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Based on kernel version 4.1. Page generated on 2015-06-28 12:14 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 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	    - Basic utilities, which are helpful but not required
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	
27	Smack kernels use the CIPSO IP option. Some network
28	configurations are intolerant of IP options and can impede
29	access to systems that use them as Smack does.
30	
31	The current git repository for Smack user space is:
32	
33		git://github.com/smack-team/smack.git
34	
35	This should make and install on most modern distributions.
36	There are five commands included in smackutil:
37	
38	chsmack    - display or set Smack extended attribute values
39	smackctl   - load the Smack access rules
40	smackaccess - report if a process with one label has access
41	              to an object with another
42	
43	These two commands are obsolete with the introduction of
44	the smackfs/load2 and smackfs/cipso2 interfaces.
45	
46	smackload  - properly formats data for writing to smackfs/load
47	smackcipso - properly formats data for writing to smackfs/cipso
48	
49	In keeping with the intent of Smack, configuration data is
50	minimal and not strictly required. The most important
51	configuration step is mounting the smackfs pseudo filesystem.
52	If smackutil is installed the startup script will take care
53	of this, but it can be manually as well.
54	
55	Add this line to /etc/fstab:
56	
57	    smackfs /sys/fs/smackfs smackfs defaults 0 0
58	
59	The /sys/fs/smackfs directory is created by the kernel.
60	
61	Smack uses extended attributes (xattrs) to store labels on filesystem
62	objects. The attributes are stored in the extended attribute security
63	name space. A process must have CAP_MAC_ADMIN to change any of these
64	attributes.
65	
66	The extended attributes that Smack uses are:
67	
68	SMACK64
69		Used to make access control decisions. In almost all cases
70		the label given to a new filesystem object will be the label
71		of the process that created it.
72	SMACK64EXEC
73		The Smack label of a process that execs a program file with
74		this attribute set will run with this attribute's value.
75	SMACK64MMAP
76		Don't allow the file to be mmapped by a process whose Smack
77		label does not allow all of the access permitted to a process
78		with the label contained in this attribute. This is a very
79		specific use case for shared libraries.
80	SMACK64TRANSMUTE
81		Can only have the value "TRUE". If this attribute is present
82		on a directory when an object is created in the directory and
83		the Smack rule (more below) that permitted the write access
84		to the directory includes the transmute ("t") mode the object
85		gets the label of the directory instead of the label of the
86		creating process. If the object being created is a directory
87		the SMACK64TRANSMUTE attribute is set as well.
88	SMACK64IPIN
89		This attribute is only available on file descriptors for sockets.
90		Use the Smack label in this attribute for access control
91		decisions on packets being delivered to this socket.
92	SMACK64IPOUT
93		This attribute is only available on file descriptors for sockets.
94		Use the Smack label in this attribute for access control
95		decisions on packets coming from this socket.
96	
97	There are multiple ways to set a Smack label on a file:
98	
99	    # attr -S -s SMACK64 -V "value" path
100	    # chsmack -a value path
101	
102	A process can see the Smack label it is running with by
103	reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
104	can set the process Smack by writing there.
105	
106	Most Smack configuration is accomplished by writing to files
107	in the smackfs filesystem. This pseudo-filesystem is mounted
108	on /sys/fs/smackfs.
109	
110	access
111		This interface reports whether a subject with the specified
112		Smack label has a particular access to an object with a
113		specified Smack label. Write a fixed format access rule to
114		this file. The next read will indicate whether the access
115		would be permitted. The text will be either "1" indicating
116		access, or "0" indicating denial.
117	access2
118		This interface reports whether a subject with the specified
119		Smack label has a particular access to an object with a
120		specified Smack label. Write a long format access rule to
121		this file. The next read will indicate whether the access
122		would be permitted. The text will be either "1" indicating
123		access, or "0" indicating denial.
124	ambient
125		This contains the Smack label applied to unlabeled network
126		packets.
