3 "Good for you, you've decided to clean the elevator!"
4 - The Elevator, from Dark Star
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.
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
16 Smack consists of three major components:
18 - Basic utilities, which are helpful but not required
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.
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.
31 The current git repository for Smack user space is:
33 git://github.com/smack-team/smack.git
35 This should make and install on most modern distributions.
36 There are three commands included in smackutil:
38 smackload - properly formats data for writing to /smack/load
39 smackcipso - properly formats data for writing to /smack/cipso
40 chsmack - display or set Smack extended attribute values
42 In keeping with the intent of Smack, configuration data is
43 minimal and not strictly required. The most important
44 configuration step is mounting the smackfs pseudo filesystem.
45 If smackutil is installed the startup script will take care
46 of this, but it can be manually as well.
48 Add this line to /etc/fstab:
50 smackfs /smack smackfs smackfsdef=* 0 0
52 and create the /smack directory for mounting.
54 Smack uses extended attributes (xattrs) to store labels on filesystem
55 objects. The attributes are stored in the extended attribute security
56 name space. A process must have CAP_MAC_ADMIN to change any of these
59 The extended attributes that Smack uses are:
62 Used to make access control decisions. In almost all cases
63 the label given to a new filesystem object will be the label
64 of the process that created it.
66 The Smack label of a process that execs a program file with
67 this attribute set will run with this attribute's value.
69 Don't allow the file to be mmapped by a process whose Smack
70 label does not allow all of the access permitted to a process
71 with the label contained in this attribute. This is a very
72 specific use case for shared libraries.
74 Can only have the value "TRUE". If this attribute is present
75 on a directory when an object is created in the directory and
76 the Smack rule (more below) that permitted the write access
77 to the directory includes the transmute ("t") mode the object
78 gets the label of the directory instead of the label of the
79 creating process. If the object being created is a directory
80 the SMACK64TRANSMUTE attribute is set as well.
82 This attribute is only available on file descriptors for sockets.
83 Use the Smack label in this attribute for access control
84 decisions on packets being delivered to this socket.
86 This attribute is only available on file descriptors for sockets.
87 Use the Smack label in this attribute for access control
88 decisions on packets coming from this socket.
90 There are multiple ways to set a Smack label on a file:
92 # attr -S -s SMACK64 -V "value" path
93 # chsmack -a value path
95 A process can see the smack label it is running with by
96 reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
97 can set the process smack by writing there.
99 Most Smack configuration is accomplished by writing to files
100 in the smackfs filesystem. This pseudo-filesystem is usually
104 This interface reports whether a subject with the specified
105 Smack label has a particular access to an object with a
106 specified Smack label. Write a fixed format access rule to
107 this file. The next read will indicate whether the access
108 would be permitted. The text will be either "1" indicating
109 access, or "0" indicating denial.
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 long 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.
118 This contains the Smack label applied to unlabeled network
121 This interface allows modification of existing access control rules.
122 The format accepted on write is:
124 where the first string is the subject label, the second the
125 object label, the third the access to allow and the fourth the
126 access to deny. The access strings may contain only the characters
127 "rwxat-". If a rule for a given subject and object exists it will be
128 modified by enabling the permissions in the third string and disabling
129 those in the fourth string. If there is no such rule it will be
130 created using the access specified in the third and the fourth strings.
132 This interface allows a specific CIPSO header to be assigned
133 to a Smack label. The format accepted on write is:
134 "%24s%4d%4d"["%4d"]...
135 The first string is a fixed Smack label. The first number is
136 the level to use. The second number is the number of categories.
137 The following numbers are the categories.
138 "level-3-cats-5-19 3 2 5 19"
140 This interface allows a specific CIPSO header to be assigned
141 to a Smack label. The format accepted on write is:
143 The first string is a long Smack label. The first number is
144 the level to use. The second number is the number of categories.
145 The following numbers are the categories.
146 "level-3-cats-5-19 3 2 5 19"
148 This contains the CIPSO level used for Smack direct label
149 representation in network packets.
151 This contains the CIPSO domain of interpretation used in
154 This interface allows access control rules in addition to
155 the system defined rules to be specified. The format accepted
158 where the first string is the subject label, the second the
159 object label, and the third the requested access. The access
160 string may contain only the characters "rwxat-", and specifies
161 which sort of access is allowed. The "-" is a placeholder for
162 permissions that are not allowed. The string "r-x--" would
163 specify read and execute access. Labels are limited to 23
164 characters in length.
