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 =====
 Smack
 =====
 
 
     "Good for you, you've decided to clean the elevator!"
     - The Elevator, from Dark Star
 
 Smack is 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
     - Basic utilities, which are helpful but not required
     - 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.
 
 Smack is used in the Tizen operating system. Please
 go to http://wiki.tizen.org for information about how
 Smack is used in Tizen.
 
 The current git repository for Smack user space is:
 
         git://github.com/smack-team/smack.git
 
 This should make and install on most modern distributions.
 There are five commands included in smackutil:
 
 chsmack:
         display or set Smack extended attribute values
 
 smackctl:
         load the Smack access rules
 
 smackaccess:
         report if a process with one label has access
         to an object with another
 
 These two commands are obsolete with the introduction of
 the smackfs/load2 and smackfs/cipso2 interfaces.
 
 smackload:
         properly formats data for writing to smackfs/load
 
 smackcipso:
         properly formats data for writing to smackfs/cipso
 
 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.
 If smackutil is installed the startup script will take care
 of this, but it can be manually as well.
 
 Add this line to ``/etc/fstab``::
 
     smackfs /sys/fs/smackfs smackfs defaults 0 0
 
 The ``/sys/fs/smackfs`` directory is created by the kernel.
 
 Smack uses extended attributes (xattrs) to store labels on filesystem
 objects. The attributes are stored in the extended attribute security
 name space. A process must have ``CAP_MAC_ADMIN`` to change any of these
 attributes.
 
 The extended attributes that Smack uses are:
 
 SMACK64
         Used to make access control decisions. In almost all cases
         the label given to a new filesystem object will be the label
         of the process that created it.
 
 SMACK64EXEC
         The Smack label of a process that execs a program file with
         this attribute set will run with this attribute's value.
 
 SMACK64MMAP
         Don't allow the file to be mmapped by a process whose Smack
         label does not allow all of the access permitted to a process
         with the label contained in this attribute. This is a very
         specific use case for shared libraries.
 
 SMACK64TRANSMUTE
         Can only have the value "TRUE". If this attribute is present
         on a directory when an object is created in the directory and
         the Smack rule (more below) that permitted the write access
         to the directory includes the transmute ("t") mode the object
         gets the label of the directory instead of the label of the
         creating process. If the object being created is a directory
         the SMACK64TRANSMUTE attribute is set as well.
 
 SMACK64IPIN
         This attribute is only available on file descriptors for sockets.
         Use the Smack label in this attribute for access control
         decisions on packets being delivered to this socket.
 
 SMACK64IPOUT
         This attribute is only available on file descriptors for sockets.
         Use the Smack label in this attribute for access control
         decisions on packets coming from this socket.
 
 There are multiple ways to set a Smack label on a file::
 
     # attr -S -s SMACK64 -V "value" path
     # chsmack -a value path
 
 A process can see the Smack label it is running with by
 reading ``/proc/self/attr/current``. A process with ``CAP_MAC_ADMIN``
 can set the process Smack by writing there.
 
 Most Smack configuration is accomplished by writing to files
 in the smackfs filesystem. This pseudo-filesystem is mounted
 on ``/sys/fs/smackfs``.
 
 access
         Provided for backward compatibility. The access2 interface
         is preferred and should be used instead.
         This interface reports whether a subject with the specified
         Smack label has a particular access to an object with a
         specified Smack label. Write a fixed format access rule to
         this file. The next read will indicate whether the access
         would be permitted. The text will be either "1" indicating
         access, or "0" indicating denial.
 
 access2
         This interface reports whether a subject with the specified
         Smack label has a particular access to an object with a
         specified Smack label. Write a long format access rule to
         this file. The next read will indicate whether the access
         would be permitted. The text will be either "1" indicating
         access, or "0" indicating denial.
 
 ambient
         This contains the Smack label applied to unlabeled network
         packets.
 
 change-rule
         This interface allows modification of existing access control rules.
         The format accepted on write is::
 
                 "%s %s %s %s"
 
         where the first string is the subject label, the second the
         object label, the third the access to allow and the fourth the
         access to deny. The access strings may contain only the characters
         "rwxat-". If a rule for a given subject and object exists it will be
         modified by enabling the permissions in the third string and disabling
         those in the fourth string. If there is no such rule it will be
         created using the access specified in the third and the fourth strings.
 
 cipso
         Provided for backward compatibility. The cipso2 interface
         is preferred and should be used instead.
         This interface allows a specific CIPSO header to be assigned
         to a Smack label. The format accepted on write is::
 
                 "%24s%4d%4d"["%4d"]...
 
         The first string is a fixed Smack label. The first number is
         the level to use. The second number is the number of categories.
         The following numbers are the categories::
 
                 "level-3-cats-5-19          3   2   5  19"
 
 cipso2
         This interface allows a specific CIPSO header to be assigned
         to a Smack label. The format accepted on write is::
 
                 "%s%4d%4d"["%4d"]...
 
         The first string is a long Smack label. The first number is
         the level to use. The second number is the number of categories.
         The following numbers are the categories::
 
                 "level-3-cats-5-19   3   2   5  19"
 
 direct
         This contains the CIPSO level used for Smack direct label
         representation in network packets.
 
 doi
         This contains the CIPSO domain of interpretation used in
         network packets.
 
 ipv6host
         This interface allows specific IPv6 internet addresses to be
         treated as single label hosts. Packets are sent to single
         label hosts only from processes that have Smack write access
         to the host label. All packets received from single label hosts
         are given the specified label. The format accepted on write is::
 
                 "%h:%h:%h:%h:%h:%h:%h:%h label" or
                 "%h:%h:%h:%h:%h:%h:%h:%h/%d label".
 