127	change-rule
128		This interface allows modification of existing access control rules.
129		The format accepted on write is:
130			"%s %s %s %s"
131		where the first string is the subject label, the second the
132		object label, the third the access to allow and the fourth the
133		access to deny. The access strings may contain only the characters
134		"rwxat-". If a rule for a given subject and object exists it will be
135		modified by enabling the permissions in the third string and disabling
136		those in the fourth string. If there is no such rule it will be
137		created using the access specified in the third and the fourth strings.
138	cipso
139		This interface allows a specific CIPSO header to be assigned
140		to a Smack label. The format accepted on write is:
141			"%24s%4d%4d"["%4d"]...
142		The first string is a fixed Smack label. The first number is
143		the level to use. The second number is the number of categories.
144		The following numbers are the categories.
145		"level-3-cats-5-19          3   2   5  19"
146	cipso2
147		This interface allows a specific CIPSO header to be assigned
148		to a Smack label. The format accepted on write is:
149		"%s%4d%4d"["%4d"]...
150		The first string is a long Smack label. The first number is
151		the level to use. The second number is the number of categories.
152		The following numbers are the categories.
153		"level-3-cats-5-19   3   2   5  19"
154	direct
155		This contains the CIPSO level used for Smack direct label
156		representation in network packets.
157	doi
158		This contains the CIPSO domain of interpretation used in
159		network packets.
160	load
161		This interface allows access control rules in addition to
162		the system defined rules to be specified. The format accepted
163		on write is:
164			"%24s%24s%5s"
165		where the first string is the subject label, the second the
166		object label, and the third the requested access. The access
167		string may contain only the characters "rwxat-", and specifies
168		which sort of access is allowed. The "-" is a placeholder for
169		permissions that are not allowed. The string "r-x--" would
170		specify read and execute access. Labels are limited to 23
171		characters in length.
172	load2
173		This interface allows access control rules in addition to
174		the system defined rules to be specified. The format accepted
175		on write is:
176			"%s %s %s"
177		where the first string is the subject label, the second the
178		object label, and the third the requested access. The access
179		string may contain only the characters "rwxat-", and specifies
180		which sort of access is allowed. The "-" is a placeholder for
181		permissions that are not allowed. The string "r-x--" would
182		specify read and execute access.
183	load-self
184		This interface allows process specific access rules to be
185		defined. These rules are only consulted if access would
186		otherwise be permitted, and are intended to provide additional
187		restrictions on the process. The format is the same as for
188		the load interface.
189	load-self2
190		This interface allows process specific access rules to be
191		defined. These rules are only consulted if access would
192		otherwise be permitted, and are intended to provide additional
193		restrictions on the process. The format is the same as for
194		the load2 interface.
195	logging
196		This contains the Smack logging state.
197	mapped
198		This contains the CIPSO level used for Smack mapped label
199		representation in network packets.
200	netlabel
201		This interface allows specific internet addresses to be
202		treated as single label hosts. Packets are sent to single
203		label hosts without CIPSO headers, but only from processes
204		that have Smack write access to the host label. All packets
205		received from single label hosts are given the specified
206		label. The format accepted on write is:
207			"%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
208	onlycap
209		This contains the label processes must have for CAP_MAC_ADMIN
210		and CAP_MAC_OVERRIDE to be effective. If this file is empty
211		these capabilities are effective at for processes with any
212		label. The value is set by writing the desired label to the
213		file or cleared by writing "-" to the file.
214	ptrace
215		This is used to define the current ptrace policy
216		0 - default: this is the policy that relies on Smack access rules.
217		    For the PTRACE_READ a subject needs to have a read access on
218		    object. For the PTRACE_ATTACH a read-write access is required.
219		1 - exact: this is the policy that limits PTRACE_ATTACH. Attach is
220		    only allowed when subject's and object's labels are equal.
