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. Smack labels
 188cannot begin with a '-', which is reserved for special options.
 190There are some predefined labels:
 192        _       Pronounced "floor", a single underscore character.
 193        ^       Pronounced "hat", a single circumflex character.
 194        *       Pronounced "star", a single asterisk character.
 195        ?       Pronounced "huh", a single question mark character.
 196        @       Pronounced "Internet", a single at sign character.
 198Every task on a Smack system is assigned a label. System tasks, such as
 199init(8) and systems daemons, are run with the floor ("_") label. User tasks
 200are assigned labels according to the specification found in the
 201/etc/smack/user configuration file.
 203Access Rules
 205Smack uses the traditional access modes of Linux. These modes are read,
 206execute, write, and occasionally append. There are a few cases where the
 207access mode may not be obvious. These include:
 209        Signals: A signal is a write operation from the subject task to
 210        the object task.
 211        Internet Domain IPC: Transmission of a packet is considered a
 212        write operation from the source task to the destination task.
 214Smack restricts access based on the label attached to a subject and the label
 215attached to the object it is trying to access. The rules enforced are, in
 218        1. Any access requested by a task labeled "*" is denied.
 219        2. A read or execute access requested by a task labeled "^"
 220           is permitted.
 221        3. A read or execute access requested on an object labeled "_"
 222           is permitted.
 223        4. Any access requested on an object labeled "*" is permitted.
 224        5. Any access requested by a task on an object with the same
 225           label is permitted.
 226        6. Any access requested that is explicitly defined in the loaded
 227           rule set is permitted.
 228        7. Any other access is denied.
 230Smack Access Rules
 232With the isolation provided by Smack access separation is simple. There are
 233many interesting cases where limited access by subjects to objects with
 234different labels is desired. One example is the familiar spy model of
 235sensitivity, where a scientist working on a highly classified project would be
 236able to read documents of lower classifications and anything she writes will
 237be "born" highly classified. To accommodate such schemes Smack includes a
 238mechanism for specifying rules allowing access between labels.
 240Access Rule Format
 242The format of an access rule is:
 244        subject-label object-label access
 246Where subject-label is the Smack label of the task, object-label is the Smack
 247label of the thing being accessed, and access is a string specifying the sort
 248of access allowed. The Smack labels are limited to 23 characters. The access
 249specification is searched for letters that describe access modes:
 251        a: indicates that append access should be granted.
 252        r: indicates that read access should be granted.
 253        w: indicates that write access should be granted.
 254        x: indicates that execute access should be granted.
 256Uppercase values for the specification letters are allowed as well.
 257Access mode specifications can be in any order. Examples of acceptable rules
 260        TopSecret Secret  rx
 261        Secret    Unclass R
 262        Manager   Game    x
 263        User      HR      w
 264        New       Old     rRrRr
 265        Closed    Off     -
 267Examples of unacceptable rules are:
 269        Top Secret Secret     rx
 270        Ace        Ace        r
 271        Odd        spells     waxbeans
 273Spaces are not allowed in labels. Since a subject always has access to files
 274with the same label specifying a rule for that case is pointless. Only
 275valid letters (rwxaRWXA) and the dash ('-') character are allowed in
 276access specifications. The dash is a placeholder, so "a-r" is the same
 277as "ar". A lone dash is used to specify that no access should be allowed.
 279Applying Access Rules
 281The developers of Linux rarely define new sorts of things, usually importing
 282schemes and concepts from other systems. Most often, the other systems are
 283variants of Unix. Unix has many endearing properties, but consistency of
 284access control models is not one of them. Smack strives to treat accesses as
 285uniformly as is sensible while keeping with the spirit of the underlying
 288File system objects including files, directories, named pipes, symbolic links,
 289and devices require access permissions that closely match those used by mode
 290bit access. To open a file for reading read access is required on the file. To
 291search a directory requires execute access. Creating a file with write access
 292requires both read and write access on the containing directory. Deleting a
 293file requires read and write access to the file and to the containing
 294directory. It is possible that a user may be able to see that a file exists
 295but not any of its attributes by the circumstance of having read access to the
 296containing directory but not to the differently labeled file. This is an
 297artifact of the file name being data in the directory, not a part of the file.
