1 =====
2 Smack
3 =====
4
5
6 "Good for you, you've decided to clean the elevator!"
7 - The Elevator, from Dark Star
8
9 Smack is the Simplified Mandatory Access Control Kernel.
10 Smack is a kernel based implementation of mandatory access
11 control that includes simplicity in its primary design goals.
12
13 Smack is not the only Mandatory Access Control scheme
14 available for Linux. Those new to Mandatory Access Control
15 are encouraged to compare Smack with the other mechanisms
16 available to determine which is best suited to the problem
17 at hand.
18
19 Smack consists of three major components:
20
21 - The kernel
22 - Basic utilities, which are helpful but not required
23 - Configuration data
24
25 The kernel component of Smack is implemented as a Linux
26 Security Modules (LSM) module. It requires netlabel and
27 works best with file systems that support extended attributes,
28 although xattr support is not strictly required.
29 It is safe to run a Smack kernel under a "vanilla" distribution.
30
31 Smack kernels use the CIPSO IP option. Some network
32 configurations are intolerant of IP options and can impede
33 access to systems that use them as Smack does.
34
35 Smack is used in the Tizen operating system. Please
36 go to http://wiki.tizen.org for information about how
37 Smack is used in Tizen.
38
39 The current git repository for Smack user space is:
40
41 git://github.com/smack-team/smack.git
42
43 This should make and install on most modern distributions.
44 There are five commands included in smackutil:
45
46 chsmack:
47 display or set Smack extended attribute values
48
49 smackctl:
50 load the Smack access rules
51
52 smackaccess:
53 report if a process with one label has access
54 to an object with another
55
56 These two commands are obsolete with the introduction of
57 the smackfs/load2 and smackfs/cipso2 interfaces.
58
59 smackload:
60 properly formats data for writing to smackfs/load
61
62 smackcipso:
63 properly formats data for writing to smackfs/cipso
64
65 In keeping with the intent of Smack, configuration data is
66 minimal and not strictly required. The most important
67 configuration step is mounting the smackfs pseudo filesystem.
68 If smackutil is installed the startup script will take care
69 of this, but it can be manually as well.
70
71 Add this line to ``/etc/fstab``::
72
73 smackfs /sys/fs/smackfs smackfs defaults 0 0
74
75 The ``/sys/fs/smackfs`` directory is created by the kernel.
76
77 Smack uses extended attributes (xattrs) to store labels on filesystem
78 objects. The attributes are stored in the extended attribute security
79 name space. A process must have ``CAP_MAC_ADMIN`` to change any of these
80 attributes.
81
82 The extended attributes that Smack uses are:
83
84 SMACK64
85 Used to make access control decisions. In almost all cases
86 the label given to a new filesystem object will be the label
87 of the process that created it.
88
89 SMACK64EXEC
90 The Smack label of a process that execs a program file with
91 this attribute set will run with this attribute's value.
92
93 SMACK64MMAP
94 Don't allow the file to be mmapped by a process whose Smack
95 label does not allow all of the access permitted to a process
96 with the label contained in this attribute. This is a very
97 specific use case for shared libraries.
98
99 SMACK64TRANSMUTE
100 Can only have the value "TRUE". If this attribute is present
101 on a directory when an object is created in the directory and
102 the Smack rule (more below) that permitted the write access
103 to the directory includes the transmute ("t") mode the object
104 gets the label of the directory instead of the label of the
105 creating process. If the object being created is a directory
106 the SMACK64TRANSMUTE attribute is set as well.
107
108 SMACK64IPIN
109 This attribute is only available on file descriptors for sockets.
110 Use the Smack label in this attribute for access control
111 decisions on packets being delivered to this socket.
112
113 SMACK64IPOUT
114 This attribute is only available on file descriptors for sockets.
115 Use the Smack label in this attribute for access control
116 decisions on packets coming from this socket.
117
118 There are multiple ways to set a Smack label on a file::
119
120 # attr -S -s SMACK64 -V "value" path
121 # chsmack -a value path
122
123 A process can see the Smack label it is running with by
124 reading ``/proc/self/attr/current``. A process with ``CAP_MAC_ADMIN``
125 can set the process Smack by writing there.
126
127 Most Smack configuration is accomplished by writing to files
128 in the smackfs filesystem. This pseudo-filesystem is mounted
129 on ``/sys/fs/smackfs``.
130
131 access
132 Provided for backward compatibility. The access2 interface
133 is preferred and should be used instead.
