3 "Good for you, you've decided to clean the elevator!"
4 - The Elevator, from Dark Star
6 Smack is the the Simplified Mandatory Access Control Kernel.
7 Smack is a kernel based implementation of mandatory access
8 control that includes simplicity in its primary design goals.
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 a specific CIPSO header to be assigned
122 to a Smack label. The format accepted on write is:
123 "%24s%4d%4d"["%4d"]...
124 The first string is a fixed Smack label. The first number is
125 the level to use. The second number is the number of categories.
126 The following numbers are the categories.
127 "level-3-cats-5-19 3 2 5 19"
129 This interface allows a specific CIPSO header to be assigned
130 to a Smack label. The format accepted on write is:
132 The first string is a long Smack label. The first number is
133 the level to use. The second number is the number of categories.
134 The following numbers are the categories.
135 "level-3-cats-5-19 3 2 5 19"
137 This contains the CIPSO level used for Smack direct label
138 representation in network packets.
140 This contains the CIPSO domain of interpretation used in
143 This interface allows access control rules in addition to
144 the system defined rules to be specified. The format accepted
147 where the first string is the subject label, the second the
148 object label, and the third the requested access. The access
149 string may contain only the characters "rwxat-", and specifies
150 which sort of access is allowed. The "-" is a placeholder for
151 permissions that are not allowed. The string "r-x--" would
152 specify read and execute access. Labels are limited to 23
153 characters in length.
155 This interface allows access control rules in addition to
156 the system defined rules to be specified. The format accepted
159 where the first string is the subject label, the second the
160 object label, and the third the requested access. The access
161 string may contain only the characters "rwxat-", and specifies
162 which sort of access is allowed. The "-" is a placeholder for
163 permissions that are not allowed. The string "r-x--" would
164 specify read and execute access.
166 This interface allows process specific access rules to be
167 defined. These rules are only consulted if access would
168 otherwise be permitted, and are intended to provide additional
169 restrictions on the process. The format is the same as for
172 This interface allows process specific access rules to be
173 defined. These rules are only consulted if access would
174 otherwise be permitted, and are intended to provide additional
175 restrictions on the process. The format is the same as for
178 This contains the Smack logging state.
180 This contains the CIPSO level used for Smack mapped label
181 representation in network packets.
183 This interface allows specific internet addresses to be
184 treated as single label hosts. Packets are sent to single
185 label hosts without CIPSO headers, but only from processes
186 that have Smack write access to the host label. All packets
187 received from single label hosts are given the specified
188 label. The format accepted on write is:
189 "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
191 This contains the label processes must have for CAP_MAC_ADMIN
192 and CAP_MAC_OVERRIDE to be effective. If this file is empty
193 these capabilities are effective at for processes with any
194 label. The value is set by writing the desired label to the
195 file or cleared by writing "-" to the file.
197 Writing a Smack label here sets the access to '-' for all access
198 rules with that subject label.
200 You can add access rules in /etc/smack/accesses. They take the form:
202 subjectlabel objectlabel access
204 access is a combination of the letters rwxa which specify the
205 kind of access permitted a subject with subjectlabel on an
206 object with objectlabel. If there is no rule no access is allowed.
208 Look for additional programs on http://schaufler-ca.com
210 From the Smack Whitepaper:
212 The Simplified Mandatory Access Control Kernel
215 casey@schaufler-ca.com
217 Mandatory Access Control
219 Computer systems employ a variety of schemes to constrain how information is
220 shared among the people and services using the machine. Some of these schemes
221 allow the program or user to decide what other programs or users are allowed
222 access to pieces of data. These schemes are called discretionary access
223 control mechanisms because the access control is specified at the discretion
224 of the user. Other schemes do not leave the decision regarding what a user or
225 program can access up to users or programs. These schemes are called mandatory
226 access control mechanisms because you don't have a choice regarding the users
227 or programs that have access to pieces of data.
231 From the middle of the 1980's until the turn of the century Mandatory Access
232 Control (MAC) was very closely associated with the Bell & LaPadula security
233 model, a mathematical description of the United States Department of Defense
234 policy for marking paper documents. MAC in this form enjoyed a following
235 within the Capital Beltway and Scandinavian supercomputer centers but was
236 often sited as failing to address general needs.
238 Domain Type Enforcement
240 Around the turn of the century Domain Type Enforcement (DTE) became popular.
241 This scheme organizes users, programs, and data into domains that are
242 protected from each other. This scheme has been widely deployed as a component
243 of popular Linux distributions. The administrative overhead required to
244 maintain this scheme and the detailed understanding of the whole system
245 necessary to provide a secure domain mapping leads to the scheme being
246 disabled or used in limited ways in the majority of cases.