166 This interface allows access control rules in addition to
167 the system defined rules to be specified. The format accepted
170 where the first string is the subject label, the second the
171 object label, and the third the requested access. The access
172 string may contain only the characters "rwxat-", and specifies
173 which sort of access is allowed. The "-" is a placeholder for
174 permissions that are not allowed. The string "r-x--" would
175 specify read and execute access.
177 This interface allows process specific access rules to be
178 defined. These rules are only consulted if access would
179 otherwise be permitted, and are intended to provide additional
180 restrictions on the process. The format is the same as for
183 This interface allows process specific access rules to be
184 defined. These rules are only consulted if access would
185 otherwise be permitted, and are intended to provide additional
186 restrictions on the process. The format is the same as for
189 This contains the Smack logging state.
191 This contains the CIPSO level used for Smack mapped label
192 representation in network packets.
194 This interface allows specific internet addresses to be
195 treated as single label hosts. Packets are sent to single
196 label hosts without CIPSO headers, but only from processes
197 that have Smack write access to the host label. All packets
198 received from single label hosts are given the specified
199 label. The format accepted on write is:
200 "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
202 This contains the label processes must have for CAP_MAC_ADMIN
203 and CAP_MAC_OVERRIDE to be effective. If this file is empty
204 these capabilities are effective at for processes with any
205 label. The value is set by writing the desired label to the
206 file or cleared by writing "-" to the file.
208 Writing a Smack label here sets the access to '-' for all access
209 rules with that subject label.
211 You can add access rules in /etc/smack/accesses. They take the form:
213 subjectlabel objectlabel access
215 access is a combination of the letters rwxa which specify the
216 kind of access permitted a subject with subjectlabel on an
217 object with objectlabel. If there is no rule no access is allowed.
219 Look for additional programs on http://schaufler-ca.com
221 From the Smack Whitepaper:
223 The Simplified Mandatory Access Control Kernel
226 casey@schaufler-ca.com
228 Mandatory Access Control
230 Computer systems employ a variety of schemes to constrain how information is
231 shared among the people and services using the machine. Some of these schemes
232 allow the program or user to decide what other programs or users are allowed
233 access to pieces of data. These schemes are called discretionary access
234 control mechanisms because the access control is specified at the discretion
235 of the user. Other schemes do not leave the decision regarding what a user or
236 program can access up to users or programs. These schemes are called mandatory
237 access control mechanisms because you don't have a choice regarding the users
238 or programs that have access to pieces of data.
242 From the middle of the 1980's until the turn of the century Mandatory Access
243 Control (MAC) was very closely associated with the Bell & LaPadula security
244 model, a mathematical description of the United States Department of Defense
245 policy for marking paper documents. MAC in this form enjoyed a following
246 within the Capital Beltway and Scandinavian supercomputer centers but was
247 often sited as failing to address general needs.
249 Domain Type Enforcement
251 Around the turn of the century Domain Type Enforcement (DTE) became popular.
252 This scheme organizes users, programs, and data into domains that are
253 protected from each other. This scheme has been widely deployed as a component
254 of popular Linux distributions. The administrative overhead required to
255 maintain this scheme and the detailed understanding of the whole system
256 necessary to provide a secure domain mapping leads to the scheme being
257 disabled or used in limited ways in the majority of cases.
261 Smack is a Mandatory Access Control mechanism designed to provide useful MAC
262 while avoiding the pitfalls of its predecessors. The limitations of Bell &
263 LaPadula are addressed by providing a scheme whereby access can be controlled
264 according to the requirements of the system and its purpose rather than those
265 imposed by an arcane government policy. The complexity of Domain Type
266 Enforcement and avoided by defining access controls in terms of the access
267 modes already in use.
271 The jargon used to talk about Smack will be familiar to those who have dealt
272 with other MAC systems and shouldn't be too difficult for the uninitiated to
273 pick up. There are four terms that are used in a specific way and that are
274 especially important:
276 Subject: A subject is an active entity on the computer system.
277 On Smack a subject is a task, which is in turn the basic unit
280 Object: An object is a passive entity on the computer system.
281 On Smack files of all types, IPC, and tasks can be objects.
283 Access: Any attempt by a subject to put information into or get
284 information from an object is an access.
