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Casey Schauflere114e472008-02-04 22:29:50 -08001
2
3 "Good for you, you've decided to clean the elevator!"
4 - The Elevator, from Dark Star
5
6Smack is the the Simplified Mandatory Access Control Kernel.
7Smack is a kernel based implementation of mandatory access
8control that includes simplicity in its primary design goals.
9
10Smack is not the only Mandatory Access Control scheme
11available for Linux. Those new to Mandatory Access Control
12are encouraged to compare Smack with the other mechanisms
13available to determine which is best suited to the problem
14at hand.
15
16Smack consists of three major components:
17 - The kernel
18 - A start-up script and a few modified applications
19 - Configuration data
20
21The kernel component of Smack is implemented as a Linux
22Security Modules (LSM) module. It requires netlabel and
23works best with file systems that support extended attributes,
24although xattr support is not strictly required.
25It is safe to run a Smack kernel under a "vanilla" distribution.
26Smack kernels use the CIPSO IP option. Some network
27configurations are intolerant of IP options and can impede
28access to systems that use them as Smack does.
29
30The startup script etc-init.d-smack should be installed
31in /etc/init.d/smack and should be invoked early in the
32start-up process. On Fedora rc5.d/S02smack is recommended.
33This script ensures that certain devices have the correct
34Smack attributes and loads the Smack configuration if
35any is defined. This script invokes two programs that
36ensure configuration data is properly formatted. These
37programs are /usr/sbin/smackload and /usr/sin/smackcipso.
38The system will run just fine without these programs,
39but it will be difficult to set access rules properly.
40
41A version of "ls" that provides a "-M" option to display
42Smack labels on long listing is available.
43
44A hacked version of sshd that allows network logins by users
45with specific Smack labels is available. This version does
46not work for scp. You must set the /etc/ssh/sshd_config
47line:
48 UsePrivilegeSeparation no
49
50The format of /etc/smack/usr is:
51
52 username smack
53
54In keeping with the intent of Smack, configuration data is
55minimal and not strictly required. The most important
56configuration step is mounting the smackfs pseudo filesystem.
57
58Add this line to /etc/fstab:
59
60 smackfs /smack smackfs smackfsdef=* 0 0
61
62and create the /smack directory for mounting.
63
64Smack uses extended attributes (xattrs) to store file labels.
65The command to set a Smack label on a file is:
66
67 # attr -S -s SMACK64 -V "value" path
68
69NOTE: Smack labels are limited to 23 characters. The attr command
70 does not enforce this restriction and can be used to set
71 invalid Smack labels on files.
72
73If you don't do anything special all users will get the floor ("_")
74label when they log in. If you do want to log in via the hacked ssh
75at other labels use the attr command to set the smack value on the
76home directory and it's contents.
77
78You can add access rules in /etc/smack/accesses. They take the form:
79
80 subjectlabel objectlabel access
81
82access is a combination of the letters rwxa which specify the
83kind of access permitted a subject with subjectlabel on an
84object with objectlabel. If there is no rule no access is allowed.
85
86A process can see the smack label it is running with by
87reading /proc/self/attr/current. A privileged process can
88set the process smack by writing there.
89
90Look for additional programs on http://schaufler-ca.com
91
92From the Smack Whitepaper:
93
94The Simplified Mandatory Access Control Kernel
95
96Casey Schaufler
97casey@schaufler-ca.com
98
99Mandatory Access Control
100
101Computer systems employ a variety of schemes to constrain how information is
102shared among the people and services using the machine. Some of these schemes
103allow the program or user to decide what other programs or users are allowed
104access to pieces of data. These schemes are called discretionary access
105control mechanisms because the access control is specified at the discretion
106of the user. Other schemes do not leave the decision regarding what a user or
107program can access up to users or programs. These schemes are called mandatory
108access control mechanisms because you don't have a choice regarding the users
109or programs that have access to pieces of data.
110
111Bell & LaPadula
112
113From the middle of the 1980's until the turn of the century Mandatory Access
114Control (MAC) was very closely associated with the Bell & LaPadula security
115model, a mathematical description of the United States Department of Defense
116policy for marking paper documents. MAC in this form enjoyed a following
117within the Capital Beltway and Scandinavian supercomputer centers but was
118often sited as failing to address general needs.
119
120Domain Type Enforcement
121
122Around the turn of the century Domain Type Enforcement (DTE) became popular.