250 Smack is a Mandatory Access Control mechanism designed to provide useful MAC
251 while avoiding the pitfalls of its predecessors. The limitations of Bell &
252 LaPadula are addressed by providing a scheme whereby access can be controlled
253 according to the requirements of the system and its purpose rather than those
254 imposed by an arcane government policy. The complexity of Domain Type
255 Enforcement and avoided by defining access controls in terms of the access
256 modes already in use.
260 The jargon used to talk about Smack will be familiar to those who have dealt
261 with other MAC systems and shouldn't be too difficult for the uninitiated to
262 pick up. There are four terms that are used in a specific way and that are
263 especially important:
265 Subject: A subject is an active entity on the computer system.
266 On Smack a subject is a task, which is in turn the basic unit
269 Object: An object is a passive entity on the computer system.
270 On Smack files of all types, IPC, and tasks can be objects.
272 Access: Any attempt by a subject to put information into or get
273 information from an object is an access.
275 Label: Data that identifies the Mandatory Access Control
276 characteristics of a subject or an object.
278 These definitions are consistent with the traditional use in the security
279 community. There are also some terms from Linux that are likely to crop up:
281 Capability: A task that possesses a capability has permission to
282 violate an aspect of the system security policy, as identified by
283 the specific capability. A task that possesses one or more
284 capabilities is a privileged task, whereas a task with no
285 capabilities is an unprivileged task.
287 Privilege: A task that is allowed to violate the system security
288 policy is said to have privilege. As of this writing a task can
289 have privilege either by possessing capabilities or by having an
290 effective user of root.
294 Smack is an extension to a Linux system. It enforces additional restrictions
295 on what subjects can access which objects, based on the labels attached to
296 each of the subject and the object.
300 Smack labels are ASCII character strings, one to twenty-three characters in
301 length. Single character labels using special characters, that being anything
302 other than a letter or digit, are reserved for use by the Smack development
303 team. Smack labels are unstructured, case sensitive, and the only operation
304 ever performed on them is comparison for equality. Smack labels cannot
305 contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
306 (quote) and '"' (double-quote) characters.
307 Smack labels cannot begin with a '-'. This is reserved for special options.
309 There are some predefined labels:
311 _ Pronounced "floor", a single underscore character.
312 ^ Pronounced "hat", a single circumflex character.
313 * Pronounced "star", a single asterisk character.
314 ? Pronounced "huh", a single question mark character.
315 @ Pronounced "web", a single at sign character.
317 Every task on a Smack system is assigned a label. System tasks, such as
318 init(8) and systems daemons, are run with the floor ("_") label. User tasks
319 are assigned labels according to the specification found in the
320 /etc/smack/user configuration file.
324 Smack uses the traditional access modes of Linux. These modes are read,
325 execute, write, and occasionally append. There are a few cases where the
326 access mode may not be obvious. These include:
328 Signals: A signal is a write operation from the subject task to
330 Internet Domain IPC: Transmission of a packet is considered a
331 write operation from the source task to the destination task.
333 Smack restricts access based on the label attached to a subject and the label
334 attached to the object it is trying to access. The rules enforced are, in
337 1. Any access requested by a task labeled "*" is denied.
338 2. A read or execute access requested by a task labeled "^"
340 3. A read or execute access requested on an object labeled "_"
342 4. Any access requested on an object labeled "*" is permitted.
343 5. Any access requested by a task on an object with the same
345 6. Any access requested that is explicitly defined in the loaded
346 rule set is permitted.
347 7. Any other access is denied.
351 With the isolation provided by Smack access separation is simple. There are
352 many interesting cases where limited access by subjects to objects with
353 different labels is desired. One example is the familiar spy model of
354 sensitivity, where a scientist working on a highly classified project would be
355 able to read documents of lower classifications and anything she writes will
356 be "born" highly classified. To accommodate such schemes Smack includes a
357 mechanism for specifying rules allowing access between labels.
361 The format of an access rule is:
363 subject-label object-label access
365 Where subject-label is the Smack label of the task, object-label is the Smack
366 label of the thing being accessed, and access is a string specifying the sort
367 of access allowed. The access specification is searched for letters that
368 describe access modes:
370 a: indicates that append access should be granted.
371 r: indicates that read access should be granted.
372 w: indicates that write access should be granted.
373 x: indicates that execute access should be granted.
374 t: indicates that the rule requests transmutation.
