"Good for you, you've decided to clean the elevator!"
      - The Elevator, from Dark Star
  
  Smack is the the Simplified Mandatory Access Control Kernel.
  Smack is a kernel based implementation of mandatory access
  control that includes simplicity in its primary design goals.
  
  Smack is not the only Mandatory Access Control scheme
  available for Linux. Those new to Mandatory Access Control
  are encouraged to compare Smack with the other mechanisms
  available to determine which is best suited to the problem
  at hand.
  
  Smack consists of three major components:
      - The kernel
      - A start-up script and a few modified applications
      - Configuration data
  
  The kernel component of Smack is implemented as a Linux
  Security Modules (LSM) module. It requires netlabel and
  works best with file systems that support extended attributes,
  although xattr support is not strictly required.
  It is safe to run a Smack kernel under a "vanilla" distribution.
  Smack kernels use the CIPSO IP option. Some network
  configurations are intolerant of IP options and can impede
  access to systems that use them as Smack does.
  
  The startup script etc-init.d-smack should be installed
  in /etc/init.d/smack and should be invoked early in the
  start-up process. On Fedora rc5.d/S02smack is recommended.
  This script ensures that certain devices have the correct
  Smack attributes and loads the Smack configuration if
  any is defined. This script invokes two programs that
  ensure configuration data is properly formatted. These
  programs are /usr/sbin/smackload and /usr/sin/smackcipso.
  The system will run just fine without these programs,
  but it will be difficult to set access rules properly.
  
  A version of "ls" that provides a "-M" option to display
  Smack labels on long listing is available.
  
  A hacked version of sshd that allows network logins by users
  with specific Smack labels is available. This version does
  not work for scp. You must set the /etc/ssh/sshd_config
  line:
     UsePrivilegeSeparation no
  
  The format of /etc/smack/usr is:
  
     username smack
  
  In keeping with the intent of Smack, configuration data is
  minimal and not strictly required. The most important
  configuration step is mounting the smackfs pseudo filesystem.
  
  Add this line to /etc/fstab:
  
      smackfs /smack smackfs smackfsdef=* 0 0
  
  and create the /smack directory for mounting.
  
  Smack uses extended attributes (xattrs) to store file labels.
  The command to set a Smack label on a file is:
  
      # attr -S -s SMACK64 -V "value" path
  
  NOTE: Smack labels are limited to 23 characters. The attr command
        does not enforce this restriction and can be used to set
        invalid Smack labels on files.
  
  If you don't do anything special all users will get the floor ("_")
  label when they log in. If you do want to log in via the hacked ssh
  at other labels use the attr command to set the smack value on the

  home directory and its contents.

  
  You can add access rules in /etc/smack/accesses. They take the form:
  
      subjectlabel objectlabel access
  
  access is a combination of the letters rwxa which specify the
  kind of access permitted a subject with subjectlabel on an
  object with objectlabel. If there is no rule no access is allowed.
  
  A process can see the smack label it is running with by
  reading /proc/self/attr/current. A privileged process can
  set the process smack by writing there.
  
  Look for additional programs on http://schaufler-ca.com
  
  From the Smack Whitepaper:
  
  The Simplified Mandatory Access Control Kernel
  
  Casey Schaufler
  casey@schaufler-ca.com
  
  Mandatory Access Control
  
  Computer systems employ a variety of schemes to constrain how information is
  shared among the people and services using the machine. Some of these schemes
  allow the program or user to decide what other programs or users are allowed
  access to pieces of data. These schemes are called discretionary access
  control mechanisms because the access control is specified at the discretion
  of the user. Other schemes do not leave the decision regarding what a user or
  program can access up to users or programs. These schemes are called mandatory
  access control mechanisms because you don't have a choice regarding the users
  or programs that have access to pieces of data.
  