123This scheme organizes users, programs, and data into domains that are
124protected from each other. This scheme has been widely deployed as a component
125of popular Linux distributions. The administrative overhead required to
126maintain this scheme and the detailed understanding of the whole system
127necessary to provide a secure domain mapping leads to the scheme being
128disabled or used in limited ways in the majority of cases.
129
130Smack
131
132Smack is a Mandatory Access Control mechanism designed to provide useful MAC
133while avoiding the pitfalls of its predecessors. The limitations of Bell &
134LaPadula are addressed by providing a scheme whereby access can be controlled
135according to the requirements of the system and its purpose rather than those
136imposed by an arcane government policy. The complexity of Domain Type
137Enforcement and avoided by defining access controls in terms of the access
138modes already in use.
139
140Smack Terminology
141
142The jargon used to talk about Smack will be familiar to those who have dealt
143with other MAC systems and shouldn't be too difficult for the uninitiated to
144pick up. There are four terms that are used in a specific way and that are
145especially important:
146
147 Subject: A subject is an active entity on the computer system.
148 On Smack a subject is a task, which is in turn the basic unit
149 of execution.
150
151 Object: An object is a passive entity on the computer system.
152 On Smack files of all types, IPC, and tasks can be objects.
153
154 Access: Any attempt by a subject to put information into or get
155 information from an object is an access.
156
157 Label: Data that identifies the Mandatory Access Control
158 characteristics of a subject or an object.
159
160These definitions are consistent with the traditional use in the security
161community. There are also some terms from Linux that are likely to crop up:
162
163 Capability: A task that possesses a capability has permission to
164 violate an aspect of the system security policy, as identified by
165 the specific capability. A task that possesses one or more
166 capabilities is a privileged task, whereas a task with no
167 capabilities is an unprivileged task.
168
169 Privilege: A task that is allowed to violate the system security
170 policy is said to have privilege. As of this writing a task can
171 have privilege either by possessing capabilities or by having an
172 effective user of root.
173
174Smack Basics
175
176Smack is an extension to a Linux system. It enforces additional restrictions
177on what subjects can access which objects, based on the labels attached to
178each of the subject and the object.
179
180Labels
181
182Smack labels are ASCII character strings, one to twenty-three characters in
183length. Single character labels using special characters, that being anything
184other than a letter or digit, are reserved for use by the Smack development
185team. Smack labels are unstructured, case sensitive, and the only operation
186ever performed on them is comparison for equality. Smack labels cannot
187contain unprintable characters or the "/" (slash) character.
188
189There are some predefined labels:
190
191 _ Pronounced "floor", a single underscore character.
192 ^ Pronounced "hat", a single circumflex character.
193 * Pronounced "star", a single asterisk character.
194 ? Pronounced "huh", a single question mark character.
195
196Every task on a Smack system is assigned a label. System tasks, such as
197init(8) and systems daemons, are run with the floor ("_") label. User tasks
198are assigned labels according to the specification found in the
199/etc/smack/user configuration file.
200
201Access Rules
202
203Smack uses the traditional access modes of Linux. These modes are read,
204execute, write, and occasionally append. There are a few cases where the
205access mode may not be obvious. These include:
206
207 Signals: A signal is a write operation from the subject task to
208 the object task.
209 Internet Domain IPC: Transmission of a packet is considered a
210 write operation from the source task to the destination task.
211
212Smack restricts access based on the label attached to a subject and the label
213attached to the object it is trying to access. The rules enforced are, in
214order:
215
216 1. Any access requested by a task labeled "*" is denied.
217 2. A read or execute access requested by a task labeled "^"
218 is permitted.
219 3. A read or execute access requested on an object labeled "_"
220 is permitted.
221 4. Any access requested on an object labeled "*" is permitted.
222 5. Any access requested by a task on an object with the same
223 label is permitted.
224 6. Any access requested that is explicitly defined in the loaded
225 rule set is permitted.
226 7. Any other access is denied.
227
228Smack Access Rules
229
230With the isolation provided by Smack access separation is simple. There are
231many interesting cases where limited access by subjects to objects with
232different labels is desired. One example is the familiar spy model of
233sensitivity, where a scientist working on a highly classified project would be
234able to read documents of lower classifications and anything she writes will
235be "born" highly classified. To accommodate such schemes Smack includes a
236mechanism for specifying rules allowing access between labels.