 299IPC objects, message queues, semaphore sets, and memory segments exist in flat
 300namespaces and access requests are only required to match the object in
 303Process objects reflect tasks on the system and the Smack label used to access
 304them is the same Smack label that the task would use for its own access
 305attempts. Sending a signal via the kill() system call is a write operation
 306from the signaler to the recipient. Debugging a process requires both reading
 307and writing. Creating a new task is an internal operation that results in two
 308tasks with identical Smack labels and requires no access checks.
 310Sockets are data structures attached to processes and sending a packet from
 311one process to another requires that the sender have write access to the
 312receiver. The receiver is not required to have read access to the sender.
 314Setting Access Rules
 316The configuration file /etc/smack/accesses contains the rules to be set at
 317system startup. The contents are written to the special file /smack/load.
 318Rules can be written to /smack/load at any time and take effect immediately.
 319For any pair of subject and object labels there can be only one rule, with the
 320most recently specified overriding any earlier specification.
 322The program smackload is provided to ensure data is formatted
 323properly when written to /smack/load. This program reads lines
 324of the form
 326    subjectlabel objectlabel mode.
 328Task Attribute
 330The Smack label of a process can be read from /proc/<pid>/attr/current. A
 331process can read its own Smack label from /proc/self/attr/current. A
 332privileged process can change its own Smack label by writing to
 333/proc/self/attr/current but not the label of another process.
 335File Attribute
 337The Smack label of a filesystem object is stored as an extended attribute
 338named SMACK64 on the file. This attribute is in the security namespace. It can
 339only be changed by a process with privilege.
 343A process with CAP_MAC_OVERRIDE is privileged.
 345Smack Networking
 347As mentioned before, Smack enforces access control on network protocol
 348transmissions. Every packet sent by a Smack process is tagged with its Smack
 349label. This is done by adding a CIPSO tag to the header of the IP packet. Each
 350packet received is expected to have a CIPSO tag that identifies the label and
 351if it lacks such a tag the network ambient label is assumed. Before the packet
 352is delivered a check is made to determine that a subject with the label on the
 353packet has write access to the receiving process and if that is not the case
 354the packet is dropped.
 356CIPSO Configuration
 358It is normally unnecessary to specify the CIPSO configuration. The default
 359values used by the system handle all internal cases. Smack will compose CIPSO
 360label values to match the Smack labels being used without administrative
 361intervention. Unlabeled packets that come into the system will be given the
 362ambient label.
 364Smack requires configuration in the case where packets from a system that is
 365not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
 366Solaris system, but there are other, less widely deployed systems out there.
 367CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
 368and a category set with each packet. The DOI is intended to identify a group
 369of systems that use compatible labeling schemes, and the DOI specified on the
 370smack system must match that of the remote system or packets will be
 371discarded. The DOI is 3 by default. The value can be read from /smack/doi and
 372can be changed by writing to /smack/doi.
 374The label and category set are mapped to a Smack label as defined in
 377A Smack/CIPSO mapping has the form:
 379        smack level [category [category]*]
 381Smack does not expect the level or category sets to be related in any
 382particular way and does not assume or assign accesses based on them. Some
 383examples of mappings:
 385        TopSecret 7
 386        TS:A,B    7 1 2
 387        SecBDE    5 2 4 6
 388        RAFTERS   7 12 26
 390The ":" and "," characters are permitted in a Smack label but have no special
 393The mapping of Smack labels to CIPSO values is defined by writing to
 394/smack/cipso. Again, the format of data written to this special file
 395is highly restrictive, so the program smackcipso is provided to
 396ensure the writes are done properly. This program takes mappings
 397on the standard input and sends them to /smack/cipso properly.
