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 repositories for Smack user space are:
  36These should make and install on most modern distributions.
  37There are three commands included in smackutil:
  39smackload  - properly formats data for writing to /smack/load
  40smackcipso - properly formats data for writing to /smack/cipso
  41chsmack    - display or set Smack extended attribute values
  43In keeping with the intent of Smack, configuration data is
  44minimal and not strictly required. The most important
  45configuration step is mounting the smackfs pseudo filesystem.
  46If smackutil is installed the startup script will take care
  47of this, but it can be manually as well.
  49Add this line to /etc/fstab:
  51    smackfs /smack smackfs smackfsdef=* 0 0
  53and create the /smack directory for mounting.
  55Smack uses extended attributes (xattrs) to store labels on filesystem
  56objects. The attributes are stored in the extended attribute security
  57name space. A process must have CAP_MAC_ADMIN to change any of these
  60The extended attributes that Smack uses are:
  63        Used to make access control decisions. In almost all cases
  64        the label given to a new filesystem object will be the label
  65        of the process that created it.
  67        The Smack label of a process that execs a program file with
  68        this attribute set will run with this attribute's value.
  70        Don't allow the file to be mmapped by a process whose Smack
  71        label does not allow all of the access permitted to a process
  72        with the label contained in this attribute. This is a very
  73        specific use case for shared libraries.
  75        Can only have the value "TRUE". If this attribute is present
  76        on a directory when an object is created in the directory and
  77        the Smack rule (more below) that permitted the write access
  78        to the directory includes the transmute ("t") mode the object
  79        gets the label of the directory instead of the label of the
  80        creating process. If the object being created is a directory
  81        the SMACK64TRANSMUTE attribute is set as well.
  83        This attribute is only available on file descriptors for sockets.
  84        Use the Smack label in this attribute for access control
  85        decisions on packets being delivered to this socket.
  87        This attribute is only available on file descriptors for sockets.
  88        Use the Smack label in this attribute for access control
  89        decisions on packets coming from this socket.
  91There are multiple ways to set a Smack label on a file:
  93    # attr -S -s SMACK64 -V "value" path
  94    # chsmack -a value path
  96A process can see the smack label it is running with by
  97reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
  98can set the process smack by writing there.
 100Most Smack configuration is accomplished by writing to files
 101in the smackfs filesystem. This pseudo-filesystem is usually
 102mounted on /smack.
 105        This interface reports whether a subject with the specified
 106        Smack label has a particular access to an object with a
 107        specified Smack label. Write a fixed format access rule to
 108        this file. The next read will indicate whether the access
 109        would be permitted. The text will be either "1" indicating
 110        access, or "0" indicating denial.
 112        This interface reports whether a subject with the specified
 113        Smack label has a particular access to an object with a
 114        specified Smack label. Write a long format access rule to
 115        this file. The next read will indicate whether the access
 116        would be permitted. The text will be either "1" indicating
 117        access, or "0" indicating denial.
 119        This contains the Smack label applied to unlabeled network
 120        packets.
 122        This interface allows a specific CIPSO header to be assigned
 123        to a Smack label. The format accepted on write is:
 124                "%24s%4d%4d"["%4d"]...
 125        The first string is a fixed Smack label. The first number is
 126        the level to use. The second number is the number of categories.
 127        The following numbers are the categories.
 128        "level-3-cats-5-19          3   2   5  19"
 130        This interface allows a specific CIPSO header to be assigned
 131        to a Smack label. The format accepted on write is:
 132        "%s%4d%4d"["%4d"]...
 133        The first string is a long Smack label. The first number is
 134        the level to use. The second number is the number of categories.
 135        The following numbers are the categories.
 136        "level-3-cats-5-19   3   2   5  19"
 138        This contains the CIPSO level used for Smack direct label
 139        representation in network packets.
 141        This contains the CIPSO domain of interpretation used in
 142        network packets.
 144        This interface allows access control rules in addition to
 145        the system defined rules to be specified. The format accepted
 146        on write is:
 147                "%24s%24s%5s"
 148        where the first string is the subject label, the second the
 149        object label, and the third the requested access. The access
 150        string may contain only the characters "rwxat-", and specifies
 151        which sort of access is allowed. The "-" is a placeholder for
 152        permissions that are not allowed. The string "r-x--" would
 153        specify read and execute access. Labels are limited to 23
 154        characters in length.
