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    - Basic utilities, which are helpful but not required
  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.
  27Smack kernels use the CIPSO IP option. Some network
  28configurations are intolerant of IP options and can impede
  29access to systems that use them as Smack does.
  31The current git repository for Smack user space is:
  33        git://
  35This should make and install on most modern distributions.
  36There are three commands included in smackutil:
  38smackload  - properly formats data for writing to /smack/load
  39smackcipso - properly formats data for writing to /smack/cipso
  40chsmack    - display or set Smack extended attribute values
  42In keeping with the intent of Smack, configuration data is
  43minimal and not strictly required. The most important
  44configuration step is mounting the smackfs pseudo filesystem.
  45If smackutil is installed the startup script will take care
  46of this, but it can be manually as well.
  48Add this line to /etc/fstab:
  50    smackfs /smack smackfs smackfsdef=* 0 0
  52and create the /smack directory for mounting.
  54Smack uses extended attributes (xattrs) to store labels on filesystem
  55objects. The attributes are stored in the extended attribute security
  56name space. A process must have CAP_MAC_ADMIN to change any of these
  59The 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.
  90There are multiple ways to set a Smack label on a file:
  92    # attr -S -s SMACK64 -V "value" path
  93    # chsmack -a value path
  95A process can see the smack label it is running with by
  96reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
  97can set the process smack by writing there.
  99Most Smack configuration is accomplished by writing to files
 100in the smackfs filesystem. This pseudo-filesystem is usually
 101mounted on /smack.
 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
 119        packets.
 121        This interface allows modification of existing access control rules.
 122        The format accepted on write is:
 123                "%s %s %s %s"
 124        where the first string is the subject label, the second the
 125        object label, the third the access to allow and the fourth the
 126        access to deny. The access strings may contain only the characters
 127        "rwxat-". If a rule for a given subject and object exists it will be
 128        modified by enabling the permissions in the third string and disabling
 129        those in the fourth string. If there is no such rule it will be
 130        created using the access specified in the third and the fourth strings.
 132        This interface allows a specific CIPSO header to be assigned
 133        to a Smack label. The format accepted on write is:
 134                "%24s%4d%4d"["%4d"]...
 135        The first string is a fixed Smack label. The first number is
 136        the level to use. The second number is the number of categories.
 137        The following numbers are the categories.
 138        "level-3-cats-5-19          3   2   5  19"
 140        This interface allows a specific CIPSO header to be assigned
 141        to a Smack label. The format accepted on write is:
 142        "%s%4d%4d"["%4d"]...
 143        The first string is a long Smack label. The first number is
 144        the level to use. The second number is the number of categories.
 145        The following numbers are the categories.
 146        "level-3-cats-5-19   3   2   5  19"
 148        This contains the CIPSO level used for Smack direct label
 149        representation in network packets.
 151        This contains the CIPSO domain of interpretation used in
 152        network packets.
 154        This interface allows access control rules in addition to
 155        the system defined rules to be specified. The format accepted
 156        on write is:
 157                "%24s%24s%5s"
 158        where the first string is the subject label, the second the
 159        object label, and the third the requested access. The access
 160        string may contain only the characters "rwxat-", and specifies
 161        which sort of access is allowed. The "-" is a placeholder for
 162        permissions that are not allowed. The string "r-x--" would
 163        specify read and execute access. Labels are limited to 23
 164        characters in length.
 166        This interface allows access control rules in addition to
 167        the system defined rules to be specified. The format accepted
 168        on write is:
 169                "%s %s %s"
 170        where the first string is the subject label, the second the
 171        object label, and the third the requested access. The access
 172        string may contain only the characters "rwxat-", and specifies
 173        which sort of access is allowed. The "-" is a placeholder for
 174        permissions that are not allowed. The string "r-x--" would
 175        specify read and execute access.
 177        This interface allows process specific access rules to be
 178        defined. These rules are only consulted if access would
 179        otherwise be permitted, and are intended to provide additional
 180        restrictions on the process. The format is the same as for
 181        the load interface.
 183        This interface allows process specific access rules to be
 184        defined. These rules are only consulted if access would
 185        otherwise be permitted, and are intended to provide additional
 186        restrictions on the process. The format is the same as for
 187        the load2 interface.
 189        This contains the Smack logging state.
 191        This contains the CIPSO level used for Smack mapped label
 192        representation in network packets.