221		    PTRACE_READ is not affected. Can be overridden with CAP_SYS_PTRACE.
222		2 - draconian: this policy behaves like the 'exact' above with an
223		    exception that it can't be overridden with CAP_SYS_PTRACE.
224	revoke-subject
225		Writing a Smack label here sets the access to '-' for all access
226		rules with that subject label.
227	unconfined
228		If the kernel is configured with CONFIG_SECURITY_SMACK_BRINGUP
229		a process with CAP_MAC_ADMIN can write a label into this interface.
230		Thereafter, accesses that involve that label will be logged and
231		the access permitted if it wouldn't be otherwise. Note that this
232		is dangerous and can ruin the proper labeling of your system.
233		It should never be used in production.
234	
235	You can add access rules in /etc/smack/accesses. They take the form:
236	
237	    subjectlabel objectlabel access
238	
239	access is a combination of the letters rwxatb which specify the
240	kind of access permitted a subject with subjectlabel on an
241	object with objectlabel. If there is no rule no access is allowed.
242	
243	Look for additional programs on http://schaufler-ca.com
244	
245	From the Smack Whitepaper:
246	
247	The Simplified Mandatory Access Control Kernel
248	
249	Casey Schaufler
250	casey@schaufler-ca.com
251	
252	Mandatory Access Control
253	
254	Computer systems employ a variety of schemes to constrain how information is
255	shared among the people and services using the machine. Some of these schemes
256	allow the program or user to decide what other programs or users are allowed
257	access to pieces of data. These schemes are called discretionary access
258	control mechanisms because the access control is specified at the discretion
259	of the user. Other schemes do not leave the decision regarding what a user or
260	program can access up to users or programs. These schemes are called mandatory
261	access control mechanisms because you don't have a choice regarding the users
262	or programs that have access to pieces of data.
263	
264	Bell & LaPadula
265	
266	From the middle of the 1980's until the turn of the century Mandatory Access
267	Control (MAC) was very closely associated with the Bell & LaPadula security
268	model, a mathematical description of the United States Department of Defense
269	policy for marking paper documents. MAC in this form enjoyed a following
270	within the Capital Beltway and Scandinavian supercomputer centers but was
271	often sited as failing to address general needs.
272	
273	Domain Type Enforcement
274	
275	Around the turn of the century Domain Type Enforcement (DTE) became popular.
276	This scheme organizes users, programs, and data into domains that are
277	protected from each other. This scheme has been widely deployed as a component
278	of popular Linux distributions. The administrative overhead required to
279	maintain this scheme and the detailed understanding of the whole system
280	necessary to provide a secure domain mapping leads to the scheme being
281	disabled or used in limited ways in the majority of cases.
282	
283	Smack
284	
285	Smack is a Mandatory Access Control mechanism designed to provide useful MAC
286	while avoiding the pitfalls of its predecessors. The limitations of Bell &
287	LaPadula are addressed by providing a scheme whereby access can be controlled
288	according to the requirements of the system and its purpose rather than those
289	imposed by an arcane government policy. The complexity of Domain Type
290	Enforcement and avoided by defining access controls in terms of the access
291	modes already in use.
292	
293	Smack Terminology
294	
295	The jargon used to talk about Smack will be familiar to those who have dealt
296	with other MAC systems and shouldn't be too difficult for the uninitiated to
297	pick up. There are four terms that are used in a specific way and that are
298	especially important:
299	
300		Subject: A subject is an active entity on the computer system.
301		On Smack a subject is a task, which is in turn the basic unit
302		of execution.
303	
304		Object: An object is a passive entity on the computer system.
305		On Smack files of all types, IPC, and tasks can be objects.
306	
307		Access: Any attempt by a subject to put information into or get
308		information from an object is an access.
309	
310		Label: Data that identifies the Mandatory Access Control
311		characteristics of a subject or an object.