134 This interface reports whether a subject with the specified
135 Smack label has a particular access to an object with a
136 specified Smack label. Write a fixed format access rule to
137 this file. The next read will indicate whether the access
138 would be permitted. The text will be either "1" indicating
139 access, or "0" indicating denial.
140
141 access2
142 This interface reports whether a subject with the specified
143 Smack label has a particular access to an object with a
144 specified Smack label. Write a long format access rule to
145 this file. The next read will indicate whether the access
146 would be permitted. The text will be either "1" indicating
147 access, or "0" indicating denial.
148
149 ambient
150 This contains the Smack label applied to unlabeled network
151 packets.
152
153 change-rule
154 This interface allows modification of existing access control rules.
155 The format accepted on write is::
156
157 "%s %s %s %s"
158
159 where the first string is the subject label, the second the
160 object label, the third the access to allow and the fourth the
161 access to deny. The access strings may contain only the characters
162 "rwxat-". If a rule for a given subject and object exists it will be
163 modified by enabling the permissions in the third string and disabling
164 those in the fourth string. If there is no such rule it will be
165 created using the access specified in the third and the fourth strings.
166
167 cipso
168 Provided for backward compatibility. The cipso2 interface
169 is preferred and should be used instead.
170 This interface allows a specific CIPSO header to be assigned
171 to a Smack label. The format accepted on write is::
172
173 "%24s%4d%4d"["%4d"]...
174
175 The first string is a fixed Smack label. The first number is
176 the level to use. The second number is the number of categories.
177 The following numbers are the categories::
178
179 "level-3-cats-5-19 3 2 5 19"
180
181 cipso2
182 This interface allows a specific CIPSO header to be assigned
183 to a Smack label. The format accepted on write is::
184
185 "%s%4d%4d"["%4d"]...
186
187 The first string is a long Smack label. The first number is
188 the level to use. The second number is the number of categories.
189 The following numbers are the categories::
190
191 "level-3-cats-5-19 3 2 5 19"
192
193 direct
194 This contains the CIPSO level used for Smack direct label
195 representation in network packets.
196
197 doi
198 This contains the CIPSO domain of interpretation used in
199 network packets.
200
201 ipv6host
202 This interface allows specific IPv6 internet addresses to be
203 treated as single label hosts. Packets are sent to single
204 label hosts only from processes that have Smack write access
205 to the host label. All packets received from single label hosts
206 are given the specified label. The format accepted on write is::
207
208 "%h:%h:%h:%h:%h:%h:%h:%h label" or
209 "%h:%h:%h:%h:%h:%h:%h:%h/%d label".
210
211 The "::" address shortcut is not supported.
212 If label is "-DELETE" a matched entry will be deleted.
213
214 load
215 Provided for backward compatibility. The load2 interface
216 is preferred and should be used instead.
217 This interface allows access control rules in addition to
218 the system defined rules to be specified. The format accepted
219 on write is::
220
221 "%24s%24s%5s"
222
223 where the first string is the subject label, the second the
224 object label, and the third the requested access. The access
225 string may contain only the characters "rwxat-", and specifies
226 which sort of access is allowed. The "-" is a placeholder for
227 permissions that are not allowed. The string "r-x--" would
228 specify read and execute access. Labels are limited to 23
229 characters in length.
230
231 load2
232 This interface allows access control rules in addition to
233 the system defined rules to be specified. The format accepted
234 on write is::
235
236 "%s %s %s"
237
238 where the first string is the subject label, the second the
239 object label, and the third the requested access. The access
240 string may contain only the characters "rwxat-", and specifies
241 which sort of access is allowed. The "-" is a placeholder for
242 permissions that are not allowed. The string "r-x--" would
243 specify read and execute access.
244
245 load-self
246 Provided for backward compatibility. The load-self2 interface
247 is preferred and should be used instead.
248 This interface allows process specific access rules to be
249 defined. These rules are only consulted if access would
250 otherwise be permitted, and are intended to provide additional
251 restrictions on the process. The format is the same as for
252 the load interface.
253
254 load-self2
255 This interface allows process specific access rules to be
256 defined. These rules are only consulted if access would
257 otherwise be permitted, and are intended to provide additional
258 restrictions on the process. The format is the same as for
259 the load2 interface.
260
261 logging
262 This contains the Smack logging state.
263
264 mapped
265 This contains the CIPSO level used for Smack mapped label
266 representation in network packets.