 194        This interface allows specific internet addresses to be
 195        treated as single label hosts. Packets are sent to single
 196        label hosts without CIPSO headers, but only from processes
 197        that have Smack write access to the host label. All packets
 198        received from single label hosts are given the specified
 199        label. The format accepted on write is:
 200                "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
 202        This contains the label processes must have for CAP_MAC_ADMIN
 203        and CAP_MAC_OVERRIDE to be effective. If this file is empty
 204        these capabilities are effective at for processes with any
 205        label. The value is set by writing the desired label to the
 206        file or cleared by writing "-" to the file.
 208        Writing a Smack label here sets the access to '-' for all access
 209        rules with that subject label.
 211You can add access rules in /etc/smack/accesses. They take the form:
 213    subjectlabel objectlabel access
 215access is a combination of the letters rwxa which specify the
 216kind of access permitted a subject with subjectlabel on an
 217object with objectlabel. If there is no rule no access is allowed.
 219Look for additional programs on
 221From the Smack Whitepaper:
 223The Simplified Mandatory Access Control Kernel
 225Casey Schaufler
 228Mandatory Access Control
 230Computer systems employ a variety of schemes to constrain how information is
 231shared among the people and services using the machine. Some of these schemes
 232allow the program or user to decide what other programs or users are allowed
 233access to pieces of data. These schemes are called discretionary access
 234control mechanisms because the access control is specified at the discretion
 235of the user. Other schemes do not leave the decision regarding what a user or
 236program can access up to users or programs. These schemes are called mandatory
 237access control mechanisms because you don't have a choice regarding the users
 238or programs that have access to pieces of data.
 240Bell & LaPadula
 242From the middle of the 1980's until the turn of the century Mandatory Access
 243Control (MAC) was very closely associated with the Bell & LaPadula security
 244model, a mathematical description of the United States Department of Defense
 245policy for marking paper documents. MAC in this form enjoyed a following
 246within the Capital Beltway and Scandinavian supercomputer centers but was
 247often sited as failing to address general needs.
 249Domain Type Enforcement
 251Around the turn of the century Domain Type Enforcement (DTE) became popular.
 252This scheme organizes users, programs, and data into domains that are
 253protected from each other. This scheme has been widely deployed as a component
 254of popular Linux distributions. The administrative overhead required to
 255maintain this scheme and the detailed understanding of the whole system
 256necessary to provide a secure domain mapping leads to the scheme being
 257disabled or used in limited ways in the majority of cases.
 261Smack is a Mandatory Access Control mechanism designed to provide useful MAC
 262while avoiding the pitfalls of its predecessors. The limitations of Bell &
 263LaPadula are addressed by providing a scheme whereby access can be controlled
 264according to the requirements of the system and its purpose rather than those
 265imposed by an arcane government policy. The complexity of Domain Type
 266Enforcement and avoided by defining access controls in terms of the access
 267modes already in use.
 269Smack Terminology
 271The jargon used to talk about Smack will be familiar to those who have dealt
 272with other MAC systems and shouldn't be too difficult for the uninitiated to
 273pick up. There are four terms that are used in a specific way and that are
 274especially important:
 276        Subject: A subject is an active entity on the computer system.
 277        On Smack a subject is a task, which is in turn the basic unit
 278        of execution.
 280        Object: An object is a passive entity on the computer system.
 281        On Smack files of all types, IPC, and tasks can be objects.
 283        Access: Any attempt by a subject to put information into or get
 284        information from an object is an access.
 286        Label: Data that identifies the Mandatory Access Control
 287        characteristics of a subject or an object.
 289These definitions are consistent with the traditional use in the security
 290community. There are also some terms from Linux that are likely to crop up:
 292        Capability: A task that possesses a capability has permission to
 293        violate an aspect of the system security policy, as identified by
 294        the specific capability. A task that possesses one or more
 295        capabilities is a privileged task, whereas a task with no
 296        capabilities is an unprivileged task.
 298        Privilege: A task that is allowed to violate the system security
 299        policy is said to have privilege. As of this writing a task can
 300        have privilege either by possessing capabilities or by having an
 301        effective user of root.
 303Smack Basics
 305Smack is an extension to a Linux system. It enforces additional restrictions
 306on what subjects can access which objects, based on the labels attached to
 307each of the subject and the object.