286 Label: Data that identifies the Mandatory Access Control
287 characteristics of a subject or an object.
289 These definitions are consistent with the traditional use in the security
290 community. There are also some terms from Linux that are likely to crop up:
292 Capability: A task that possesses a capability has permission to
293 violate an aspect of the system security policy, as identified by
294 the specific capability. A task that possesses one or more
295 capabilities is a privileged task, whereas a task with no
296 capabilities is an unprivileged task.
298 Privilege: A task that is allowed to violate the system security
299 policy is said to have privilege. As of this writing a task can
300 have privilege either by possessing capabilities or by having an
301 effective user of root.
305 Smack is an extension to a Linux system. It enforces additional restrictions
306 on what subjects can access which objects, based on the labels attached to
307 each of the subject and the object.
311 Smack labels are ASCII character strings, one to twenty-three characters in
312 length. Single character labels using special characters, that being anything
313 other than a letter or digit, are reserved for use by the Smack development
314 team. Smack labels are unstructured, case sensitive, and the only operation
315 ever performed on them is comparison for equality. Smack labels cannot
316 contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
317 (quote) and '"' (double-quote) characters.
318 Smack labels cannot begin with a '-'. This is reserved for special options.
320 There are some predefined labels:
322 _ Pronounced "floor", a single underscore character.
323 ^ Pronounced "hat", a single circumflex character.
324 * Pronounced "star", a single asterisk character.
325 ? Pronounced "huh", a single question mark character.
326 @ Pronounced "web", a single at sign character.
328 Every task on a Smack system is assigned a label. System tasks, such as
329 init(8) and systems daemons, are run with the floor ("_") label. User tasks
330 are assigned labels according to the specification found in the
331 /etc/smack/user configuration file.
335 Smack uses the traditional access modes of Linux. These modes are read,
336 execute, write, and occasionally append. There are a few cases where the
337 access mode may not be obvious. These include:
339 Signals: A signal is a write operation from the subject task to
341 Internet Domain IPC: Transmission of a packet is considered a
342 write operation from the source task to the destination task.
344 Smack restricts access based on the label attached to a subject and the label
345 attached to the object it is trying to access. The rules enforced are, in
348 1. Any access requested by a task labeled "*" is denied.
349 2. A read or execute access requested by a task labeled "^"
351 3. A read or execute access requested on an object labeled "_"
353 4. Any access requested on an object labeled "*" is permitted.
354 5. Any access requested by a task on an object with the same
356 6. Any access requested that is explicitly defined in the loaded
357 rule set is permitted.
358 7. Any other access is denied.
362 With the isolation provided by Smack access separation is simple. There are
363 many interesting cases where limited access by subjects to objects with
364 different labels is desired. One example is the familiar spy model of
365 sensitivity, where a scientist working on a highly classified project would be
366 able to read documents of lower classifications and anything she writes will
367 be "born" highly classified. To accommodate such schemes Smack includes a
368 mechanism for specifying rules allowing access between labels.
372 The format of an access rule is:
374 subject-label object-label access
376 Where subject-label is the Smack label of the task, object-label is the Smack
377 label of the thing being accessed, and access is a string specifying the sort
378 of access allowed. The access specification is searched for letters that
379 describe access modes:
381 a: indicates that append access should be granted.
382 r: indicates that read access should be granted.
383 w: indicates that write access should be granted.
384 x: indicates that execute access should be granted.
385 t: indicates that the rule requests transmutation.
387 Uppercase values for the specification letters are allowed as well.
388 Access mode specifications can be in any order. Examples of acceptable rules
398 Examples of unacceptable rules are:
404 Spaces are not allowed in labels. Since a subject always has access to files
405 with the same label specifying a rule for that case is pointless. Only
406 valid letters (rwxatRWXAT) and the dash ('-') character are allowed in
407 access specifications. The dash is a placeholder, so "a-r" is the same
408 as "ar". A lone dash is used to specify that no access should be allowed.