312	
313	These definitions are consistent with the traditional use in the security
314	community. There are also some terms from Linux that are likely to crop up:
315	
316		Capability: A task that possesses a capability has permission to
317		violate an aspect of the system security policy, as identified by
318		the specific capability. A task that possesses one or more
319		capabilities is a privileged task, whereas a task with no
320		capabilities is an unprivileged task.
321	
322		Privilege: A task that is allowed to violate the system security
323		policy is said to have privilege. As of this writing a task can
324		have privilege either by possessing capabilities or by having an
325		effective user of root.
326	
327	Smack Basics
328	
329	Smack is an extension to a Linux system. It enforces additional restrictions
330	on what subjects can access which objects, based on the labels attached to
331	each of the subject and the object.
332	
333	Labels
334	
335	Smack labels are ASCII character strings. They can be up to 255 characters
336	long, but keeping them to twenty-three characters is recommended.
337	Single character labels using special characters, that being anything
338	other than a letter or digit, are reserved for use by the Smack development
339	team. Smack labels are unstructured, case sensitive, and the only operation
340	ever performed on them is comparison for equality. Smack labels cannot
341	contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
342	(quote) and '"' (double-quote) characters.
343	Smack labels cannot begin with a '-'. This is reserved for special options.
344	
345	There are some predefined labels:
346	
347		_ 	Pronounced "floor", a single underscore character.
348		^ 	Pronounced "hat", a single circumflex character.
349		* 	Pronounced "star", a single asterisk character.
350		? 	Pronounced "huh", a single question mark character.
351		@ 	Pronounced "web", a single at sign character.
352	
353	Every task on a Smack system is assigned a label. The Smack label
354	of a process will usually be assigned by the system initialization
355	mechanism.
356	
357	Access Rules
358	
359	Smack uses the traditional access modes of Linux. These modes are read,
360	execute, write, and occasionally append. There are a few cases where the
361	access mode may not be obvious. These include:
362	
363		Signals: A signal is a write operation from the subject task to
364		the object task.
365		Internet Domain IPC: Transmission of a packet is considered a
366		write operation from the source task to the destination task.
367	
368	Smack restricts access based on the label attached to a subject and the label
369	attached to the object it is trying to access. The rules enforced are, in
370	order:
371	
372		1. Any access requested by a task labeled "*" is denied.
373		2. A read or execute access requested by a task labeled "^"
374		   is permitted.
375		3. A read or execute access requested on an object labeled "_"
376		   is permitted.
377		4. Any access requested on an object labeled "*" is permitted.
378		5. Any access requested by a task on an object with the same
379		   label is permitted.
380		6. Any access requested that is explicitly defined in the loaded
381		   rule set is permitted.
382		7. Any other access is denied.
383	
384	Smack Access Rules
385	
386	With the isolation provided by Smack access separation is simple. There are
387	many interesting cases where limited access by subjects to objects with
388	different labels is desired. One example is the familiar spy model of
389	sensitivity, where a scientist working on a highly classified project would be
390	able to read documents of lower classifications and anything she writes will
391	be "born" highly classified. To accommodate such schemes Smack includes a
392	mechanism for specifying rules allowing access between labels.
393	
394	Access Rule Format
395	
396	The format of an access rule is:
397	
398		subject-label object-label access
399	
400	Where subject-label is the Smack label of the task, object-label is the Smack
401	label of the thing being accessed, and access is a string specifying the sort
402	of access allowed. The access specification is searched for letters that
403	describe access modes:
404	
405		a: indicates that append access should be granted.
406		r: indicates that read access should be granted.
407		w: indicates that write access should be granted.
408		x: indicates that execute access should be granted.
409		t: indicates that the rule requests transmutation.
410		b: indicates that the rule should be reported for bring-up.