376 Uppercase values for the specification letters are allowed as well.
377 Access mode specifications can be in any order. Examples of acceptable rules
387 Examples of unacceptable rules are:
393 Spaces are not allowed in labels. Since a subject always has access to files
394 with the same label specifying a rule for that case is pointless. Only
395 valid letters (rwxatRWXAT) and the dash ('-') character are allowed in
396 access specifications. The dash is a placeholder, so "a-r" is the same
397 as "ar". A lone dash is used to specify that no access should be allowed.
399 Applying Access Rules
401 The developers of Linux rarely define new sorts of things, usually importing
402 schemes and concepts from other systems. Most often, the other systems are
403 variants of Unix. Unix has many endearing properties, but consistency of
404 access control models is not one of them. Smack strives to treat accesses as
405 uniformly as is sensible while keeping with the spirit of the underlying
408 File system objects including files, directories, named pipes, symbolic links,
409 and devices require access permissions that closely match those used by mode
410 bit access. To open a file for reading read access is required on the file. To
411 search a directory requires execute access. Creating a file with write access
412 requires both read and write access on the containing directory. Deleting a
413 file requires read and write access to the file and to the containing
414 directory. It is possible that a user may be able to see that a file exists
415 but not any of its attributes by the circumstance of having read access to the
416 containing directory but not to the differently labeled file. This is an
417 artifact of the file name being data in the directory, not a part of the file.
419 If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
420 access rule that allows a process to create an object in that directory
421 includes 't' access the label assigned to the new object will be that
422 of the directory, not the creating process. This makes it much easier
423 for two processes with different labels to share data without granting
424 access to all of their files.
426 IPC objects, message queues, semaphore sets, and memory segments exist in flat
427 namespaces and access requests are only required to match the object in
430 Process objects reflect tasks on the system and the Smack label used to access
431 them is the same Smack label that the task would use for its own access
432 attempts. Sending a signal via the kill() system call is a write operation
433 from the signaler to the recipient. Debugging a process requires both reading
434 and writing. Creating a new task is an internal operation that results in two
435 tasks with identical Smack labels and requires no access checks.
437 Sockets are data structures attached to processes and sending a packet from
438 one process to another requires that the sender have write access to the
439 receiver. The receiver is not required to have read access to the sender.
443 The configuration file /etc/smack/accesses contains the rules to be set at
444 system startup. The contents are written to the special file /smack/load.
445 Rules can be written to /smack/load at any time and take effect immediately.
446 For any pair of subject and object labels there can be only one rule, with the
447 most recently specified overriding any earlier specification.
449 The program smackload is provided to ensure data is formatted
450 properly when written to /smack/load. This program reads lines
453 subjectlabel objectlabel mode.
457 The Smack label of a process can be read from /proc/<pid>/attr/current. A
458 process can read its own Smack label from /proc/self/attr/current. A
459 privileged process can change its own Smack label by writing to
460 /proc/self/attr/current but not the label of another process.
464 The Smack label of a filesystem object is stored as an extended attribute
465 named SMACK64 on the file. This attribute is in the security namespace. It can
466 only be changed by a process with privilege.
470 A process with CAP_MAC_OVERRIDE is privileged.
474 As mentioned before, Smack enforces access control on network protocol
475 transmissions. Every packet sent by a Smack process is tagged with its Smack
476 label. This is done by adding a CIPSO tag to the header of the IP packet. Each
477 packet received is expected to have a CIPSO tag that identifies the label and
478 if it lacks such a tag the network ambient label is assumed. Before the packet
479 is delivered a check is made to determine that a subject with the label on the
480 packet has write access to the receiving process and if that is not the case
481 the packet is dropped.
485 It is normally unnecessary to specify the CIPSO configuration. The default
486 values used by the system handle all internal cases. Smack will compose CIPSO
487 label values to match the Smack labels being used without administrative
488 intervention. Unlabeled packets that come into the system will be given the
491 Smack requires configuration in the case where packets from a system that is
492 not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
493 Solaris system, but there are other, less widely deployed systems out there.
494 CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
495 and a category set with each packet. The DOI is intended to identify a group
496 of systems that use compatible labeling schemes, and the DOI specified on the
497 smack system must match that of the remote system or packets will be
498 discarded. The DOI is 3 by default. The value can be read from /smack/doi and
499 can be changed by writing to /smack/doi.
501 The label and category set are mapped to a Smack label as defined in
504 A Smack/CIPSO mapping has the form:
506 smack level [category [category]*]
508 Smack does not expect the level or category sets to be related in any
509 particular way and does not assume or assign accesses based on them. Some
510 examples of mappings:
517 The ":" and "," characters are permitted in a Smack label but have no special
520 The mapping of Smack labels to CIPSO values is defined by writing to
521 /smack/cipso. Again, the format of data written to this special file
522 is highly restrictive, so the program smackcipso is provided to
523 ensure the writes are done properly. This program takes mappings
524 on the standard input and sends them to /smack/cipso properly.