 311Smack labels are ASCII character strings, one to twenty-three characters in
 312length. Single character labels using special characters, that being anything
 313other than a letter or digit, are reserved for use by the Smack development
 314team. Smack labels are unstructured, case sensitive, and the only operation
 315ever performed on them is comparison for equality. Smack labels cannot
 316contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
 317(quote) and '"' (double-quote) characters.
 318Smack labels cannot begin with a '-'. This is reserved for special options.
 320There are some predefined labels:
 322        _       Pronounced "floor", a single underscore character.
 323        ^       Pronounced "hat", a single circumflex character.
 324        *       Pronounced "star", a single asterisk character.
 325        ?       Pronounced "huh", a single question mark character.
 326        @       Pronounced "web", a single at sign character.
 328Every task on a Smack system is assigned a label. System tasks, such as
 329init(8) and systems daemons, are run with the floor ("_") label. User tasks
 330are assigned labels according to the specification found in the
 331/etc/smack/user configuration file.
 333Access Rules
 335Smack uses the traditional access modes of Linux. These modes are read,
 336execute, write, and occasionally append. There are a few cases where the
 337access mode may not be obvious. These include:
 339        Signals: A signal is a write operation from the subject task to
 340        the object task.
 341        Internet Domain IPC: Transmission of a packet is considered a
 342        write operation from the source task to the destination task.
 344Smack restricts access based on the label attached to a subject and the label
 345attached to the object it is trying to access. The rules enforced are, in
 348        1. Any access requested by a task labeled "*" is denied.
 349        2. A read or execute access requested by a task labeled "^"
 350           is permitted.
 351        3. A read or execute access requested on an object labeled "_"
 352           is permitted.
 353        4. Any access requested on an object labeled "*" is permitted.
 354        5. Any access requested by a task on an object with the same
 355           label is permitted.
 356        6. Any access requested that is explicitly defined in the loaded
 357           rule set is permitted.
 358        7. Any other access is denied.
 360Smack Access Rules
 362With the isolation provided by Smack access separation is simple. There are
 363many interesting cases where limited access by subjects to objects with
 364different labels is desired. One example is the familiar spy model of
 365sensitivity, where a scientist working on a highly classified project would be
 366able to read documents of lower classifications and anything she writes will
 367be "born" highly classified. To accommodate such schemes Smack includes a
 368mechanism for specifying rules allowing access between labels.
 370Access Rule Format
 372The format of an access rule is:
 374        subject-label object-label access
 376Where subject-label is the Smack label of the task, object-label is the Smack
 377label of the thing being accessed, and access is a string specifying the sort
 378of access allowed. The access specification is searched for letters that
 379describe access modes:
 381        a: indicates that append access should be granted.
 382        r: indicates that read access should be granted.
 383        w: indicates that write access should be granted.
 384        x: indicates that execute access should be granted.
 385        t: indicates that the rule requests transmutation.
 387Uppercase values for the specification letters are allowed as well.
 388Access mode specifications can be in any order. Examples of acceptable rules
 391        TopSecret Secret  rx
 392        Secret    Unclass R
 393        Manager   Game    x
 394        User      HR      w
 395        New       Old     rRrRr
 396        Closed    Off     -
 398Examples of unacceptable rules are:
 400        Top Secret Secret     rx
 401        Ace        Ace        r
 402        Odd        spells     waxbeans
 404Spaces are not allowed in labels. Since a subject always has access to files
 405with the same label specifying a rule for that case is pointless. Only
 406valid letters (rwxatRWXAT) and the dash ('-') character are allowed in
 407access specifications. The dash is a placeholder, so "a-r" is the same
 408as "ar". A lone dash is used to specify that no access should be allowed.
 410Applying Access Rules
 412The developers of Linux rarely define new sorts of things, usually importing
 413schemes and concepts from other systems. Most often, the other systems are
 414variants of Unix. Unix has many endearing properties, but consistency of
 415access control models is not one of them. Smack strives to treat accesses as
 416uniformly as is sensible while keeping with the spirit of the underlying
 419File system objects including files, directories, named pipes, symbolic links,
 420and devices require access permissions that closely match those used by mode
 421bit access. To open a file for reading read access is required on the file. To
 422search a directory requires execute access. Creating a file with write access
 423requires both read and write access on the containing directory. Deleting a
 424file requires read and write access to the file and to the containing
 425directory. It is possible that a user may be able to see that a file exists
 426but not any of its attributes by the circumstance of having read access to the
 427containing directory but not to the differently labeled file. This is an
 428artifact of the file name being data in the directory, not a part of the file.