267
268 netlabel
269 This interface allows specific internet addresses to be
270 treated as single label hosts. Packets are sent to single
271 label hosts without CIPSO headers, but only from processes
272 that have Smack write access to the host label. All packets
273 received from single label hosts are given the specified
274 label. The format accepted on write is::
275
276 "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
277
278 If the label specified is "-CIPSO" the address is treated
279 as a host that supports CIPSO headers.
280
281 onlycap
282 This contains labels processes must have for CAP_MAC_ADMIN
283 and ``CAP_MAC_OVERRIDE`` to be effective. If this file is empty
284 these capabilities are effective at for processes with any
285 label. The values are set by writing the desired labels, separated
286 by spaces, to the file or cleared by writing "-" to the file.
287
288 ptrace
289 This is used to define the current ptrace policy
290
291 0 - default:
292 this is the policy that relies on Smack access rules.
293 For the ``PTRACE_READ`` a subject needs to have a read access on
294 object. For the ``PTRACE_ATTACH`` a read-write access is required.
295
296 1 - exact:
297 this is the policy that limits ``PTRACE_ATTACH``. Attach is
298 only allowed when subject's and object's labels are equal.
299 ``PTRACE_READ`` is not affected. Can be overridden with ``CAP_SYS_PTRACE``.
300
301 2 - draconian:
302 this policy behaves like the 'exact' above with an
303 exception that it can't be overridden with ``CAP_SYS_PTRACE``.
304
305 revoke-subject
306 Writing a Smack label here sets the access to '-' for all access
307 rules with that subject label.
308
309 unconfined
310 If the kernel is configured with ``CONFIG_SECURITY_SMACK_BRINGUP``
311 a process with ``CAP_MAC_ADMIN`` can write a label into this interface.
312 Thereafter, accesses that involve that label will be logged and
313 the access permitted if it wouldn't be otherwise. Note that this
314 is dangerous and can ruin the proper labeling of your system.
315 It should never be used in production.
316
317 relabel-self
318 This interface contains a list of labels to which the process can
319 transition to, by writing to ``/proc/self/attr/current``.
320 Normally a process can change its own label to any legal value, but only
321 if it has ``CAP_MAC_ADMIN``. This interface allows a process without
322 ``CAP_MAC_ADMIN`` to relabel itself to one of labels from predefined list.
323 A process without ``CAP_MAC_ADMIN`` can change its label only once. When it
324 does, this list will be cleared.
325 The values are set by writing the desired labels, separated
326 by spaces, to the file or cleared by writing "-" to the file.
327
328 If you are using the smackload utility
329 you can add access rules in ``/etc/smack/accesses``. They take the form::
330
331 subjectlabel objectlabel access
332
333 access is a combination of the letters rwxatb which specify the
334 kind of access permitted a subject with subjectlabel on an
335 object with objectlabel. If there is no rule no access is allowed.
336
337 Look for additional programs on http://schaufler-ca.com
338
339 The Simplified Mandatory Access Control Kernel (Whitepaper)
340 ===========================================================
341
342 Casey Schaufler
343 casey@schaufler-ca.com
344
345 Mandatory Access Control
346 ------------------------
347
348 Computer systems employ a variety of schemes to constrain how information is
349 shared among the people and services using the machine. Some of these schemes
350 allow the program or user to decide what other programs or users are allowed
351 access to pieces of data. These schemes are called discretionary access
352 control mechanisms because the access control is specified at the discretion
353 of the user. Other schemes do not leave the decision regarding what a user or
354 program can access up to users or programs. These schemes are called mandatory
355 access control mechanisms because you don't have a choice regarding the users
356 or programs that have access to pieces of data.
357
358 Bell & LaPadula
359 ---------------
360
361 From the middle of the 1980's until the turn of the century Mandatory Access
362 Control (MAC) was very closely associated with the Bell & LaPadula security
363 model, a mathematical description of the United States Department of Defense
364 policy for marking paper documents. MAC in this form enjoyed a following
365 within the Capital Beltway and Scandinavian supercomputer centers but was
366 often sited as failing to address general needs.
367
368 Domain Type Enforcement
369 -----------------------
370
371 Around the turn of the century Domain Type Enforcement (DTE) became popular.
372 This scheme organizes users, programs, and data into domains that are
373 protected from each other. This scheme has been widely deployed as a component
374 of popular Linux distributions. The administrative overhead required to
375 maintain this scheme and the detailed understanding of the whole system
376 necessary to provide a secure domain mapping leads to the scheme being
377 disabled or used in limited ways in the majority of cases.