 156        This interface allows access control rules in addition to
 157        the system defined rules to be specified. The format accepted
 158        on write is:
 159                "%s %s %s"
 160        where the first string is the subject label, the second the
 161        object label, and the third the requested access. The access
 162        string may contain only the characters "rwxat-", and specifies
 163        which sort of access is allowed. The "-" is a placeholder for
 164        permissions that are not allowed. The string "r-x--" would
 165        specify read and execute access.
 167        This interface allows process specific access rules to be
 168        defined. These rules are only consulted if access would
 169        otherwise be permitted, and are intended to provide additional
 170        restrictions on the process. The format is the same as for
 171        the load interface.
 173        This interface allows process specific access rules to be
 174        defined. These rules are only consulted if access would
 175        otherwise be permitted, and are intended to provide additional
 176        restrictions on the process. The format is the same as for
 177        the load2 interface.
 179        This contains the Smack logging state.
 181        This contains the CIPSO level used for Smack mapped label
 182        representation in network packets.
 184        This interface allows specific internet addresses to be
 185        treated as single label hosts. Packets are sent to single
 186        label hosts without CIPSO headers, but only from processes
 187        that have Smack write access to the host label. All packets
 188        received from single label hosts are given the specified
 189        label. The format accepted on write is:
 190                "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
 192        This contains the label processes must have for CAP_MAC_ADMIN
 193        and CAP_MAC_OVERRIDE to be effective. If this file is empty
 194        these capabilities are effective at for processes with any
 195        label. The value is set by writing the desired label to the
 196        file or cleared by writing "-" to the file.
 198You can add access rules in /etc/smack/accesses. They take the form:
 200    subjectlabel objectlabel access
 202access is a combination of the letters rwxa which specify the
 203kind of access permitted a subject with subjectlabel on an
 204object with objectlabel. If there is no rule no access is allowed.
 206Look for additional programs on
 208From the Smack Whitepaper:
 210The Simplified Mandatory Access Control Kernel
 212Casey Schaufler
 215Mandatory Access Control
 217Computer systems employ a variety of schemes to constrain how information is
 218shared among the people and services using the machine. Some of these schemes
 219allow the program or user to decide what other programs or users are allowed
 220access to pieces of data. These schemes are called discretionary access
 221control mechanisms because the access control is specified at the discretion
 222of the user. Other schemes do not leave the decision regarding what a user or
 223program can access up to users or programs. These schemes are called mandatory
 224access control mechanisms because you don't have a choice regarding the users
 225or programs that have access to pieces of data.
 227Bell & LaPadula
 229From the middle of the 1980's until the turn of the century Mandatory Access
 230Control (MAC) was very closely associated with the Bell & LaPadula security
 231model, a mathematical description of the United States Department of Defense
 232policy for marking paper documents. MAC in this form enjoyed a following
 233within the Capital Beltway and Scandinavian supercomputer centers but was
 234often sited as failing to address general needs.
 236Domain Type Enforcement
 238Around the turn of the century Domain Type Enforcement (DTE) became popular.
 239This scheme organizes users, programs, and data into domains that are
 240protected from each other. This scheme has been widely deployed as a component
 241of popular Linux distributions. The administrative overhead required to
 242maintain this scheme and the detailed understanding of the whole system
 243necessary to provide a secure domain mapping leads to the scheme being
 244disabled or used in limited ways in the majority of cases.
 248Smack is a Mandatory Access Control mechanism designed to provide useful MAC
 249while avoiding the pitfalls of its predecessors. The limitations of Bell &
 250LaPadula are addressed by providing a scheme whereby access can be controlled
 251according to the requirements of the system and its purpose rather than those
 252imposed by an arcane government policy. The complexity of Domain Type
 253Enforcement and avoided by defining access controls in terms of the access
 254modes already in use.
 256Smack Terminology
 258The jargon used to talk about Smack will be familiar to those who have dealt
 259with other MAC systems and shouldn't be too difficult for the uninitiated to
 260pick up. There are four terms that are used in a specific way and that are
 261especially important:
 263        Subject: A subject is an active entity on the computer system.
 264        On Smack a subject is a task, which is in turn the basic unit
 265        of execution.
 267        Object: An object is a passive entity on the computer system.
 268        On Smack files of all types, IPC, and tasks can be objects.
 270        Access: Any attempt by a subject to put information into or get
 271        information from an object is an access.
 273        Label: Data that identifies the Mandatory Access Control
 274        characteristics of a subject or an object.
 276These definitions are consistent with the traditional use in the security
 277community. There are also some terms from Linux that are likely to crop up:
 279        Capability: A task that possesses a capability has permission to
 280        violate an aspect of the system security policy, as identified by
 281        the specific capability. A task that possesses one or more
 282        capabilities is a privileged task, whereas a task with no
 283        capabilities is an unprivileged task.