 399In addition to explicit mappings Smack supports direct CIPSO mappings. One
 400CIPSO level is used to indicate that the category set passed in the packet is
 401in fact an encoding of the Smack label. The level used is 250 by default. The
 402value can be read from /smack/direct and changed by writing to /smack/direct.
 404Socket Attributes
 406There are two attributes that are associated with sockets. These attributes
 407can only be set by privileged tasks, but any task can read them for their own
 410        SMACK64IPIN: The Smack label of the task object. A privileged
 411        program that will enforce policy may set this to the star label.
 413        SMACK64IPOUT: The Smack label transmitted with outgoing packets.
 414        A privileged program may set this to match the label of another
 415        task with which it hopes to communicate.
 417Smack Netlabel Exceptions
 419You will often find that your labeled application has to talk to the outside,
 420unlabeled world. To do this there's a special file /smack/netlabel where you can
 421add some exceptions in the form of :
 422@IP1       LABEL1 or
 425It means that your application will have unlabeled access to @IP1 if it has
 426write access on LABEL1, and access to the subnet @IP2/MASK if it has write
 427access on LABEL2.
 429Entries in the /smack/netlabel file are matched by longest mask first, like in
 430classless IPv4 routing.
 432A special label '@' and an option '-CIPSO' can be used there :
 433@      means Internet, any application with any label has access to it
 434-CIPSO means standard CIPSO networking
 436If you don't know what CIPSO is and don't plan to use it, you can just do :
 437echo -CIPSO > /smack/netlabel
 438echo @      > /smack/netlabel
 440If you use CIPSO on your local network and need also unlabeled
 441Internet access, you can have :
 442echo      -CIPSO > /smack/netlabel
 443echo -CIPSO > /smack/netlabel
 444echo      @      > /smack/netlabel
 447Writing Applications for Smack
 449There are three sorts of applications that will run on a Smack system. How an
 450application interacts with Smack will determine what it will have to do to
 451work properly under Smack.
 453Smack Ignorant Applications
 455By far the majority of applications have no reason whatever to care about the
 456unique properties of Smack. Since invoking a program has no impact on the
 457Smack label associated with the process the only concern likely to arise is
 458whether the process has execute access to the program.
 460Smack Relevant Applications
 462Some programs can be improved by teaching them about Smack, but do not make
 463any security decisions themselves. The utility ls(1) is one example of such a
 466Smack Enforcing Applications
 468These are special programs that not only know about Smack, but participate in
 469the enforcement of system policy. In most cases these are the programs that
 470set up user sessions. There are also network services that provide information
 471to processes running with various labels.
 473File System Interfaces
 475Smack maintains labels on file system objects using extended attributes. The
 476Smack label of a file, directory, or other file system object can be obtained
 477using getxattr(2).
 479        len = getxattr("/", "security.SMACK64", value, sizeof (value));
 481will put the Smack label of the root directory into value. A privileged
 482process can set the Smack label of a file system object with setxattr(2).
 484        len = strlen("Rubble");
 485        rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
 487will set the Smack label of /foo to "Rubble" if the program has appropriate
 490Socket Interfaces
 492The socket attributes can be read using fgetxattr(2).
 494A privileged process can set the Smack label of outgoing packets with
 497        len = strlen("Rubble");
 498        rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
 500will set the Smack label "Rubble" on packets going out from the socket if the
 501program has appropriate privilege.
 503        rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
 505will set the Smack label "*" as the object label against which incoming
 506packets will be checked if the program has appropriate privilege.
 510Smack supports some mount options:
 512        smackfsdef=label: specifies the label to give files that lack
 513        the Smack label extended attribute.
 515        smackfsroot=label: specifies the label to assign the root of the
 516        file system if it lacks the Smack extended attribute.
 518        smackfshat=label: specifies a label that must have read access to
 519        all labels set on the filesystem. Not yet enforced.
 521        smackfsfloor=label: specifies a label to which all labels set on the
 522        filesystem must have read access. Not yet enforced.
 524These mount options apply to all file system types.