237
238Access Rule Format
239
240The format of an access rule is:
241
242 subject-label object-label access
243
244Where subject-label is the Smack label of the task, object-label is the Smack
245label of the thing being accessed, and access is a string specifying the sort
246of access allowed. The Smack labels are limited to 23 characters. The access
247specification is searched for letters that describe access modes:
248
249 a: indicates that append access should be granted.
250 r: indicates that read access should be granted.
251 w: indicates that write access should be granted.
252 x: indicates that execute access should be granted.
253
254Uppercase values for the specification letters are allowed as well.
255Access mode specifications can be in any order. Examples of acceptable rules
256are:
257
258 TopSecret Secret rx
259 Secret Unclass R
260 Manager Game x
261 User HR w
262 New Old rRrRr
263 Closed Off -
264
265Examples of unacceptable rules are:
266
267 Top Secret Secret rx
268 Ace Ace r
269 Odd spells waxbeans
270
271Spaces are not allowed in labels. Since a subject always has access to files
272with the same label specifying a rule for that case is pointless. Only
273valid letters (rwxaRWXA) and the dash ('-') character are allowed in
274access specifications. The dash is a placeholder, so "a-r" is the same
275as "ar". A lone dash is used to specify that no access should be allowed.
276
277Applying Access Rules
278
279The developers of Linux rarely define new sorts of things, usually importing
280schemes and concepts from other systems. Most often, the other systems are
281variants of Unix. Unix has many endearing properties, but consistency of
282access control models is not one of them. Smack strives to treat accesses as
283uniformly as is sensible while keeping with the spirit of the underlying
284mechanism.
285
286File system objects including files, directories, named pipes, symbolic links,
287and devices require access permissions that closely match those used by mode
288bit access. To open a file for reading read access is required on the file. To
289search a directory requires execute access. Creating a file with write access
290requires both read and write access on the containing directory. Deleting a
291file requires read and write access to the file and to the containing
292directory. It is possible that a user may be able to see that a file exists
293but not any of its attributes by the circumstance of having read access to the
294containing directory but not to the differently labeled file. This is an
295artifact of the file name being data in the directory, not a part of the file.
296
297IPC objects, message queues, semaphore sets, and memory segments exist in flat
298namespaces and access requests are only required to match the object in
299question.
300
301Process objects reflect tasks on the system and the Smack label used to access
302them is the same Smack label that the task would use for its own access
303attempts. Sending a signal via the kill() system call is a write operation
304from the signaler to the recipient. Debugging a process requires both reading
305and writing. Creating a new task is an internal operation that results in two
306tasks with identical Smack labels and requires no access checks.
307
308Sockets are data structures attached to processes and sending a packet from
309one process to another requires that the sender have write access to the
310receiver. The receiver is not required to have read access to the sender.
311
312Setting Access Rules
313
314The configuration file /etc/smack/accesses contains the rules to be set at
315system startup. The contents are written to the special file /smack/load.
316Rules can be written to /smack/load at any time and take effect immediately.
317For any pair of subject and object labels there can be only one rule, with the
318most recently specified overriding any earlier specification.
319
320The program smackload is provided to ensure data is formatted
321properly when written to /smack/load. This program reads lines
322of the form
323
324 subjectlabel objectlabel mode.
325
326Task Attribute
327
328The Smack label of a process can be read from /proc/<pid>/attr/current. A
329process can read its own Smack label from /proc/self/attr/current. A
330privileged process can change its own Smack label by writing to
331/proc/self/attr/current but not the label of another process.
332
333File Attribute
334
335The Smack label of a filesystem object is stored as an extended attribute
336named SMACK64 on the file. This attribute is in the security namespace. It can
337only be changed by a process with privilege.
338
339Privilege
340
341A process with CAP_MAC_OVERRIDE is privileged.
342
343Smack Networking
344
345As mentioned before, Smack enforces access control on network protocol
346transmissions. Every packet sent by a Smack process is tagged with its Smack
347label. This is done by adding a CIPSO tag to the header of the IP packet. Each
348packet received is expected to have a CIPSO tag that identifies the label and
349if it lacks such a tag the network ambient label is assumed. Before the packet
350is delivered a check is made to determine that a subject with the label on the
351packet has write access to the receiving process and if that is not the case
352the packet is dropped.
353
354CIPSO Configuration
355
356It is normally unnecessary to specify the CIPSO configuration. The default
357values used by the system handle all internal cases. Smack will compose CIPSO
358label values to match the Smack labels being used without administrative
359intervention. Unlabeled packets that come into the system will be given the
360ambient label.