378
379 Smack
380 -----
381
382 Smack is a Mandatory Access Control mechanism designed to provide useful MAC
383 while avoiding the pitfalls of its predecessors. The limitations of Bell &
384 LaPadula are addressed by providing a scheme whereby access can be controlled
385 according to the requirements of the system and its purpose rather than those
386 imposed by an arcane government policy. The complexity of Domain Type
387 Enforcement and avoided by defining access controls in terms of the access
388 modes already in use.
389
390 Smack Terminology
391 -----------------
392
393 The jargon used to talk about Smack will be familiar to those who have dealt
394 with other MAC systems and shouldn't be too difficult for the uninitiated to
395 pick up. There are four terms that are used in a specific way and that are
396 especially important:
397
398 Subject:
399 A subject is an active entity on the computer system.
400 On Smack a subject is a task, which is in turn the basic unit
401 of execution.
402
403 Object:
404 An object is a passive entity on the computer system.
405 On Smack files of all types, IPC, and tasks can be objects.
406
407 Access:
408 Any attempt by a subject to put information into or get
409 information from an object is an access.
410
411 Label:
412 Data that identifies the Mandatory Access Control
413 characteristics of a subject or an object.
414
415 These definitions are consistent with the traditional use in the security
416 community. There are also some terms from Linux that are likely to crop up:
417
418 Capability:
419 A task that possesses a capability has permission to
420 violate an aspect of the system security policy, as identified by
421 the specific capability. A task that possesses one or more
422 capabilities is a privileged task, whereas a task with no
423 capabilities is an unprivileged task.
424
425 Privilege:
426 A task that is allowed to violate the system security
427 policy is said to have privilege. As of this writing a task can
428 have privilege either by possessing capabilities or by having an
429 effective user of root.
430
431 Smack Basics
432 ------------
433
434 Smack is an extension to a Linux system. It enforces additional restrictions
435 on what subjects can access which objects, based on the labels attached to
436 each of the subject and the object.
437
438 Labels
439 ~~~~~~
440
441 Smack labels are ASCII character strings. They can be up to 255 characters
442 long, but keeping them to twenty-three characters is recommended.
443 Single character labels using special characters, that being anything
444 other than a letter or digit, are reserved for use by the Smack development
445 team. Smack labels are unstructured, case sensitive, and the only operation
446 ever performed on them is comparison for equality. Smack labels cannot
447 contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
448 (quote) and '"' (double-quote) characters.
449 Smack labels cannot begin with a '-'. This is reserved for special options.
450
451 There are some predefined labels::
452
453 _ Pronounced "floor", a single underscore character.
454 ^ Pronounced "hat", a single circumflex character.
455 * Pronounced "star", a single asterisk character.
456 ? Pronounced "huh", a single question mark character.
457 @ Pronounced "web", a single at sign character.
458
459 Every task on a Smack system is assigned a label. The Smack label
460 of a process will usually be assigned by the system initialization
461 mechanism.
462
463 Access Rules
464 ~~~~~~~~~~~~
465
466 Smack uses the traditional access modes of Linux. These modes are read,
467 execute, write, and occasionally append. There are a few cases where the
468 access mode may not be obvious. These include:
469
470 Signals:
471 A signal is a write operation from the subject task to
472 the object task.
473
474 Internet Domain IPC:
475 Transmission of a packet is considered a
476 write operation from the source task to the destination task.
477
478 Smack restricts access based on the label attached to a subject and the label
479 attached to the object it is trying to access. The rules enforced are, in
480 order:
481
482 1. Any access requested by a task labeled "*" is denied.
483 2. A read or execute access requested by a task labeled "^"
484 is permitted.
485 3. A read or execute access requested on an object labeled "_"
486 is permitted.
487 4. Any access requested on an object labeled "*" is permitted.
488 5. Any access requested by a task on an object with the same
489 label is permitted.
490 6. Any access requested that is explicitly defined in the loaded
491 rule set is permitted.
492 7. Any other access is denied.
493
494 Smack Access Rules
495 ~~~~~~~~~~~~~~~~~~
496
497 With the isolation provided by Smack access separation is simple. There are
498 many interesting cases where limited access by subjects to objects with
499 different labels is desired. One example is the familiar spy model of
500 sensitivity, where a scientist working on a highly classified project would be
501 able to read documents of lower classifications and anything she writes will
502 be "born" highly classified. To accommodate such schemes Smack includes a
503 mechanism for specifying rules allowing access between labels.