410 Applying Access Rules
412 The developers of Linux rarely define new sorts of things, usually importing
413 schemes and concepts from other systems. Most often, the other systems are
414 variants of Unix. Unix has many endearing properties, but consistency of
415 access control models is not one of them. Smack strives to treat accesses as
416 uniformly as is sensible while keeping with the spirit of the underlying
419 File system objects including files, directories, named pipes, symbolic links,
420 and devices require access permissions that closely match those used by mode
421 bit access. To open a file for reading read access is required on the file. To
422 search a directory requires execute access. Creating a file with write access
423 requires both read and write access on the containing directory. Deleting a
424 file requires read and write access to the file and to the containing
425 directory. It is possible that a user may be able to see that a file exists
426 but not any of its attributes by the circumstance of having read access to the
427 containing directory but not to the differently labeled file. This is an
428 artifact of the file name being data in the directory, not a part of the file.
430 If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
431 access rule that allows a process to create an object in that directory
432 includes 't' access the label assigned to the new object will be that
433 of the directory, not the creating process. This makes it much easier
434 for two processes with different labels to share data without granting
435 access to all of their files.
437 IPC objects, message queues, semaphore sets, and memory segments exist in flat
438 namespaces and access requests are only required to match the object in
441 Process objects reflect tasks on the system and the Smack label used to access
442 them is the same Smack label that the task would use for its own access
443 attempts. Sending a signal via the kill() system call is a write operation
444 from the signaler to the recipient. Debugging a process requires both reading
445 and writing. Creating a new task is an internal operation that results in two
446 tasks with identical Smack labels and requires no access checks.
448 Sockets are data structures attached to processes and sending a packet from
449 one process to another requires that the sender have write access to the
450 receiver. The receiver is not required to have read access to the sender.
454 The configuration file /etc/smack/accesses contains the rules to be set at
455 system startup. The contents are written to the special file /smack/load.
456 Rules can be written to /smack/load at any time and take effect immediately.
457 For any pair of subject and object labels there can be only one rule, with the
458 most recently specified overriding any earlier specification.
460 The program smackload is provided to ensure data is formatted
461 properly when written to /smack/load. This program reads lines
464 subjectlabel objectlabel mode.
468 The Smack label of a process can be read from /proc/<pid>/attr/current. A
469 process can read its own Smack label from /proc/self/attr/current. A
470 privileged process can change its own Smack label by writing to
471 /proc/self/attr/current but not the label of another process.
475 The Smack label of a filesystem object is stored as an extended attribute
476 named SMACK64 on the file. This attribute is in the security namespace. It can
477 only be changed by a process with privilege.
481 A process with CAP_MAC_OVERRIDE is privileged.
485 As mentioned before, Smack enforces access control on network protocol
486 transmissions. Every packet sent by a Smack process is tagged with its Smack
487 label. This is done by adding a CIPSO tag to the header of the IP packet. Each
488 packet received is expected to have a CIPSO tag that identifies the label and
489 if it lacks such a tag the network ambient label is assumed. Before the packet
490 is delivered a check is made to determine that a subject with the label on the
491 packet has write access to the receiving process and if that is not the case
492 the packet is dropped.
496 It is normally unnecessary to specify the CIPSO configuration. The default
497 values used by the system handle all internal cases. Smack will compose CIPSO
498 label values to match the Smack labels being used without administrative
499 intervention. Unlabeled packets that come into the system will be given the
502 Smack requires configuration in the case where packets from a system that is
503 not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
504 Solaris system, but there are other, less widely deployed systems out there.
505 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
506 and a category set with each packet. The DOI is intended to identify a group
507 of systems that use compatible labeling schemes, and the DOI specified on the
508 smack system must match that of the remote system or packets will be
509 discarded. The DOI is 3 by default. The value can be read from /smack/doi and
510 can be changed by writing to /smack/doi.
512 The label and category set are mapped to a Smack label as defined in
515 A Smack/CIPSO mapping has the form:
517 smack level [category [category]*]
519 Smack does not expect the level or category sets to be related in any
520 particular way and does not assume or assign accesses based on them. Some
521 examples of mappings:
528 The ":" and "," characters are permitted in a Smack label but have no special
531 The mapping of Smack labels to CIPSO values is defined by writing to
532 /smack/cipso. Again, the format of data written to this special file
533 is highly restrictive, so the program smackcipso is provided to
534 ensure the writes are done properly. This program takes mappings
535 on the standard input and sends them to /smack/cipso properly.