526 In addition to explicit mappings Smack supports direct CIPSO mappings. One
527 CIPSO level is used to indicate that the category set passed in the packet is
528 in fact an encoding of the Smack label. The level used is 250 by default. The
529 value can be read from /smack/direct and changed by writing to /smack/direct.
533 There are two attributes that are associated with sockets. These attributes
534 can only be set by privileged tasks, but any task can read them for their own
537 SMACK64IPIN: The Smack label of the task object. A privileged
538 program that will enforce policy may set this to the star label.
540 SMACK64IPOUT: The Smack label transmitted with outgoing packets.
541 A privileged program may set this to match the label of another
542 task with which it hopes to communicate.
544 Smack Netlabel Exceptions
546 You will often find that your labeled application has to talk to the outside,
547 unlabeled world. To do this there's a special file /smack/netlabel where you can
548 add some exceptions in the form of :
552 It means that your application will have unlabeled access to @IP1 if it has
553 write access on LABEL1, and access to the subnet @IP2/MASK if it has write
556 Entries in the /smack/netlabel file are matched by longest mask first, like in
557 classless IPv4 routing.
559 A special label '@' and an option '-CIPSO' can be used there :
560 @ means Internet, any application with any label has access to it
561 -CIPSO means standard CIPSO networking
563 If you don't know what CIPSO is and don't plan to use it, you can just do :
564 echo 127.0.0.1 -CIPSO > /smack/netlabel
565 echo 0.0.0.0/0 @ > /smack/netlabel
567 If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
568 Internet access, you can have :
569 echo 127.0.0.1 -CIPSO > /smack/netlabel
570 echo 192.168.0.0/16 -CIPSO > /smack/netlabel
571 echo 0.0.0.0/0 @ > /smack/netlabel
574 Writing Applications for Smack
576 There are three sorts of applications that will run on a Smack system. How an
577 application interacts with Smack will determine what it will have to do to
578 work properly under Smack.
580 Smack Ignorant Applications
582 By far the majority of applications have no reason whatever to care about the
583 unique properties of Smack. Since invoking a program has no impact on the
584 Smack label associated with the process the only concern likely to arise is
585 whether the process has execute access to the program.
587 Smack Relevant Applications
589 Some programs can be improved by teaching them about Smack, but do not make
590 any security decisions themselves. The utility ls(1) is one example of such a
593 Smack Enforcing Applications
595 These are special programs that not only know about Smack, but participate in
596 the enforcement of system policy. In most cases these are the programs that
597 set up user sessions. There are also network services that provide information
598 to processes running with various labels.
600 File System Interfaces
602 Smack maintains labels on file system objects using extended attributes. The
603 Smack label of a file, directory, or other file system object can be obtained
606 len = getxattr("/", "security.SMACK64", value, sizeof (value));
608 will put the Smack label of the root directory into value. A privileged
609 process can set the Smack label of a file system object with setxattr(2).
611 len = strlen("Rubble");
612 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
614 will set the Smack label of /foo to "Rubble" if the program has appropriate
619 The socket attributes can be read using fgetxattr(2).
621 A privileged process can set the Smack label of outgoing packets with
624 len = strlen("Rubble");
625 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
627 will set the Smack label "Rubble" on packets going out from the socket if the
628 program has appropriate privilege.
630 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
632 will set the Smack label "*" as the object label against which incoming
633 packets will be checked if the program has appropriate privilege.
637 Smack supports some mount options:
639 smackfsdef=label: specifies the label to give files that lack
640 the Smack label extended attribute.
642 smackfsroot=label: specifies the label to assign the root of the
643 file system if it lacks the Smack extended attribute.
645 smackfshat=label: specifies a label that must have read access to
646 all labels set on the filesystem. Not yet enforced.
648 smackfsfloor=label: specifies a label to which all labels set on the
649 filesystem must have read access. Not yet enforced.
651 These mount options apply to all file system types.
655 If you want Smack auditing of security events, you need to set CONFIG_AUDIT
656 in your kernel configuration.
657 By default, all denied events will be audited. You can change this behavior by
658 writing a single character to the /smack/logging file :
660 1 : log denied (default)
662 3 : log denied & accepted
664 Events are logged as 'key=value' pairs, for each event you at least will get
665 the subject, the object, the rights requested, the action, the kernel function
666 that triggered the event, plus other pairs depending on the type of event
projects / karo-tx-linux.git / blob