 285        Privilege: A task that is allowed to violate the system security
 286        policy is said to have privilege. As of this writing a task can
 287        have privilege either by possessing capabilities or by having an
 288        effective user of root.
 290Smack Basics
 292Smack is an extension to a Linux system. It enforces additional restrictions
 293on what subjects can access which objects, based on the labels attached to
 294each of the subject and the object.
 298Smack labels are ASCII character strings, one to twenty-three characters in
 299length. Single character labels using special characters, that being anything
 300other than a letter or digit, are reserved for use by the Smack development
 301team. Smack labels are unstructured, case sensitive, and the only operation
 302ever performed on them is comparison for equality. Smack labels cannot
 303contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
 304(quote) and '"' (double-quote) characters.
 305Smack labels cannot begin with a '-'. This is reserved for special options.
 307There are some predefined labels:
 309        _       Pronounced "floor", a single underscore character.
 310        ^       Pronounced "hat", a single circumflex character.
 311        *       Pronounced "star", a single asterisk character.
 312        ?       Pronounced "huh", a single question mark character.
 313        @       Pronounced "web", a single at sign character.
 315Every task on a Smack system is assigned a label. System tasks, such as
 316init(8) and systems daemons, are run with the floor ("_") label. User tasks
 317are assigned labels according to the specification found in the
 318/etc/smack/user configuration file.
 320Access Rules
 322Smack uses the traditional access modes of Linux. These modes are read,
 323execute, write, and occasionally append. There are a few cases where the
 324access mode may not be obvious. These include:
 326        Signals: A signal is a write operation from the subject task to
 327        the object task.
 328        Internet Domain IPC: Transmission of a packet is considered a
 329        write operation from the source task to the destination task.
 331Smack restricts access based on the label attached to a subject and the label
 332attached to the object it is trying to access. The rules enforced are, in
 335        1. Any access requested by a task labeled "*" is denied.
 336        2. A read or execute access requested by a task labeled "^"
 337           is permitted.
 338        3. A read or execute access requested on an object labeled "_"
 339           is permitted.
 340        4. Any access requested on an object labeled "*" is permitted.
 341        5. Any access requested by a task on an object with the same
 342           label is permitted.
 343        6. Any access requested that is explicitly defined in the loaded
 344           rule set is permitted.
 345        7. Any other access is denied.
 347Smack Access Rules
 349With the isolation provided by Smack access separation is simple. There are
 350many interesting cases where limited access by subjects to objects with
 351different labels is desired. One example is the familiar spy model of
 352sensitivity, where a scientist working on a highly classified project would be
 353able to read documents of lower classifications and anything she writes will
 354be "born" highly classified. To accommodate such schemes Smack includes a
 355mechanism for specifying rules allowing access between labels.
 357Access Rule Format
 359The format of an access rule is:
 361        subject-label object-label access
 363Where subject-label is the Smack label of the task, object-label is the Smack
 364label of the thing being accessed, and access is a string specifying the sort
 365of access allowed. The access specification is searched for letters that
 366describe access modes:
 368        a: indicates that append access should be granted.
 369        r: indicates that read access should be granted.
 370        w: indicates that write access should be granted.
 371        x: indicates that execute access should be granted.
 372        t: indicates that the rule requests transmutation.
 374Uppercase values for the specification letters are allowed as well.
 375Access mode specifications can be in any order. Examples of acceptable rules
 378        TopSecret Secret  rx
 379        Secret    Unclass R
 380        Manager   Game    x
 381        User      HR      w
 382        New       Old     rRrRr
 383        Closed    Off     -
 385Examples of unacceptable rules are:
 387        Top Secret Secret     rx
 388        Ace        Ace        r
 389        Odd        spells     waxbeans
 391Spaces are not allowed in labels. Since a subject always has access to files
 392with the same label specifying a rule for that case is pointless. Only
 393valid letters (rwxatRWXAT) and the dash ('-') character are allowed in
 394access specifications. The dash is a placeholder, so "a-r" is the same
 395as "ar". A lone dash is used to specify that no access should be allowed.