537 In addition to explicit mappings Smack supports direct CIPSO mappings. One
538 CIPSO level is used to indicate that the category set passed in the packet is
539 in fact an encoding of the Smack label. The level used is 250 by default. The
540 value can be read from /smack/direct and changed by writing to /smack/direct.
544 There are two attributes that are associated with sockets. These attributes
545 can only be set by privileged tasks, but any task can read them for their own
548 SMACK64IPIN: The Smack label of the task object. A privileged
549 program that will enforce policy may set this to the star label.
551 SMACK64IPOUT: The Smack label transmitted with outgoing packets.
552 A privileged program may set this to match the label of another
553 task with which it hopes to communicate.
555 Smack Netlabel Exceptions
557 You will often find that your labeled application has to talk to the outside,
558 unlabeled world. To do this there's a special file /smack/netlabel where you can
559 add some exceptions in the form of :
563 It means that your application will have unlabeled access to @IP1 if it has
564 write access on LABEL1, and access to the subnet @IP2/MASK if it has write
567 Entries in the /smack/netlabel file are matched by longest mask first, like in
568 classless IPv4 routing.
570 A special label '@' and an option '-CIPSO' can be used there :
571 @ means Internet, any application with any label has access to it
572 -CIPSO means standard CIPSO networking
574 If you don't know what CIPSO is and don't plan to use it, you can just do :
575 echo 127.0.0.1 -CIPSO > /smack/netlabel
576 echo 0.0.0.0/0 @ > /smack/netlabel
578 If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
579 Internet access, you can have :
580 echo 127.0.0.1 -CIPSO > /smack/netlabel
581 echo 192.168.0.0/16 -CIPSO > /smack/netlabel
582 echo 0.0.0.0/0 @ > /smack/netlabel
585 Writing Applications for Smack
587 There are three sorts of applications that will run on a Smack system. How an
588 application interacts with Smack will determine what it will have to do to
589 work properly under Smack.
591 Smack Ignorant Applications
593 By far the majority of applications have no reason whatever to care about the
594 unique properties of Smack. Since invoking a program has no impact on the
595 Smack label associated with the process the only concern likely to arise is
596 whether the process has execute access to the program.
598 Smack Relevant Applications
600 Some programs can be improved by teaching them about Smack, but do not make
601 any security decisions themselves. The utility ls(1) is one example of such a
604 Smack Enforcing Applications
606 These are special programs that not only know about Smack, but participate in
607 the enforcement of system policy. In most cases these are the programs that
608 set up user sessions. There are also network services that provide information
609 to processes running with various labels.
611 File System Interfaces
613 Smack maintains labels on file system objects using extended attributes. The
614 Smack label of a file, directory, or other file system object can be obtained
617 len = getxattr("/", "security.SMACK64", value, sizeof (value));
619 will put the Smack label of the root directory into value. A privileged
620 process can set the Smack label of a file system object with setxattr(2).
622 len = strlen("Rubble");
623 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
625 will set the Smack label of /foo to "Rubble" if the program has appropriate
630 The socket attributes can be read using fgetxattr(2).
632 A privileged process can set the Smack label of outgoing packets with
635 len = strlen("Rubble");
636 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
638 will set the Smack label "Rubble" on packets going out from the socket if the
639 program has appropriate privilege.
641 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
643 will set the Smack label "*" as the object label against which incoming
644 packets will be checked if the program has appropriate privilege.
648 Smack supports some mount options:
650 smackfsdef=label: specifies the label to give files that lack
651 the Smack label extended attribute.
653 smackfsroot=label: specifies the label to assign the root of the
654 file system if it lacks the Smack extended attribute.
656 smackfshat=label: specifies a label that must have read access to
657 all labels set on the filesystem. Not yet enforced.
659 smackfsfloor=label: specifies a label to which all labels set on the
660 filesystem must have read access. Not yet enforced.
662 These mount options apply to all file system types.
666 If you want Smack auditing of security events, you need to set CONFIG_AUDIT
667 in your kernel configuration.
668 By default, all denied events will be audited. You can change this behavior by
669 writing a single character to the /smack/logging file :
671 1 : log denied (default)
673 3 : log denied & accepted
675 Events are logged as 'key=value' pairs, for each event you at least will get
676 the subject, the object, the rights requested, the action, the kernel function
677 that triggered the event, plus other pairs depending on the type of event
git.samba.org / sfrench / cifs-2.6.git / blob