411	
412	Uppercase values for the specification letters are allowed as well.
413	Access mode specifications can be in any order. Examples of acceptable rules
414	are:
415	
416		TopSecret Secret  rx
417		Secret    Unclass R
418		Manager   Game    x
419		User      HR      w
420		Snap      Crackle rwxatb
421		New       Old     rRrRr
422		Closed    Off     -
423	
424	Examples of unacceptable rules are:
425	
426		Top Secret Secret     rx
427		Ace        Ace        r
428		Odd        spells     waxbeans
429	
430	Spaces are not allowed in labels. Since a subject always has access to files
431	with the same label specifying a rule for that case is pointless. Only
432	valid letters (rwxatbRWXATB) and the dash ('-') character are allowed in
433	access specifications. The dash is a placeholder, so "a-r" is the same
434	as "ar". A lone dash is used to specify that no access should be allowed.
435	
436	Applying Access Rules
437	
438	The developers of Linux rarely define new sorts of things, usually importing
439	schemes and concepts from other systems. Most often, the other systems are
440	variants of Unix. Unix has many endearing properties, but consistency of
441	access control models is not one of them. Smack strives to treat accesses as
442	uniformly as is sensible while keeping with the spirit of the underlying
443	mechanism.
444	
445	File system objects including files, directories, named pipes, symbolic links,
446	and devices require access permissions that closely match those used by mode
447	bit access. To open a file for reading read access is required on the file. To
448	search a directory requires execute access. Creating a file with write access
449	requires both read and write access on the containing directory. Deleting a
450	file requires read and write access to the file and to the containing
451	directory. It is possible that a user may be able to see that a file exists
452	but not any of its attributes by the circumstance of having read access to the
453	containing directory but not to the differently labeled file. This is an
454	artifact of the file name being data in the directory, not a part of the file.
455	
456	If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
457	access rule that allows a process to create an object in that directory
458	includes 't' access the label assigned to the new object will be that
459	of the directory, not the creating process. This makes it much easier
460	for two processes with different labels to share data without granting
461	access to all of their files.
462	
463	IPC objects, message queues, semaphore sets, and memory segments exist in flat
464	namespaces and access requests are only required to match the object in
465	question.
466	
467	Process objects reflect tasks on the system and the Smack label used to access
468	them is the same Smack label that the task would use for its own access
469	attempts. Sending a signal via the kill() system call is a write operation
470	from the signaler to the recipient. Debugging a process requires both reading
471	and writing. Creating a new task is an internal operation that results in two
472	tasks with identical Smack labels and requires no access checks.
473	
474	Sockets are data structures attached to processes and sending a packet from
475	one process to another requires that the sender have write access to the
476	receiver. The receiver is not required to have read access to the sender.
477	
478	Setting Access Rules
479	
480	The configuration file /etc/smack/accesses contains the rules to be set at
481	system startup. The contents are written to the special file
482	/sys/fs/smackfs/load2. Rules can be added at any time and take effect
483	immediately. For any pair of subject and object labels there can be only
484	one rule, with the most recently specified overriding any earlier
485	specification.
486	
487	Task Attribute
488	
489	The Smack label of a process can be read from /proc/<pid>/attr/current. A
490	process can read its own Smack label from /proc/self/attr/current. A
491	privileged process can change its own Smack label by writing to
492	/proc/self/attr/current but not the label of another process.
493	
494	File Attribute
495	
496	The Smack label of a filesystem object is stored as an extended attribute
497	named SMACK64 on the file. This attribute is in the security namespace. It can
498	only be changed by a process with privilege.
499	
500	Privilege
501	
502	A process with CAP_MAC_OVERRIDE or CAP_MAC_ADMIN is privileged.
503	CAP_MAC_OVERRIDE allows the process access to objects it would
504	be denied otherwise. CAP_MAC_ADMIN allows a process to change
505	Smack data, including rules and attributes.
506	
507	Smack Networking
508	
509	As mentioned before, Smack enforces access control on network protocol
510	transmissions. Every packet sent by a Smack process is tagged with its Smack
511	label. This is done by adding a CIPSO tag to the header of the IP packet. Each
512	packet received is expected to have a CIPSO tag that identifies the label and
513	if it lacks such a tag the network ambient label is assumed. Before the packet
514	is delivered a check is made to determine that a subject with the label on the
515	packet has write access to the receiving process and if that is not the case
516	the packet is dropped.
