understanding of the whole system
127	necessary to provide a secure domain mapping leads to the scheme being
128	disabled or used in limited ways in the majority of cases.
129	
130	Smack
131	
132	Smack is a Mandatory Access Control mechanism designed to provide useful MAC
133	while avoiding the pitfalls of its predecessors. The limitations of Bell &
134	LaPadula are addressed by providing a scheme whereby access can be controlled
135	according to the requirements of the system and its purpose rather than those
136	imposed by an arcane government policy. The complexity of Domain Type
137	Enforcement and avoided by defining access controls in terms of the access
138	modes already in use.
139	
140	Smack Terminology
141	
142	The jargon used to talk about Smack will be familiar to those who have dealt
143	with other MAC systems and shouldn't be too difficult for the uninitiated to
144	pick up. There are four terms that are used in a specific way and that are
145	especially important:
146	
147		Subject: A subject is an active entity on the computer system.
148		On Smack a subject is a task, which is in turn the basic unit
149		of execution.
150	
151		Object: An object is a passive entity on the computer system.
152		On Smack files of all types, IPC, and tasks can be objects.
153	
154		Access: Any attempt by a subject to put information into or get
155		information from an object is an access.
156	
157		Label: Data that identifies the Mandatory Access Control
158		characteristics of a subject or an object.
159	
160	These definitions are consistent with the traditional use in the security
161	community. There are also some terms from Linux that are likely to crop up:
162	
163		Capability: A task that possesses a capability has permission to
164		violate an aspect of the system security policy, as identified by
165		the specific capability. A task that possesses one or more
166		capabilities is a privileged task, whereas a task with no
167		capabilities is an unprivileged task.
168	
169		Privilege: A task that is allowed to violate the system security
170		policy is said to have privilege. As of this writing a task can
171		have privilege either by possessing capabilities or by having an
172		effective user of root.
173	
174	Smack Basics
175	
176	Smack is an extension to a Linux system. It enforces additional restrictions
177	on what subjects can access which objects, based on the labels attached to
178	each of the subject and the object.
179	
180	Labels
181	
182	Smack labels are ASCII character strings, one to twenty-three characters in
183	length. Single character labels using special characters, that being anything
184	other than a letter or digit, are reserved for use by the Smack development
185	team. Smack labels are unstructured, case sensitive, and the only operation
186	ever performed on them is comparison for equality. Smack labels cannot
187	contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
188	(quote) and '"' (double-quote) characters.
189	Smack labels cannot begin with a '-', which is reserved for special options.
190	
191	There are some predefined labels:
192	
193		_ 	Pronounced "floor", a single underscore character.
194		^ 	Pronounced "hat", a single circumflex character.
195		* 	Pronounced "star", a single asterisk character.
196		? 	Pronounced "huh", a single question mark character.
197		@ 	Pronounced "Internet", a single at sign character.
198	
199	Every task on a Smack system is assigned a label. System tasks, such as
200	init(8) and systems daemons, are run with the floor ("_") label. User tasks
201	are assigned labels according to the specification found in the
202	/etc/smack/user configuration file.
203	
204	Access Rules
205	
206	Smack uses the traditional access modes of Linux. These modes are read,
207	execute, write, and occasionally append. There are a few cases where the
208	access mode may not be obvious. These include:
209	
210		Signals: A signal is a write operation from the subject task to
211		the object task.
212		Internet Domain IPC: Transmission of a packet is considered a
213		write operation from the source task to the destination task.
214	
215	Smack restricts access based on the label attached to a subject and the label
216	attached to the object it is trying to access. The rules enforced are, in
217	order:
218	
219		1. Any access requested by a task labeled "*" is denied.
220		2. A read or execute access requested by a task labeled "^"
221		   is permitted.
222		3. A read or execute access requested on an object labeled "_"
223		   is permitted.
224		4. Any access requested on an object labeled "*" is permitted.
225		5. Any access requested by a task on an object with the same
226		   label is permitted.