361
362Smack requires configuration in the case where packets from a system that is
363not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
364Solaris system, but there are other, less widely deployed systems out there.
365CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
366and a category set with each packet. The DOI is intended to identify a group
367of systems that use compatible labeling schemes, and the DOI specified on the
368smack system must match that of the remote system or packets will be
369discarded. The DOI is 3 by default. The value can be read from /smack/doi and
370can be changed by writing to /smack/doi.
371
372The label and category set are mapped to a Smack label as defined in
373/etc/smack/cipso.
374
375A Smack/CIPSO mapping has the form:
376
377 smack level [category [category]*]
378
379Smack does not expect the level or category sets to be related in any
380particular way and does not assume or assign accesses based on them. Some
381examples of mappings:
382
383 TopSecret 7
384 TS:A,B 7 1 2
385 SecBDE 5 2 4 6
386 RAFTERS 7 12 26
387
388The ":" and "," characters are permitted in a Smack label but have no special
389meaning.
390
391The mapping of Smack labels to CIPSO values is defined by writing to
392/smack/cipso. Again, the format of data written to this special file
393is highly restrictive, so the program smackcipso is provided to
394ensure the writes are done properly. This program takes mappings
395on the standard input and sends them to /smack/cipso properly.
396
397In addition to explicit mappings Smack supports direct CIPSO mappings. One
398CIPSO level is used to indicate that the category set passed in the packet is
399in fact an encoding of the Smack label. The level used is 250 by default. The
400value can be read from /smack/direct and changed by writing to /smack/direct.
401
402Socket Attributes
403
404There are two attributes that are associated with sockets. These attributes
405can only be set by privileged tasks, but any task can read them for their own
406sockets.
407
408 SMACK64IPIN: The Smack label of the task object. A privileged
409 program that will enforce policy may set this to the star label.
410
411 SMACK64IPOUT: The Smack label transmitted with outgoing packets.
412 A privileged program may set this to match the label of another
413 task with which it hopes to communicate.
414
415Writing Applications for Smack
416
417There are three sorts of applications that will run on a Smack system. How an
418application interacts with Smack will determine what it will have to do to
419work properly under Smack.
420
421Smack Ignorant Applications
422
423By far the majority of applications have no reason whatever to care about the
424unique properties of Smack. Since invoking a program has no impact on the
425Smack label associated with the process the only concern likely to arise is
426whether the process has execute access to the program.
427
428Smack Relevant Applications
429
430Some programs can be improved by teaching them about Smack, but do not make
431any security decisions themselves. The utility ls(1) is one example of such a
432program.
433
434Smack Enforcing Applications
435
436These are special programs that not only know about Smack, but participate in
437the enforcement of system policy. In most cases these are the programs that
438set up user sessions. There are also network services that provide information
439to processes running with various labels.
440
441File System Interfaces
442
443Smack maintains labels on file system objects using extended attributes. The
444Smack label of a file, directory, or other file system object can be obtained
445using getxattr(2).
446
447 len = getxattr("/", "security.SMACK64", value, sizeof (value));
448
449will put the Smack label of the root directory into value. A privileged
450process can set the Smack label of a file system object with setxattr(2).
451
452 len = strlen("Rubble");
453 rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
454
455will set the Smack label of /foo to "Rubble" if the program has appropriate
456privilege.
457
458Socket Interfaces
459
460The socket attributes can be read using fgetxattr(2).
461
462A privileged process can set the Smack label of outgoing packets with
463fsetxattr(2).
464
465 len = strlen("Rubble");
466 rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
467
468will set the Smack label "Rubble" on packets going out from the socket if the
469program has appropriate privilege.
470
471 rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
472
473will set the Smack label "*" as the object label against which incoming
474packets will be checked if the program has appropriate privilege.
475
476Administration
477
478Smack supports some mount options:
479
480 smackfsdef=label: specifies the label to give files that lack
481 the Smack label extended attribute.
482
483 smackfsroot=label: specifies the label to assign the root of the
484 file system if it lacks the Smack extended attribute.
485
486 smackfshat=label: specifies a label that must have read access to
487 all labels set on the filesystem. Not yet enforced.
488
489 smackfsfloor=label: specifies a label to which all labels set on the
490 filesystem must have read access. Not yet enforced.
491
492These mount options apply to all file system types.
493