517	
518	CIPSO Configuration
519	
520	It is normally unnecessary to specify the CIPSO configuration. The default
521	values used by the system handle all internal cases. Smack will compose CIPSO
522	label values to match the Smack labels being used without administrative
523	intervention. Unlabeled packets that come into the system will be given the
524	ambient label.
525	
526	Smack requires configuration in the case where packets from a system that is
527	not Smack that speaks CIPSO may be encountered. Usually this will be a Trusted
528	Solaris system, but there are other, less widely deployed systems out there.
529	CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
530	and a category set with each packet. The DOI is intended to identify a group
531	of systems that use compatible labeling schemes, and the DOI specified on the
532	Smack system must match that of the remote system or packets will be
533	discarded. The DOI is 3 by default. The value can be read from
534	/sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/doi.
535	
536	The label and category set are mapped to a Smack label as defined in
537	/etc/smack/cipso.
538	
539	A Smack/CIPSO mapping has the form:
540	
541		smack level [category [category]*]
542	
543	Smack does not expect the level or category sets to be related in any
544	particular way and does not assume or assign accesses based on them. Some
545	examples of mappings:
546	
547		TopSecret 7
548		TS:A,B    7 1 2
549		SecBDE    5 2 4 6
550		RAFTERS   7 12 26
551	
552	The ":" and "," characters are permitted in a Smack label but have no special
553	meaning.
554	
555	The mapping of Smack labels to CIPSO values is defined by writing to
556	/sys/fs/smackfs/cipso2.
557	
558	In addition to explicit mappings Smack supports direct CIPSO mappings. One
559	CIPSO level is used to indicate that the category set passed in the packet is
560	in fact an encoding of the Smack label. The level used is 250 by default. The
561	value can be read from /sys/fs/smackfs/direct and changed by writing to
562	/sys/fs/smackfs/direct.
563	
564	Socket Attributes
565	
566	There are two attributes that are associated with sockets. These attributes
567	can only be set by privileged tasks, but any task can read them for their own
568	sockets.
569	
570		SMACK64IPIN: The Smack label of the task object. A privileged
571		program that will enforce policy may set this to the star label.
572	
573		SMACK64IPOUT: The Smack label transmitted with outgoing packets.
574		A privileged program may set this to match the label of another
575		task with which it hopes to communicate.
576	
577	Smack Netlabel Exceptions
578	
579	You will often find that your labeled application has to talk to the outside,
580	unlabeled world. To do this there's a special file /sys/fs/smackfs/netlabel
581	where you can add some exceptions in the form of :
582	@IP1	   LABEL1 or
583	@IP2/MASK  LABEL2
584	
585	It means that your application will have unlabeled access to @IP1 if it has
586	write access on LABEL1, and access to the subnet @IP2/MASK if it has write
587	access on LABEL2.
588	
589	Entries in the /sys/fs/smackfs/netlabel file are matched by longest mask
590	first, like in classless IPv4 routing.
591	
592	A special label '@' and an option '-CIPSO' can be used there :
593	@      means Internet, any application with any label has access to it
594	-CIPSO means standard CIPSO networking
595	
596	If you don't know what CIPSO is and don't plan to use it, you can just do :
597	echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel
598	echo 0.0.0.0/0 @      > /sys/fs/smackfs/netlabel
599	
600	If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
601	Internet access, you can have :
602	echo 127.0.0.1      -CIPSO > /sys/fs/smackfs/netlabel
603	echo 192.168.0.0/16 -CIPSO > /sys/fs/smackfs/netlabel
604	echo 0.0.0.0/0      @      > /sys/fs/smackfs/netlabel
605	
606	
607	Writing Applications for Smack
608	
609	There are three sorts of applications that will run on a Smack system. How an
610	application interacts with Smack will determine what it will have to do to
611	work properly under Smack.