227		6. Any access requested that is explicitly defined in the loaded
228		   rule set is permitted.
229		7. Any other access is denied.
230	
231	Smack Access Rules
232	
233	With the isolation provided by Smack access separation is simple. There are
234	many interesting cases where limited access by subjects to objects with
235	different labels is desired. One example is the familiar spy model of
236	sensitivity, where a scientist working on a highly classified project would be
237	able to read documents of lower classifications and anything she writes will
238	be "born" highly classified. To accommodate such schemes Smack includes a
239	mechanism for specifying rules allowing access between labels.
240	
241	Access Rule Format
242	
243	The format of an access rule is:
244	
245		subject-label object-label access
246	
247	Where subject-label is the Smack label of the task, object-label is the Smack
248	label of the thing being accessed, and access is a string specifying the sort
249	of access allowed. The Smack labels are limited to 23 characters. The access
250	specification is searched for letters that describe access modes:
251	
252		a: indicates that append access should be granted.
253		r: indicates that read access should be granted.
254		w: indicates that write access should be granted.
255		x: indicates that execute access should be granted.
256	
257	Uppercase values for the specification letters are allowed as well.
258	Access mode specifications can be in any order. Examples of acceptable rules
259	are:
260	
261		TopSecret Secret  rx
262		Secret    Unclass R
263		Manager   Game    x
264		User      HR      w
265		New       Old     rRrRr
266		Closed    Off     -
267	
268	Examples of unacceptable rules are:
269	
270		Top Secret Secret     rx
271		Ace        Ace        r
272		Odd        spells     waxbeans
273	
274	Spaces are not allowed in labels. Since a subject always has access to files
275	with the same label specifying a rule for that case is pointless. Only
276	valid letters (rwxaRWXA) and the dash ('-') character are allowed in
277	access specifications. The dash is a placeholder, so "a-r" is the same
278	as "ar". A lone dash is used to specify that no access should be allowed.
279	
280	Applying Access Rules
281	
282	The developers of Linux rarely define new sorts of things, usually importing
283	schemes and concepts from other systems. Most often, the other systems are
284	variants of Unix. Unix has many endearing properties, but consistency of
285	access control models is not one of them. Smack strives to treat accesses as
286	uniformly as is sensible while keeping with the spirit of the underlying
287	mechanism.
288	
289	File system objects including files, directories, named pipes, symbolic links,
290	and devices require access permissions that closely match those used by mode
291	bit access. To open a file for reading read access is required on the file. To
292	search a directory requires execute access. Creating a file with write access
293	requires both read and write access on the containing directory. Deleting a
294	file requires read and write access to the file and to the containing
295	directory. It is possible that a user may be able to see that a file exists
296	but not any of its attributes by the circumstance of having read access to the
297	containing directory but not to the differently labeled file. This is an
298	artifact of the file name being data in the directory, not a part of the file.
299	
300	IPC objects, message queues, semaphore sets, and memory segments exist in flat
301	namespaces and access requests are only required to match the object in
302	question.
303	
304	Process objects reflect tasks on the system and the Smack label used to access
305	them is the same Smack label that the task would use for its own access
306	attempts. Sending a signal via the kill() system call is a write operation
307	from the signaler to the recipient. Debugging a process requires both reading
308	and writing. Creating a new task is an internal operation that results in two
309	tasks with identical Smack labels and requires no access checks.
310	
311	Sockets are data structures attached to processes and sending a packet from
312	one process to another requires that the sender have write access to the
313	receiver. The receiver is not required to have read access to the sender.
314	
315	Setting Access Rules
316	
317	The configuration file /etc/smack/accesses contains the rules to be set at
318	system startup. The contents are written to the special file /smack/load.
319	Rules can be written to /smack/load at any time and take effect immediately.
320	For any pair of subject and object labels there can be only one rule, with the
321	most recently specified overriding any earlier specification.
322	
323	The program smackload is provided to ensure data is formatted
324	properly when written to /smack/load. This program reads lines
325	of the form
326	
327	    subjectlabel objectlabel mode.