  Bell & LaPadula
  
  From the middle of the 1980's until the turn of the century Mandatory Access
  Control (MAC) was very closely associated with the Bell & LaPadula security
  model, a mathematical description of the United States Department of Defense
  policy for marking paper documents. MAC in this form enjoyed a following
  within the Capital Beltway and Scandinavian supercomputer centers but was
  often sited as failing to address general needs.
  
  Domain Type Enforcement
  
  Around the turn of the century Domain Type Enforcement (DTE) became popular.
  This scheme organizes users, programs, and data into domains that are
  protected from each other. This scheme has been widely deployed as a component
  of popular Linux distributions. The administrative overhead required to
  maintain this scheme and the detailed understanding of the whole system
  necessary to provide a secure domain mapping leads to the scheme being
  disabled or used in limited ways in the majority of cases.
  
  Smack
  
  Smack is a Mandatory Access Control mechanism designed to provide useful MAC
  while avoiding the pitfalls of its predecessors. The limitations of Bell &
  LaPadula are addressed by providing a scheme whereby access can be controlled
  according to the requirements of the system and its purpose rather than those
  imposed by an arcane government policy. The complexity of Domain Type
  Enforcement and avoided by defining access controls in terms of the access
  modes already in use.
  
  Smack Terminology
  
  The jargon used to talk about Smack will be familiar to those who have dealt
  with other MAC systems and shouldn't be too difficult for the uninitiated to
  pick up. There are four terms that are used in a specific way and that are
  especially important:
  
  	Subject: A subject is an active entity on the computer system.
  	On Smack a subject is a task, which is in turn the basic unit
  	of execution.
  
  	Object: An object is a passive entity on the computer system.
  	On Smack files of all types, IPC, and tasks can be objects.
  
  	Access: Any attempt by a subject to put information into or get
  	information from an object is an access.
  
  	Label: Data that identifies the Mandatory Access Control
  	characteristics of a subject or an object.
  
  These definitions are consistent with the traditional use in the security
  community. There are also some terms from Linux that are likely to crop up:
  
  	Capability: A task that possesses a capability has permission to
  	violate an aspect of the system security policy, as identified by
  	the specific capability. A task that possesses one or more
  	capabilities is a privileged task, whereas a task with no
  	capabilities is an unprivileged task.
  
  	Privilege: A task that is allowed to violate the system security
  	policy is said to have privilege. As of this writing a task can
  	have privilege either by possessing capabilities or by having an
  	effective user of root.
  
  Smack Basics
  
  Smack is an extension to a Linux system. It enforces additional restrictions
  on what subjects can access which objects, based on the labels attached to
  each of the subject and the object.
  
  Labels
  
  Smack labels are ASCII character strings, one to twenty-three characters in
  length. Single character labels using special characters, that being anything
  other than a letter or digit, are reserved for use by the Smack development
  team. Smack labels are unstructured, case sensitive, and the only operation
  ever performed on them is comparison for equality. Smack labels cannot

  contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
  (quote) and '"' (double-quote) characters.
  Smack labels cannot begin with a '-', which is reserved for special options.

  
  There are some predefined labels:

  	_ 	Pronounced "floor", a single underscore character.
  	^ 	Pronounced "hat", a single circumflex character.
  	* 	Pronounced "star", a single asterisk character.
  	? 	Pronounced "huh", a single question mark character.
  	@ 	Pronounced "Internet", a single at sign character.

  
  Every task on a Smack system is assigned a label. System tasks, such as
  init(8) and systems daemons, are run with the floor ("_") label. User tasks
  are assigned labels according to the specification found in the
  /etc/smack/user configuration file.
  
  Access Rules
  
  Smack uses the traditional access modes of Linux. These modes are read,
  execute, write, and occasionally append. There are a few cases where the
  access mode may not be obvious. These include:
  
  	Signals: A signal is a write operation from the subject task to
  	the object task.
  	Internet Domain IPC: Transmission of a packet is considered a
  	write operation from the source task to the destination task.
  