504
505 Access Rule Format
506 ~~~~~~~~~~~~~~~~~~
507
508 The format of an access rule is::
509
510 subject-label object-label access
511
512 Where subject-label is the Smack label of the task, object-label is the Smack
513 label of the thing being accessed, and access is a string specifying the sort
514 of access allowed. The access specification is searched for letters that
515 describe access modes:
516
517 a: indicates that append access should be granted.
518 r: indicates that read access should be granted.
519 w: indicates that write access should be granted.
520 x: indicates that execute access should be granted.
521 t: indicates that the rule requests transmutation.
522 b: indicates that the rule should be reported for bring-up.
523
524 Uppercase values for the specification letters are allowed as well.
525 Access mode specifications can be in any order. Examples of acceptable rules
526 are::
527
528 TopSecret Secret rx
529 Secret Unclass R
530 Manager Game x
531 User HR w
532 Snap Crackle rwxatb
533 New Old rRrRr
534 Closed Off -
535
536 Examples of unacceptable rules are::
537
538 Top Secret Secret rx
539 Ace Ace r
540 Odd spells waxbeans
541
542 Spaces are not allowed in labels. Since a subject always has access to files
543 with the same label specifying a rule for that case is pointless. Only
544 valid letters (rwxatbRWXATB) and the dash ('-') character are allowed in
545 access specifications. The dash is a placeholder, so "a-r" is the same
546 as "ar". A lone dash is used to specify that no access should be allowed.
547
548 Applying Access Rules
549 ~~~~~~~~~~~~~~~~~~~~~
550
551 The developers of Linux rarely define new sorts of things, usually importing
552 schemes and concepts from other systems. Most often, the other systems are
553 variants of Unix. Unix has many endearing properties, but consistency of
554 access control models is not one of them. Smack strives to treat accesses as
555 uniformly as is sensible while keeping with the spirit of the underlying
556 mechanism.
557
558 File system objects including files, directories, named pipes, symbolic links,
559 and devices require access permissions that closely match those used by mode
560 bit access. To open a file for reading read access is required on the file. To
561 search a directory requires execute access. Creating a file with write access
562 requires both read and write access on the containing directory. Deleting a
563 file requires read and write access to the file and to the containing
564 directory. It is possible that a user may be able to see that a file exists
565 but not any of its attributes by the circumstance of having read access to the
566 containing directory but not to the differently labeled file. This is an
567 artifact of the file name being data in the directory, not a part of the file.
568
569 If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
570 access rule that allows a process to create an object in that directory
571 includes 't' access the label assigned to the new object will be that
572 of the directory, not the creating process. This makes it much easier
573 for two processes with different labels to share data without granting
574 access to all of their files.
575
576 IPC objects, message queues, semaphore sets, and memory segments exist in flat
577 namespaces and access requests are only required to match the object in
578 question.
579
580 Process objects reflect tasks on the system and the Smack label used to access
581 them is the same Smack label that the task would use for its own access
582 attempts. Sending a signal via the kill() system call is a write operation
583 from the signaler to the recipient. Debugging a process requires both reading
584 and writing. Creating a new task is an internal operation that results in two
585 tasks with identical Smack labels and requires no access checks.
586
587 Sockets are data structures attached to processes and sending a packet from
588 one process to another requires that the sender have write access to the
589 receiver. The receiver is not required to have read access to the sender.
590
591 Setting Access Rules
592 ~~~~~~~~~~~~~~~~~~~~
593
594 The configuration file /etc/smack/accesses contains the rules to be set at
595 system startup. The contents are written to the special file
596 /sys/fs/smackfs/load2. Rules can be added at any time and take effect
597 immediately. For any pair of subject and object labels there can be only
598 one rule, with the most recently specified overriding any earlier
599 specification.
600
601 Task Attribute
602 ~~~~~~~~~~~~~~
603
604 The Smack label of a process can be read from /proc/<pid>/attr/current. A
605 process can read its own Smack label from /proc/self/attr/current. A
606 privileged process can change its own Smack label by writing to
607 /proc/self/attr/current but not the label of another process.