328	
329	Task Attribute
330	
331	The Smack label of a process can be read from /proc/<pid>/attr/current. A
332	process can read its own Smack label from /proc/self/attr/current. A
333	privileged process can change its own Smack label by writing to
334	/proc/self/attr/current but not the label of another process.
335	
336	File Attribute
337	
338	The Smack label of a filesystem object is stored as an extended attribute
339	named SMACK64 on the file. This attribute is in the security namespace. It can
340	only be changed by a process with privilege.
341	
342	Privilege
343	
344	A process with CAP_MAC_OVERRIDE is privileged.
345	
346	Smack Networking
347	
348	As mentioned before, Smack enforces access control on network protocol
349	transmissions. Every packet sent by a Smack process is tagged with its Smack
350	label. This is done by adding a CIPSO tag to the header of the IP packet. Each
351	packet received is expected to have a CIPSO tag that identifies the label and
352	if it lacks such a tag the network ambient label is assumed. Before the packet
353	is delivered a check is made to determine that a subject with the label on the
354	packet has write access to the receiving process and if that is not the case
355	the packet is dropped.
356	
357	CIPSO Configuration
358	
359	It is normally unnecessary to specify the CIPSO configuration. The default
360	values used by the system handle all internal cases. Smack will compose CIPSO
361	label values to match the Smack labels being used without administrative
362	intervention. Unlabeled packets that come into the system will be given the
363	ambient label.
364	
365	Smack requires configuration in the case where packets from a system that is
366	not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
367	Solaris system, but there are other, less widely deployed systems out there.
368	CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
369	and a category set with each packet. The DOI is intended to identify a group
370	of systems that use compatible labeling schemes, and the DOI specified on the
371	smack system must match that of the remote system or packets will be
372	discarded. The DOI is 3 by default. The value can be read from /smack/doi and
373	can be changed by writing to /smack/doi.
374	
375	The label and category set are mapped to a Smack label as defined in
376	/etc/smack/cipso.
377	
378	A Smack/CIPSO mapping has the form:
379	
380		smack level [category [category]*]
381	
382	Smack does not expect the level or category sets to be related in any
383	particular way and does not assume or assign accesses based on them. Some
384	examples of mappings:
385	
386		TopSecret 7
387		TS:A,B    7 1 2
388		SecBDE    5 2 4 6
389		RAFTERS   7 12 26
390	
391	The ":" and "," characters are permitted in a Smack label but have no special
392	meaning.
393	
394	The mapping of Smack labels to CIPSO values is defined by writing to
395	/smack/cipso. Again, the format of data written to this special file
396	is highly restrictive, so the program smackcipso is provided to
397	ensure the writes are done properly. This program takes mappings
398	on the standard input and sends them to /smack/cipso properly.
399	
400	In addition to explicit mappings Smack supports direct CIPSO mappings. One
401	CIPSO level is used to indicate that the category set passed in the packet is
402	in fact an encoding of the Smack label. The level used is 250 by default. The
403	value can be read from /smack/direct and changed by writing to /smack/direct.
404	
405	Socket Attributes
406	
407	There are two attributes that are associated with sockets. These attributes
408	can only be set by privileged tasks, but any task can read them for their own
409	sockets.
410	
411		SMACK64IPIN: The Smack label of the task object. A privileged
412		program that will enforce policy may set this to the star label.
413	
414		SMACK64IPOUT: The Smack label transmitted with outgoing packets.
415		A privileged program may set this to match the label of another
416		task with which it hopes to communicate.
417	
418	Smack Netlabel Exceptions
419	
420	You will often find that your labeled application has to talk to the outside,
421	unlabeled world. To do this there's a special file /smack/netlabel where you can
422	add some exceptions in the form of :
423	@IP1	   LABEL1 or
424	@IP2/MASK  LABEL2
425	
426	It means that your application will have unlabeled access to @IP1 if it has
427	write access on LABEL1, and access to the subnet @IP2/MASK if it has write
428	access on LABEL2.