  Smack restricts access based on the label attached to a subject and the label
  attached to the object it is trying to access. The rules enforced are, in
  order:
  
  	1. Any access requested by a task labeled "*" is denied.
  	2. A read or execute access requested by a task labeled "^"
  	   is permitted.
  	3. A read or execute access requested on an object labeled "_"
  	   is permitted.
  	4. Any access requested on an object labeled "*" is permitted.
  	5. Any access requested by a task on an object with the same
  	   label is permitted.
  	6. Any access requested that is explicitly defined in the loaded
  	   rule set is permitted.
  	7. Any other access is denied.
  
  Smack Access Rules
  
  With the isolation provided by Smack access separation is simple. There are
  many interesting cases where limited access by subjects to objects with
  different labels is desired. One example is the familiar spy model of
  sensitivity, where a scientist working on a highly classified project would be
  able to read documents of lower classifications and anything she writes will
  be "born" highly classified. To accommodate such schemes Smack includes a
  mechanism for specifying rules allowing access between labels.
  
  Access Rule Format
  
  The format of an access rule is:
  
  	subject-label object-label access
  
  Where subject-label is the Smack label of the task, object-label is the Smack
  label of the thing being accessed, and access is a string specifying the sort
  of access allowed. The Smack labels are limited to 23 characters. The access
  specification is searched for letters that describe access modes:
  
  	a: indicates that append access should be granted.
  	r: indicates that read access should be granted.
  	w: indicates that write access should be granted.
  	x: indicates that execute access should be granted.
  
  Uppercase values for the specification letters are allowed as well.
  Access mode specifications can be in any order. Examples of acceptable rules
  are:
  
  	TopSecret Secret  rx
  	Secret    Unclass R
  	Manager   Game    x
  	User      HR      w
  	New       Old     rRrRr
  	Closed    Off     -
  
  Examples of unacceptable rules are:
  
  	Top Secret Secret     rx
  	Ace        Ace        r
  	Odd        spells     waxbeans
  
  Spaces are not allowed in labels. Since a subject always has access to files
  with the same label specifying a rule for that case is pointless. Only
  valid letters (rwxaRWXA) and the dash ('-') character are allowed in
  access specifications. The dash is a placeholder, so "a-r" is the same
  as "ar". A lone dash is used to specify that no access should be allowed.
  
  Applying Access Rules
  
  The developers of Linux rarely define new sorts of things, usually importing
  schemes and concepts from other systems. Most often, the other systems are
  variants of Unix. Unix has many endearing properties, but consistency of
  access control models is not one of them. Smack strives to treat accesses as
  uniformly as is sensible while keeping with the spirit of the underlying
  mechanism.
  
  File system objects including files, directories, named pipes, symbolic links,
  and devices require access permissions that closely match those used by mode
  bit access. To open a file for reading read access is required on the file. To
  search a directory requires execute access. Creating a file with write access
  requires both read and write access on the containing directory. Deleting a
  file requires read and write access to the file and to the containing
  directory. It is possible that a user may be able to see that a file exists
  but not any of its attributes by the circumstance of having read access to the
  containing directory but not to the differently labeled file. This is an
  artifact of the file name being data in the directory, not a part of the file.
  
  IPC objects, message queues, semaphore sets, and memory segments exist in flat
  namespaces and access requests are only required to match the object in
  question.
  
  Process objects reflect tasks on the system and the Smack label used to access
  them is the same Smack label that the task would use for its own access
  attempts. Sending a signal via the kill() system call is a write operation
  from the signaler to the recipient. Debugging a process requires both reading
  and writing. Creating a new task is an internal operation that results in two
  tasks with identical Smack labels and requires no access checks.
  
  Sockets are data structures attached to processes and sending a packet from
  one process to another requires that the sender have write access to the
  receiver. The receiver is not required to have read access to the sender.
  