612	
613	Smack Ignorant Applications
614	
615	By far the majority of applications have no reason whatever to care about the
616	unique properties of Smack. Since invoking a program has no impact on the
617	Smack label associated with the process the only concern likely to arise is
618	whether the process has execute access to the program.
619	
620	Smack Relevant Applications
621	
622	Some programs can be improved by teaching them about Smack, but do not make
623	any security decisions themselves. The utility ls(1) is one example of such a
624	program.
625	
626	Smack Enforcing Applications
627	
628	These are special programs that not only know about Smack, but participate in
629	the enforcement of system policy. In most cases these are the programs that
630	set up user sessions. There are also network services that provide information
631	to processes running with various labels.
632	
633	File System Interfaces
634	
635	Smack maintains labels on file system objects using extended attributes. The
636	Smack label of a file, directory, or other file system object can be obtained
637	using getxattr(2).
638	
639		len = getxattr("/", "security.SMACK64", value, sizeof (value));
640	
641	will put the Smack label of the root directory into value. A privileged
642	process can set the Smack label of a file system object with setxattr(2).
643	
644		len = strlen("Rubble");
645		rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
646	
647	will set the Smack label of /foo to "Rubble" if the program has appropriate
648	privilege.
649	
650	Socket Interfaces
651	
652	The socket attributes can be read using fgetxattr(2).
653	
654	A privileged process can set the Smack label of outgoing packets with
655	fsetxattr(2).
656	
657		len = strlen("Rubble");
658		rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
659	
660	will set the Smack label "Rubble" on packets going out from the socket if the
661	program has appropriate privilege.
662	
663		rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
664	
665	will set the Smack label "*" as the object label against which incoming
666	packets will be checked if the program has appropriate privilege.
667	
668	Administration
669	
670	Smack supports some mount options:
671	
672		smackfsdef=label: specifies the label to give files that lack
673		the Smack label extended attribute.
674	
675		smackfsroot=label: specifies the label to assign the root of the
676		file system if it lacks the Smack extended attribute.
677	
678		smackfshat=label: specifies a label that must have read access to
679		all labels set on the filesystem. Not yet enforced.
680	
681		smackfsfloor=label: specifies a label to which all labels set on the
682		filesystem must have read access. Not yet enforced.
683	
684	These mount options apply to all file system types.
685	
686	Smack auditing
687	
688	If you want Smack auditing of security events, you need to set CONFIG_AUDIT
689	in your kernel configuration.
690	By default, all denied events will be audited. You can change this behavior by
691	writing a single character to the /sys/fs/smackfs/logging file :
692	0 : no logging
693	1 : log denied (default)
694	2 : log accepted
695	3 : log denied & accepted
696	
697	Events are logged as 'key=value' pairs, for each event you at least will get
698	the subject, the object, the rights requested, the action, the kernel function
699	that triggered the event, plus other pairs depending on the type of event
700	audited.
701	
702	Bringup Mode
703	
704	Bringup mode provides logging features that can make application
705	configuration and system bringup easier. Configure the kernel with
706	CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup
707	mode is enabled accesses that succeed due to rules marked with the "b"
708	access mode will logged. When a new label is introduced for processes
709	rules can be added aggressively, marked with the "b". The logging allows
710	tracking of which rules actual get used for that label.
711	
712	Another feature of bringup mode is the "unconfined" option. Writing
713	a label to /sys/fs/smackfs/unconfined makes subjects with that label
714	able to access any object, and objects with that label accessible to
715	all subjects. Any access that is granted because a label is unconfined
716	is logged. This feature is dangerous, as files and directories may
717	be created in places they couldn't if the policy were being enforced.
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