3    "Good for you, you've decided to clean the elevator!"
   4    - The Elevator, from Dark Star
   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.
  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.
  16Smack consists of three major components:
  17    - The kernel
  18    - A start-up script and a few modified applications
  19    - Configuration data
  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.
  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.
  41A version of "ls" that provides a "-M" option to display
  42Smack labels on long listing is available.
  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
  48   UsePrivilegeSeparation no
  50The format of /etc/smack/usr is:
  52   username smack
  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.
  58Add this line to /etc/fstab:
  60    smackfs /smack smackfs smackfsdef=* 0 0
  62and create the /smack directory for mounting.
  64Smack uses extended attributes (xattrs) to store file labels.
  65The command to set a Smack label on a file is:
  67    # attr -S -s SMACK64 -V "value" path
  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.
  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.
  78You can add access rules in /etc/smack/accesses. They take the form:
  80    subjectlabel objectlabel access
  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.
  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.
  90Look for additional programs on
  92From the Smack Whitepaper:
  94The Simplified Mandatory Access Control Kernel
  96Casey Schaufler
  99Mandatory Access Control
 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.
 111Bell & LaPadula
 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.
 120Domain Type Enforcement
 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.
 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.
 140Smack Terminology
 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:
 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.
 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.
 154        Access: Any attempt by a subject to put information into or get
 155        information from an object is an access.
 157        Label: Data that identifies the Mandatory Access Control
 158        characteristics of a subject or an object.
 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:
 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.
 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.
 174Smack Basics
 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.
 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.
 189There are some predefined labels:
 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.
 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.
 201Access Rules
 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:
 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.
 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
 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.
 228Smack Access Rules
 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.
 238Access Rule Format
 240The format of an access rule is:
 242        subject-label object-label access
 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:
 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.
 254Uppercase values for the specification letters are allowed as well.
 255Access mode specifications can be in any order. Examples of acceptable rules
 258        TopSecret Secret  rx
 259        Secret    Unclass R
 260        Manager   Game    x
 261        User      HR      w
 262        New       Old     rRrRr
 263        Closed    Off     -
 265Examples of unacceptable rules are:
 267        Top Secret Secret     rx
 268        Ace        Ace        r
 269        Odd        spells     waxbeans
 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.
 277Applying Access Rules
 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
 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.
 297IPC objects, message queues, semaphore sets, and memory segments exist in flat
 298namespaces and access requests are only required to match the object in
 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.
 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.
 312Setting Access Rules
 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.
 320The program smackload is provided to ensure data is formatted
 321properly when written to /smack/load. This program reads lines
 322of the form
 324    subjectlabel objectlabel mode.
 326Task Attribute
 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.
 333File Attribute
 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.
 341A process with CAP_MAC_OVERRIDE is privileged.
 343Smack Networking
 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.
 354CIPSO Configuration
 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.
 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.
 372The label and category set are mapped to a Smack label as defined in
 375A Smack/CIPSO mapping has the form:
 377        smack level [category [category]*]
 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:
 383        TopSecret 7
 384        TS:A,B    7 1 2
 385        SecBDE    5 2 4 6
 386        RAFTERS   7 12 26
 388The ":" and "," characters are permitted in a Smack label but have no special
 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.
 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.
 402Socket Attributes
 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
 408        SMACK64IPIN: The Smack label of the task object. A privileged
 409        program that will enforce policy may set this to the star label.
 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.
 415Writing Applications for Smack
 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.
 421Smack Ignorant Applications
 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.
 428Smack Relevant Applications
 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
 434Smack Enforcing Applications
 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.
 441File System Interfaces
 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).
 447        len = getxattr("/", "security.SMACK64", value, sizeof (value));
 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).
 452        len = strlen("Rubble");
 453        rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
 455will set the Smack label of /foo to "Rubble" if the program has appropriate
 458Socket Interfaces
 460The socket attributes can be read using fgetxattr(2).
 462A privileged process can set the Smack label of outgoing packets with
 465        len = strlen("Rubble");
 466        rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
 468will set the Smack label "Rubble" on packets going out from the socket if the
 469program has appropriate privilege.
 471        rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
 473will set the Smack label "*" as the object label against which incoming
 474packets will be checked if the program has appropriate privilege.
 478Smack supports some mount options:
 480        smackfsdef=label: specifies the label to give files that lack
 481        the Smack label extended attribute.
 483        smackfsroot=label: specifies the label to assign the root of the
 484        file system if it lacks the Smack extended attribute.
 486        smackfshat=label: specifies a label that must have read access to
 487        all labels set on the filesystem. Not yet enforced.
 489        smackfsfloor=label: specifies a label to which all labels set on the
 490        filesystem must have read access. Not yet enforced.
 492These mount options apply to all file system types.