  Setting Access Rules
  
  The configuration file /etc/smack/accesses contains the rules to be set at
  system startup. The contents are written to the special file /smack/load.
  Rules can be written to /smack/load at any time and take effect immediately.
  For any pair of subject and object labels there can be only one rule, with the
  most recently specified overriding any earlier specification.
  
  The program smackload is provided to ensure data is formatted
  properly when written to /smack/load. This program reads lines
  of the form
  
      subjectlabel objectlabel mode.
  
  Task Attribute
  
  The Smack label of a process can be read from /proc/<pid>/attr/current. A
  process can read its own Smack label from /proc/self/attr/current. A
  privileged process can change its own Smack label by writing to
  /proc/self/attr/current but not the label of another process.
  
  File Attribute
  
  The Smack label of a filesystem object is stored as an extended attribute
  named SMACK64 on the file. This attribute is in the security namespace. It can
  only be changed by a process with privilege.
  
  Privilege
  
  A process with CAP_MAC_OVERRIDE is privileged.
  
  Smack Networking
  
  As mentioned before, Smack enforces access control on network protocol
  transmissions. Every packet sent by a Smack process is tagged with its Smack
  label. This is done by adding a CIPSO tag to the header of the IP packet. Each
  packet received is expected to have a CIPSO tag that identifies the label and
  if it lacks such a tag the network ambient label is assumed. Before the packet
  is delivered a check is made to determine that a subject with the label on the
  packet has write access to the receiving process and if that is not the case
  the packet is dropped.
  
  CIPSO Configuration
  
  It is normally unnecessary to specify the CIPSO configuration. The default
  values used by the system handle all internal cases. Smack will compose CIPSO
  label values to match the Smack labels being used without administrative
  intervention. Unlabeled packets that come into the system will be given the
  ambient label.
  
  Smack requires configuration in the case where packets from a system that is
  not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
  Solaris system, but there are other, less widely deployed systems out there.
  CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
  and a category set with each packet. The DOI is intended to identify a group
  of systems that use compatible labeling schemes, and the DOI specified on the
  smack system must match that of the remote system or packets will be
  discarded. The DOI is 3 by default. The value can be read from /smack/doi and
  can be changed by writing to /smack/doi.
  
  The label and category set are mapped to a Smack label as defined in
  /etc/smack/cipso.
  
  A Smack/CIPSO mapping has the form:
  
  	smack level [category [category]*]
  
  Smack does not expect the level or category sets to be related in any
  particular way and does not assume or assign accesses based on them. Some
  examples of mappings:
  
  	TopSecret 7
  	TS:A,B    7 1 2
  	SecBDE    5 2 4 6
  	RAFTERS   7 12 26
  
  The ":" and "," characters are permitted in a Smack label but have no special
  meaning.
  
  The mapping of Smack labels to CIPSO values is defined by writing to
  /smack/cipso. Again, the format of data written to this special file
  is highly restrictive, so the program smackcipso is provided to
  ensure the writes are done properly. This program takes mappings
  on the standard input and sends them to /smack/cipso properly.
  
  In addition to explicit mappings Smack supports direct CIPSO mappings. One
  CIPSO level is used to indicate that the category set passed in the packet is
  in fact an encoding of the Smack label. The level used is 250 by default. The
  value can be read from /smack/direct and changed by writing to /smack/direct.
  
  Socket Attributes
  
  There are two attributes that are associated with sockets. These attributes
  can only be set by privileged tasks, but any task can read them for their own
  sockets.
  
  	SMACK64IPIN: The Smack label of the task object. A privileged
  	program that will enforce policy may set this to the star label.
  
  	SMACK64IPOUT: The Smack label transmitted with outgoing packets.
  	A privileged program may set this to match the label of another
  	task with which it hopes to communicate.