608
609 File Attribute
610 ~~~~~~~~~~~~~~
611
612 The Smack label of a filesystem object is stored as an extended attribute
613 named SMACK64 on the file. This attribute is in the security namespace. It can
614 only be changed by a process with privilege.
615
616 Privilege
617 ~~~~~~~~~
618
619 A process with CAP_MAC_OVERRIDE or CAP_MAC_ADMIN is privileged.
620 CAP_MAC_OVERRIDE allows the process access to objects it would
621 be denied otherwise. CAP_MAC_ADMIN allows a process to change
622 Smack data, including rules and attributes.
623
624 Smack Networking
625 ~~~~~~~~~~~~~~~~
626
627 As mentioned before, Smack enforces access control on network protocol
628 transmissions. Every packet sent by a Smack process is tagged with its Smack
629 label. This is done by adding a CIPSO tag to the header of the IP packet. Each
630 packet received is expected to have a CIPSO tag that identifies the label and
631 if it lacks such a tag the network ambient label is assumed. Before the packet
632 is delivered a check is made to determine that a subject with the label on the
633 packet has write access to the receiving process and if that is not the case
634 the packet is dropped.
635
636 CIPSO Configuration
637 ~~~~~~~~~~~~~~~~~~~
638
639 It is normally unnecessary to specify the CIPSO configuration. The default
640 values used by the system handle all internal cases. Smack will compose CIPSO
641 label values to match the Smack labels being used without administrative
642 intervention. Unlabeled packets that come into the system will be given the
643 ambient label.
644
645 Smack requires configuration in the case where packets from a system that is
646 not Smack that speaks CIPSO may be encountered. Usually this will be a Trusted
647 Solaris system, but there are other, less widely deployed systems out there.
648 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
649 and a category set with each packet. The DOI is intended to identify a group
650 of systems that use compatible labeling schemes, and the DOI specified on the
651 Smack system must match that of the remote system or packets will be
652 discarded. The DOI is 3 by default. The value can be read from
653 /sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/doi.
654
655 The label and category set are mapped to a Smack label as defined in
656 /etc/smack/cipso.
657
658 A Smack/CIPSO mapping has the form::
659
660 smack level [category [category]*]
661
662 Smack does not expect the level or category sets to be related in any
663 particular way and does not assume or assign accesses based on them. Some
664 examples of mappings::
665
666 TopSecret 7
667 TS:A,B 7 1 2
668 SecBDE 5 2 4 6
669 RAFTERS 7 12 26
670
671 The ":" and "," characters are permitted in a Smack label but have no special
672 meaning.
673
674 The mapping of Smack labels to CIPSO values is defined by writing to
675 /sys/fs/smackfs/cipso2.
676
677 In addition to explicit mappings Smack supports direct CIPSO mappings. One
678 CIPSO level is used to indicate that the category set passed in the packet is
679 in fact an encoding of the Smack label. The level used is 250 by default. The
680 value can be read from /sys/fs/smackfs/direct and changed by writing to
681 /sys/fs/smackfs/direct.
682
683 Socket Attributes
684 ~~~~~~~~~~~~~~~~~
685
686 There are two attributes that are associated with sockets. These attributes
687 can only be set by privileged tasks, but any task can read them for their own
688 sockets.
689
690 SMACK64IPIN:
691 The Smack label of the task object. A privileged
692 program that will enforce policy may set this to the star label.
693
694 SMACK64IPOUT:
695 The Smack label transmitted with outgoing packets.
696 A privileged program may set this to match the label of another
697 task with which it hopes to communicate.
698
699 Smack Netlabel Exceptions
700 ~~~~~~~~~~~~~~~~~~~~~~~~~
701
702 You will often find that your labeled application has to talk to the outside,
703 unlabeled world. To do this there's a special file /sys/fs/smackfs/netlabel
704 where you can add some exceptions in the form of::
705
706 @IP1 LABEL1 or
707 @IP2/MASK LABEL2
708
709 It means that your application will have unlabeled access to @IP1 if it has
710 write access on LABEL1, and access to the subnet @IP2/MASK if it has write
711 access on LABEL2.
712
713 Entries in the /sys/fs/smackfs/netlabel file are matched by longest mask
714 first, like in classless IPv4 routing.