 397Applying Access Rules
 399The developers of Linux rarely define new sorts of things, usually importing
 400schemes and concepts from other systems. Most often, the other systems are
 401variants of Unix. Unix has many endearing properties, but consistency of
 402access control models is not one of them. Smack strives to treat accesses as
 403uniformly as is sensible while keeping with the spirit of the underlying
 406File system objects including files, directories, named pipes, symbolic links,
 407and devices require access permissions that closely match those used by mode
 408bit access. To open a file for reading read access is required on the file. To
 409search a directory requires execute access. Creating a file with write access
 410requires both read and write access on the containing directory. Deleting a
 411file requires read and write access to the file and to the containing
 412directory. It is possible that a user may be able to see that a file exists
 413but not any of its attributes by the circumstance of having read access to the
 414containing directory but not to the differently labeled file. This is an
 415artifact of the file name being data in the directory, not a part of the file.
 417If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
 418access rule that allows a process to create an object in that directory
 419includes 't' access the label assigned to the new object will be that
 420of the directory, not the creating process. This makes it much easier
 421for two processes with different labels to share data without granting
 422access to all of their files.
 424IPC objects, message queues, semaphore sets, and memory segments exist in flat
 425namespaces and access requests are only required to match the object in
 428Process objects reflect tasks on the system and the Smack label used to access
 429them is the same Smack label that the task would use for its own access
 430attempts. Sending a signal via the kill() system call is a write operation
 431from the signaler to the recipient. Debugging a process requires both reading
 432and writing. Creating a new task is an internal operation that results in two
 433tasks with identical Smack labels and requires no access checks.
 435Sockets are data structures attached to processes and sending a packet from
 436one process to another requires that the sender have write access to the
 437receiver. The receiver is not required to have read access to the sender.
 439Setting Access Rules
 441The configuration file /etc/smack/accesses contains the rules to be set at
 442system startup. The contents are written to the special file /smack/load.
 443Rules can be written to /smack/load at any time and take effect immediately.
 444For any pair of subject and object labels there can be only one rule, with the
 445most recently specified overriding any earlier specification.
 447The program smackload is provided to ensure data is formatted
 448properly when written to /smack/load. This program reads lines
 449of the form
 451    subjectlabel objectlabel mode.
 453Task Attribute
 455The Smack label of a process can be read from /proc/<pid>/attr/current. A
 456process can read its own Smack label from /proc/self/attr/current. A
 457privileged process can change its own Smack label by writing to
 458/proc/self/attr/current but not the label of another process.
 460File Attribute
 462The Smack label of a filesystem object is stored as an extended attribute
 463named SMACK64 on the file. This attribute is in the security namespace. It can
 464only be changed by a process with privilege.
 468A process with CAP_MAC_OVERRIDE is privileged.
 470Smack Networking
 472As mentioned before, Smack enforces access control on network protocol
 473transmissions. Every packet sent by a Smack process is tagged with its Smack
 474label. This is done by adding a CIPSO tag to the header of the IP packet. Each
 475packet received is expected to have a CIPSO tag that identifies the label and
 476if it lacks such a tag the network ambient label is assumed. Before the packet
 477is delivered a check is made to determine that a subject with the label on the
 478packet has write access to the receiving process and if that is not the case
 479the packet is dropped.
 481CIPSO Configuration
 483It is normally unnecessary to specify the CIPSO configuration. The default
 484values used by the system handle all internal cases. Smack will compose CIPSO
 485label values to match the Smack labels being used without administrative
 486intervention. Unlabeled packets that come into the system will be given the
 487ambient label.
 489Smack requires configuration in the case where packets from a system that is
 490not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
 491Solaris system, but there are other, less widely deployed systems out there.
 492CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
 493and a category set with each packet. The DOI is intended to identify a group
 494of systems that use compatible labeling schemes, and the DOI specified on the
 495smack system must match that of the remote system or packets will be
 496discarded. The DOI is 3 by default. The value can be read from /smack/doi and
 497can be changed by writing to /smack/doi.
 499The label and category set are mapped to a Smack label as defined in
 502A Smack/CIPSO mapping has the form:
 504        smack level [category [category]*]
 506Smack does not expect the level or category sets to be related in any
 507particular way and does not assume or assign accesses based on them. Some
 508examples of mappings:
 510        TopSecret 7
 511        TS:A,B    7 1 2
 512        SecBDE    5 2 4 6
 513        RAFTERS   7 12 26
 515The ":" and "," characters are permitted in a Smack label but have no special
 518The mapping of Smack labels to CIPSO values is defined by writing to
 519/smack/cipso. Again, the format of data written to this special file
 520is highly restrictive, so the program smackcipso is provided to
 521ensure the writes are done properly. This program takes mappings
 522on the standard input and sends them to /smack/cipso properly.