  Smack Netlabel Exceptions
  
  You will often find that your labeled application has to talk to the outside,
  unlabeled world. To do this there's a special file /smack/netlabel where you can
  add some exceptions in the form of :
  @IP1	   LABEL1 or
  @IP2/MASK  LABEL2
  
  It means that your application will have unlabeled access to @IP1 if it has
  write access on LABEL1, and access to the subnet @IP2/MASK if it has write
  access on LABEL2.
  
  Entries in the /smack/netlabel file are matched by longest mask first, like in
  classless IPv4 routing.
  
  A special label '@' and an option '-CIPSO' can be used there :
  @      means Internet, any application with any label has access to it
  -CIPSO means standard CIPSO networking
  
  If you don't know what CIPSO is and don't plan to use it, you can just do :
  echo 127.0.0.1 -CIPSO > /smack/netlabel
  echo 0.0.0.0/0 @      > /smack/netlabel
  
  If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
  Internet access, you can have :
  echo 127.0.0.1      -CIPSO > /smack/netlabel
  echo 192.168.0.0/16 -CIPSO > /smack/netlabel
  echo 0.0.0.0/0      @      > /smack/netlabel

  Writing Applications for Smack
  
  There are three sorts of applications that will run on a Smack system. How an
  application interacts with Smack will determine what it will have to do to
  work properly under Smack.
  
  Smack Ignorant Applications
  
  By far the majority of applications have no reason whatever to care about the
  unique properties of Smack. Since invoking a program has no impact on the
  Smack label associated with the process the only concern likely to arise is
  whether the process has execute access to the program.
  
  Smack Relevant Applications
  
  Some programs can be improved by teaching them about Smack, but do not make
  any security decisions themselves. The utility ls(1) is one example of such a
  program.
  
  Smack Enforcing Applications
  
  These are special programs that not only know about Smack, but participate in
  the enforcement of system policy. In most cases these are the programs that
  set up user sessions. There are also network services that provide information
  to processes running with various labels.
  
  File System Interfaces
  
  Smack maintains labels on file system objects using extended attributes. The
  Smack label of a file, directory, or other file system object can be obtained
  using getxattr(2).
  
  	len = getxattr("/", "security.SMACK64", value, sizeof (value));
  
  will put the Smack label of the root directory into value. A privileged
  process can set the Smack label of a file system object with setxattr(2).
  
  	len = strlen("Rubble");
  	rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
  
  will set the Smack label of /foo to "Rubble" if the program has appropriate
  privilege.
  
  Socket Interfaces
  
  The socket attributes can be read using fgetxattr(2).
  
  A privileged process can set the Smack label of outgoing packets with
  fsetxattr(2).
  
  	len = strlen("Rubble");
  	rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
  
  will set the Smack label "Rubble" on packets going out from the socket if the
  program has appropriate privilege.
  
  	rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
  
  will set the Smack label "*" as the object label against which incoming
  packets will be checked if the program has appropriate privilege.
  
  Administration
  
  Smack supports some mount options:
  
  	smackfsdef=label: specifies the label to give files that lack
  	the Smack label extended attribute.
  
  	smackfsroot=label: specifies the label to assign the root of the
  	file system if it lacks the Smack extended attribute.
  
  	smackfshat=label: specifies a label that must have read access to
  	all labels set on the filesystem. Not yet enforced.
  
  	smackfsfloor=label: specifies a label to which all labels set on the
  	filesystem must have read access. Not yet enforced.
  
  These mount options apply to all file system types.

  Smack auditing
  
  If you want Smack auditing of security events, you need to set CONFIG_AUDIT
  in your kernel configuration.
  By default, all denied events will be audited. You can change this behavior by
  writing a single character to the /smack/logging file :
  0 : no logging
  1 : log denied (default)
  2 : log accepted
  3 : log denied & accepted
  
  Events are logged as 'key=value' pairs, for each event you at least will get
  the subjet, the object, the rights requested, the action, the kernel function
  that triggered the event, plus other pairs depending on the type of event
  audited.