 430If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
 431access rule that allows a process to create an object in that directory
 432includes 't' access the label assigned to the new object will be that
 433of the directory, not the creating process. This makes it much easier
 434for two processes with different labels to share data without granting
 435access to all of their files.
 437IPC objects, message queues, semaphore sets, and memory segments exist in flat
 438namespaces and access requests are only required to match the object in
 441Process objects reflect tasks on the system and the Smack label used to access
 442them is the same Smack label that the task would use for its own access
 443attempts. Sending a signal via the kill() system call is a write operation
 444from the signaler to the recipient. Debugging a process requires both reading
 445and writing. Creating a new task is an internal operation that results in two
 446tasks with identical Smack labels and requires no access checks.
 448Sockets are data structures attached to processes and sending a packet from
 449one process to another requires that the sender have write access to the
 450receiver. The receiver is not required to have read access to the sender.
 452Setting Access Rules
 454The configuration file /etc/smack/accesses contains the rules to be set at
 455system startup. The contents are written to the special file /smack/load.
 456Rules can be written to /smack/load at any time and take effect immediately.
 457For any pair of subject and object labels there can be only one rule, with the
 458most recently specified overriding any earlier specification.
 460The program smackload is provided to ensure data is formatted
 461properly when written to /smack/load. This program reads lines
 462of the form
 464    subjectlabel objectlabel mode.
 466Task Attribute
 468The Smack label of a process can be read from /proc/<pid>/attr/current. A
 469process can read its own Smack label from /proc/self/attr/current. A
 470privileged process can change its own Smack label by writing to
 471/proc/self/attr/current but not the label of another process.
 473File Attribute
 475The Smack label of a filesystem object is stored as an extended attribute
 476named SMACK64 on the file. This attribute is in the security namespace. It can
 477only be changed by a process with privilege.
 481A process with CAP_MAC_OVERRIDE is privileged.
 483Smack Networking
 485As mentioned before, Smack enforces access control on network protocol
 486transmissions. Every packet sent by a Smack process is tagged with its Smack
 487label. This is done by adding a CIPSO tag to the header of the IP packet. Each
 488packet received is expected to have a CIPSO tag that identifies the label and
 489if it lacks such a tag the network ambient label is assumed. Before the packet
 490is delivered a check is made to determine that a subject with the label on the
 491packet has write access to the receiving process and if that is not the case
 492the packet is dropped.
 494CIPSO Configuration
 496It is normally unnecessary to specify the CIPSO configuration. The default
 497values used by the system handle all internal cases. Smack will compose CIPSO
 498label values to match the Smack labels being used without administrative
 499intervention. Unlabeled packets that come into the system will be given the
 500ambient label.
 502Smack requires configuration in the case where packets from a system that is
 503not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
 504Solaris system, but there are other, less widely deployed systems out there.
 505CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
 506and a category set with each packet. The DOI is intended to identify a group
 507of systems that use compatible labeling schemes, and the DOI specified on the
 508smack system must match that of the remote system or packets will be
 509discarded. The DOI is 3 by default. The value can be read from /smack/doi and
 510can be changed by writing to /smack/doi.
 512The label and category set are mapped to a Smack label as defined in
 515A Smack/CIPSO mapping has the form:
 517        smack level [category [category]*]
 519Smack does not expect the level or category sets to be related in any
 520particular way and does not assume or assign accesses based on them. Some
 521examples of mappings:
 523        TopSecret 7
 524        TS:A,B    7 1 2
 525        SecBDE    5 2 4 6
 526        RAFTERS   7 12 26
 528The ":" and "," characters are permitted in a Smack label but have no special
 531The mapping of Smack labels to CIPSO values is defined by writing to
 532/smack/cipso. Again, the format of data written to this special file
 533is highly restrictive, so the program smackcipso is provided to
 534ensure the writes are done properly. This program takes mappings
 535on the standard input and sends them to /smack/cipso properly.
 537In addition to explicit mappings Smack supports direct CIPSO mappings. One
 538CIPSO level is used to indicate that the category set passed in the packet is
 539in fact an encoding of the Smack label. The level used is 250 by default. The
 540value can be read from /smack/direct and changed by writing to /smack/direct.