715
716 A special label '@' and an option '-CIPSO' can be used there::
717
718 @ means Internet, any application with any label has access to it
719 -CIPSO means standard CIPSO networking
720
721 If you don't know what CIPSO is and don't plan to use it, you can just do::
722
723 echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel
724 echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel
725
726 If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
727 Internet access, you can have::
728
729 echo 127.0.0.1 -CIPSO > /sys/fs/smackfs/netlabel
730 echo 192.168.0.0/16 -CIPSO > /sys/fs/smackfs/netlabel
731 echo 0.0.0.0/0 @ > /sys/fs/smackfs/netlabel
732
733 Writing Applications for Smack
734 ------------------------------
735
736 There are three sorts of applications that will run on a Smack system. How an
737 application interacts with Smack will determine what it will have to do to
738 work properly under Smack.
739
740 Smack Ignorant Applications
741 ---------------------------
742
743 By far the majority of applications have no reason whatever to care about the
744 unique properties of Smack. Since invoking a program has no impact on the
745 Smack label associated with the process the only concern likely to arise is
746 whether the process has execute access to the program.
747
748 Smack Relevant Applications
749 ---------------------------
750
751 Some programs can be improved by teaching them about Smack, but do not make
752 any security decisions themselves. The utility ls(1) is one example of such a
753 program.
754
755 Smack Enforcing Applications
756 ----------------------------
757
758 These are special programs that not only know about Smack, but participate in
759 the enforcement of system policy. In most cases these are the programs that
760 set up user sessions. There are also network services that provide information
761 to processes running with various labels.
762
763 File System Interfaces
764 ----------------------
765
766 Smack maintains labels on file system objects using extended attributes. The
767 Smack label of a file, directory, or other file system object can be obtained
768 using getxattr(2)::
769
770 len = getxattr("/", "security.SMACK64", value, sizeof (value));
771
772 will put the Smack label of the root directory into value. A privileged
773 process can set the Smack label of a file system object with setxattr(2)::
774
775 len = strlen("Rubble");
776 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
777
778 will set the Smack label of /foo to "Rubble" if the program has appropriate
779 privilege.
780
781 Socket Interfaces
782 -----------------
783
784 The socket attributes can be read using fgetxattr(2).
785
786 A privileged process can set the Smack label of outgoing packets with
787 fsetxattr(2)::
788
789 len = strlen("Rubble");
790 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
791
792 will set the Smack label "Rubble" on packets going out from the socket if the
793 program has appropriate privilege::
794
795 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
796
797 will set the Smack label "*" as the object label against which incoming
798 packets will be checked if the program has appropriate privilege.
799
800 Administration
801 --------------
802
803 Smack supports some mount options:
804
805 smackfsdef=label:
806 specifies the label to give files that lack
807 the Smack label extended attribute.
808
809 smackfsroot=label:
810 specifies the label to assign the root of the
811 file system if it lacks the Smack extended attribute.
812
813 smackfshat=label:
814 specifies a label that must have read access to
815 all labels set on the filesystem. Not yet enforced.
816
817 smackfsfloor=label:
818 specifies a label to which all labels set on the
819 filesystem must have read access. Not yet enforced.
820
821 smackfstransmute=label:
822 behaves exactly like smackfsroot except that it also
823 sets the transmute flag on the root of the mount
824
825 These mount options apply to all file system types.
826
827 Smack auditing
828 --------------
829
830 If you want Smack auditing of security events, you need to set CONFIG_AUDIT
831 in your kernel configuration.
832 By default, all denied events will be audited. You can change this behavior by
833 writing a single character to the /sys/fs/smackfs/logging file::
834
835 0 : no logging
836 1 : log denied (default)
837 2 : log accepted
838 3 : log denied & accepted
839
840 Events are logged as 'key=value' pairs, for each event you at least will get
841 the subject, the object, the rights requested, the action, the kernel function
842 that triggered the event, plus other pairs depending on the type of event
843 audited.
844
845 Bringup Mode
846 ------------
847
848 Bringup mode provides logging features that can make application
849 configuration and system bringup easier. Configure the kernel with
850 CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup
851 mode is enabled accesses that succeed due to rules marked with the "b"
852 access mode will logged. When a new label is introduced for processes
853 rules can be added aggressively, marked with the "b". The logging allows
854 tracking of which rules actual get used for that label.
855
856 Another feature of bringup mode is the "unconfined" option. Writing
857 a label to /sys/fs/smackfs/unconfined makes subjects with that label
858 able to access any object, and objects with that label accessible to
859 all subjects. Any access that is granted because a label is unconfined
860 is logged. This feature is dangerous, as files and directories may
861 be created in places they couldn't if the policy were being enforced.
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