 524In addition to explicit mappings Smack supports direct CIPSO mappings. One
 525CIPSO level is used to indicate that the category set passed in the packet is
 526in fact an encoding of the Smack label. The level used is 250 by default. The
 527value can be read from /smack/direct and changed by writing to /smack/direct.
 529Socket Attributes
 531There are two attributes that are associated with sockets. These attributes
 532can only be set by privileged tasks, but any task can read them for their own
 535        SMACK64IPIN: The Smack label of the task object. A privileged
 536        program that will enforce policy may set this to the star label.
 538        SMACK64IPOUT: The Smack label transmitted with outgoing packets.
 539        A privileged program may set this to match the label of another
 540        task with which it hopes to communicate.
 542Smack Netlabel Exceptions
 544You will often find that your labeled application has to talk to the outside,
 545unlabeled world. To do this there's a special file /smack/netlabel where you can
 546add some exceptions in the form of :
 547@IP1       LABEL1 or
 550It means that your application will have unlabeled access to @IP1 if it has
 551write access on LABEL1, and access to the subnet @IP2/MASK if it has write
 552access on LABEL2.
 554Entries in the /smack/netlabel file are matched by longest mask first, like in
 555classless IPv4 routing.
 557A special label '@' and an option '-CIPSO' can be used there :
 558@      means Internet, any application with any label has access to it
 559-CIPSO means standard CIPSO networking
 561If you don't know what CIPSO is and don't plan to use it, you can just do :
 562echo -CIPSO > /smack/netlabel
 563echo @      > /smack/netlabel
 565If you use CIPSO on your local network and need also unlabeled
 566Internet access, you can have :
 567echo      -CIPSO > /smack/netlabel
 568echo -CIPSO > /smack/netlabel
 569echo      @      > /smack/netlabel
 572Writing Applications for Smack
 574There are three sorts of applications that will run on a Smack system. How an
 575application interacts with Smack will determine what it will have to do to
 576work properly under Smack.
 578Smack Ignorant Applications
 580By far the majority of applications have no reason whatever to care about the
 581unique properties of Smack. Since invoking a program has no impact on the
 582Smack label associated with the process the only concern likely to arise is
 583whether the process has execute access to the program.
 585Smack Relevant Applications
 587Some programs can be improved by teaching them about Smack, but do not make
 588any security decisions themselves. The utility ls(1) is one example of such a
 591Smack Enforcing Applications
 593These are special programs that not only know about Smack, but participate in
 594the enforcement of system policy. In most cases these are the programs that
 595set up user sessions. There are also network services that provide information
 596to processes running with various labels.
 598File System Interfaces
 600Smack maintains labels on file system objects using extended attributes. The
 601Smack label of a file, directory, or other file system object can be obtained
 602using getxattr(2).
 604        len = getxattr("/", "security.SMACK64", value, sizeof (value));
 606will put the Smack label of the root directory into value. A privileged
 607process can set the Smack label of a file system object with setxattr(2).
 609        len = strlen("Rubble");
 610        rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
 612will set the Smack label of /foo to "Rubble" if the program has appropriate
 615Socket Interfaces
 617The socket attributes can be read using fgetxattr(2).
 619A privileged process can set the Smack label of outgoing packets with
 622        len = strlen("Rubble");
 623        rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
 625will set the Smack label "Rubble" on packets going out from the socket if the
 626program has appropriate privilege.
 628        rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
 630will set the Smack label "*" as the object label against which incoming
 631packets will be checked if the program has appropriate privilege.
 635Smack supports some mount options:
 637        smackfsdef=label: specifies the label to give files that lack
 638        the Smack label extended attribute.
 640        smackfsroot=label: specifies the label to assign the root of the
 641        file system if it lacks the Smack extended attribute.
 643        smackfshat=label: specifies a label that must have read access to
 644        all labels set on the filesystem. Not yet enforced.
 646        smackfsfloor=label: specifies a label to which all labels set on the
 647        filesystem must have read access. Not yet enforced.
 649These mount options apply to all file system types.
 651Smack auditing
 653If you want Smack auditing of security events, you need to set CONFIG_AUDIT
 654in your kernel configuration.
 655By default, all denied events will be audited. You can change this behavior by
 656writing a single character to the /smack/logging file :
 6570 : no logging
 6581 : log denied (default)
 6592 : log accepted
 6603 : log denied & accepted
 662Events are logged as 'key=value' pairs, for each event you at least will get
 663the subject, the object, the rights requested, the action, the kernel function
 664that triggered the event, plus other pairs depending on the type of event
 666 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.