 542Socket Attributes
 544There are two attributes that are associated with sockets. These attributes
 545can only be set by privileged tasks, but any task can read them for their own
 548        SMACK64IPIN: The Smack label of the task object. A privileged
 549        program that will enforce policy may set this to the star label.
 551        SMACK64IPOUT: The Smack label transmitted with outgoing packets.
 552        A privileged program may set this to match the label of another
 553        task with which it hopes to communicate.
 555Smack Netlabel Exceptions
 557You will often find that your labeled application has to talk to the outside,
 558unlabeled world. To do this there's a special file /smack/netlabel where you can
 559add some exceptions in the form of :
 560@IP1       LABEL1 or
 563It means that your application will have unlabeled access to @IP1 if it has
 564write access on LABEL1, and access to the subnet @IP2/MASK if it has write
 565access on LABEL2.
 567Entries in the /smack/netlabel file are matched by longest mask first, like in
 568classless IPv4 routing.
 570A special label '@' and an option '-CIPSO' can be used there :
 571@      means Internet, any application with any label has access to it
 572-CIPSO means standard CIPSO networking
 574If you don't know what CIPSO is and don't plan to use it, you can just do :
 575echo -CIPSO > /smack/netlabel
 576echo @      > /smack/netlabel
 578If you use CIPSO on your local network and need also unlabeled
 579Internet access, you can have :
 580echo      -CIPSO > /smack/netlabel
 581echo -CIPSO > /smack/netlabel
 582echo      @      > /smack/netlabel
 585Writing Applications for Smack
 587There are three sorts of applications that will run on a Smack system. How an
 588application interacts with Smack will determine what it will have to do to
 589work properly under Smack.
 591Smack Ignorant Applications
 593By far the majority of applications have no reason whatever to care about the
 594unique properties of Smack. Since invoking a program has no impact on the
 595Smack label associated with the process the only concern likely to arise is
 596whether the process has execute access to the program.
 598Smack Relevant Applications
 600Some programs can be improved by teaching them about Smack, but do not make
 601any security decisions themselves. The utility ls(1) is one example of such a
 604Smack Enforcing Applications
 606These are special programs that not only know about Smack, but participate in
 607the enforcement of system policy. In most cases these are the programs that
 608set up user sessions. There are also network services that provide information
 609to processes running with various labels.
 611File System Interfaces
 613Smack maintains labels on file system objects using extended attributes. The
 614Smack label of a file, directory, or other file system object can be obtained
 615using getxattr(2).
 617        len = getxattr("/", "security.SMACK64", value, sizeof (value));
 619will put the Smack label of the root directory into value. A privileged
 620process can set the Smack label of a file system object with setxattr(2).
 622        len = strlen("Rubble");
 623        rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
 625will set the Smack label of /foo to "Rubble" if the program has appropriate
 628Socket Interfaces
 630The socket attributes can be read using fgetxattr(2).
 632A privileged process can set the Smack label of outgoing packets with
 635        len = strlen("Rubble");
 636        rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
 638will set the Smack label "Rubble" on packets going out from the socket if the
 639program has appropriate privilege.
 641        rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
 643will set the Smack label "*" as the object label against which incoming
 644packets will be checked if the program has appropriate privilege.
 648Smack supports some mount options:
 650        smackfsdef=label: specifies the label to give files that lack
 651        the Smack label extended attribute.
 653        smackfsroot=label: specifies the label to assign the root of the
 654        file system if it lacks the Smack extended attribute.
 656        smackfshat=label: specifies a label that must have read access to
 657        all labels set on the filesystem. Not yet enforced.
 659        smackfsfloor=label: specifies a label to which all labels set on the
 660        filesystem must have read access. Not yet enforced.
 662These mount options apply to all file system types.
 664Smack auditing
 666If you want Smack auditing of security events, you need to set CONFIG_AUDIT
 667in your kernel configuration.
 668By default, all denied events will be audited. You can change this behavior by
 669writing a single character to the /smack/logging file :
 6700 : no logging
 6711 : log denied (default)
 6722 : log accepted
 6733 : log denied & accepted
 675Events are logged as 'key=value' pairs, for each event you at least will get
 676the subject, the object, the rights requested, the action, the kernel function
 677that triggered the event, plus other pairs depending on the type of event
 679 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.