429	
430	Entries in the /smack/netlabel file are matched by longest mask first, like in
431	classless IPv4 routing.
432	
433	A special label '@' and an option '-CIPSO' can be used there :
434	@      means Internet, any application with any label has access to it
435	-CIPSO means standard CIPSO networking
436	
437	If you don't know what CIPSO is and don't plan to use it, you can just do :
438	echo 127.0.0.1 -CIPSO > /smack/netlabel
439	echo 0.0.0.0/0 @      > /smack/netlabel
440	
441	If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
442	Internet access, you can have :
443	echo 127.0.0.1      -CIPSO > /smack/netlabel
444	echo 192.168.0.0/16 -CIPSO > /smack/netlabel
445	echo 0.0.0.0/0      @      > /smack/netlabel
446	
447	
448	Writing Applications for Smack
449	
450	There are three sorts of applications that will run on a Smack system. How an
451	application interacts with Smack will determine what it will have to do to
452	work properly under Smack.
453	
454	Smack Ignorant Applications
455	
456	By far the majority of applications have no reason whatever to care about the
457	unique properties of Smack. Since invoking a program has no impact on the
458	Smack label associated with the process the only concern likely to arise is
459	whether the process has execute access to the program.
460	
461	Smack Relevant Applications
462	
463	Some programs can be improved by teaching them about Smack, but do not make
464	any security decisions themselves. The utility ls(1) is one example of such a
465	program.
466	
467	Smack Enforcing Applications
468	
469	These are special programs that not only know about Smack, but participate in
470	the enforcement of system policy. In most cases these are the programs that
471	set up user sessions. There are also network services that provide information
472	to processes running with various labels.
473	
474	File System Interfaces
475	
476	Smack maintains labels on file system objects using extended attributes. The
477	Smack label of a file, directory, or other file system object can be obtained
478	using getxattr(2).
479	
480		len = getxattr("/", "security.SMACK64", value, sizeof (value));
481	
482	will put the Smack label of the root directory into value. A privileged
483	process can set the Smack label of a file system object with setxattr(2).
484	
485		len = strlen("Rubble");
486		rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
487	
488	will set the Smack label of /foo to "Rubble" if the program has appropriate
489	privilege.
490	
491	Socket Interfaces
492	
493	The socket attributes can be read using fgetxattr(2).
494	
495	A privileged process can set the Smack label of outgoing packets with
496	fsetxattr(2).
497	
498		len = strlen("Rubble");
499		rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
500	
501	will set the Smack label "Rubble" on packets going out from the socket if the
502	program has appropriate privilege.
503	
504		rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
505	
506	will set the Smack label "*" as the object label against which incoming
507	packets will be checked if the program has appropriate privilege.
508	
509	Administration
510	
511	Smack supports some mount options:
512	
513		smackfsdef=label: specifies the label to give files that lack
514		the Smack label extended attribute.
515	
516		smackfsroot=label: specifies the label to assign the root of the
517		file system if it lacks the Smack extended attribute.
518	
519		smackfshat=label: specifies a label that must have read access to
520		all labels set on the filesystem. Not yet enforced.
521	
522		smackfsfloor=label: specifies a label to which all labels set on the
523		filesystem must have read access. Not yet enforced.
524	
525	These mount options apply to all file system types.
526	
527	Smack auditing
528	
529	If you want Smack auditing of security events, you need to set CONFIG_AUDIT
530	in your kernel configuration.
531	By default, all denied events will be audited. You can change this behavior by
532	writing a single character to the /smack/logging file :
533	0 : no logging
534	1 : log denied (default)
535	2 : log accepted
536	3 : log denied & accepted
537	
538	Events are logged as 'key=value' pairs, for each event you at least will get
539	the subjet, the object, the rights requested, the action, the kernel function
540	that triggered the event, plus other pairs depending on the type of event
541	audited.
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.