         The "::" address shortcut is not supported.
         If label is "-DELETE" a matched entry will be deleted.
 
 load
         Provided for backward compatibility. The load2 interface
         is preferred and should be used instead.
         This interface allows access control rules in addition to
         the system defined rules to be specified. The format accepted
         on write is::
 
                 "%24s%24s%5s"
 
         where the first string is the subject label, the second the
         object label, and the third the requested access. The access
         string may contain only the characters "rwxat-", and specifies
         which sort of access is allowed. The "-" is a placeholder for
         permissions that are not allowed. The string "r-x--" would
         specify read and execute access. Labels are limited to 23
         characters in length.
 
 load2
         This interface allows access control rules in addition to
         the system defined rules to be specified. The format accepted
         on write is::
 
                 "%s %s %s"
 
         where the first string is the subject label, the second the
         object label, and the third the requested access. The access
         string may contain only the characters "rwxat-", and specifies
         which sort of access is allowed. The "-" is a placeholder for
         permissions that are not allowed. The string "r-x--" would
         specify read and execute access.
 
 load-self
         Provided for backward compatibility. The load-self2 interface
         is preferred and should be used instead.
         This interface allows process specific access rules to be
         defined. These rules are only consulted if access would
         otherwise be permitted, and are intended to provide additional
         restrictions on the process. The format is the same as for
         the load interface.
 
 load-self2
         This interface allows process specific access rules to be
         defined. These rules are only consulted if access would
         otherwise be permitted, and are intended to provide additional
         restrictions on the process. The format is the same as for
         the load2 interface.
 
 logging
         This contains the Smack logging state.
 
 mapped
         This contains the CIPSO level used for Smack mapped label
         representation in network packets.
 
 netlabel
         This interface allows specific internet addresses to be
         treated as single label hosts. Packets are sent to single
         label hosts without CIPSO headers, but only from processes
         that have Smack write access to the host label. All packets
         received from single label hosts are given the specified
         label. The format accepted on write is::
 
                 "%d.%d.%d.%d label" or "%d.%d.%d.%d/%d label".
 
         If the label specified is "-CIPSO" the address is treated
         as a host that supports CIPSO headers.
 
 onlycap
         This contains labels processes must have for CAP_MAC_ADMIN
         and ``CAP_MAC_OVERRIDE`` to be effective. If this file is empty
         these capabilities are effective at for processes with any
         label. The values are set by writing the desired labels, separated
         by spaces, to the file or cleared by writing "-" to the file.
 
 ptrace
         This is used to define the current ptrace policy
 
         0 - default:
             this is the policy that relies on Smack access rules.
             For the ``PTRACE_READ`` a subject needs to have a read access on
             object. For the ``PTRACE_ATTACH`` a read-write access is required.
 
         1 - exact:
             this is the policy that limits ``PTRACE_ATTACH``. Attach is
             only allowed when subject's and object's labels are equal.
             ``PTRACE_READ`` is not affected. Can be overridden with ``CAP_SYS_PTRACE``.
 
         2 - draconian:
             this policy behaves like the 'exact' above with an
             exception that it can't be overridden with ``CAP_SYS_PTRACE``.
 
 revoke-subject
         Writing a Smack label here sets the access to '-' for all access
         rules with that subject label.
 
 unconfined
         If the kernel is configured with ``CONFIG_SECURITY_SMACK_BRINGUP``
         a process with ``CAP_MAC_ADMIN`` can write a label into this interface.
         Thereafter, accesses that involve that label will be logged and
         the access permitted if it wouldn't be otherwise. Note that this
         is dangerous and can ruin the proper labeling of your system.
         It should never be used in production.
 
 relabel-self
         This interface contains a list of labels to which the process can
         transition to, by writing to ``/proc/self/attr/current``.
         Normally a process can change its own label to any legal value, but only
         if it has ``CAP_MAC_ADMIN``. This interface allows a process without
         ``CAP_MAC_ADMIN`` to relabel itself to one of labels from predefined list.
         A process without ``CAP_MAC_ADMIN`` can change its label only once. When it
         does, this list will be cleared.
         The values are set by writing the desired labels, separated
         by spaces, to the file or cleared by writing "-" to the file.
 
 If you are using the smackload utility
 you can add access rules in ``/etc/smack/accesses``. They take the form::
 
     subjectlabel objectlabel access
 
 access is a combination of the letters rwxatb 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.
 
 Look for additional programs on http://schaufler-ca.com
 
 The Simplified Mandatory Access Control Kernel (Whitepaper)
 ===========================================================
 
 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. They can be up to 255 characters
 long, but keeping them to twenty-three characters is recommended.
 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 '-'. This 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 "web", a single at sign character.
 
 Every task on a Smack system is assigned a label. The Smack label
 of a process will usually be assigned by the system initialization
 mechanism.
 
 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 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.
         t: indicates that the rule requests transmutation.
         b: indicates that the rule should be reported for bring-up.
 
 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
         Snap      Crackle rwxatb
         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 (rwxatbRWXATB) 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.
 
 If a directory is marked as transmuting (SMACK64TRANSMUTE=TRUE) and the
 access rule that allows a process to create an object in that directory
 includes 't' access the label assigned to the new object will be that
 of the directory, not the creating process. This makes it much easier
 for two processes with different labels to share data without granting
 access to all of their files.
 
 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
 /sys/fs/smackfs/load2. Rules can be added 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.
 
 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 or CAP_MAC_ADMIN is privileged.
 CAP_MAC_OVERRIDE allows the process access to objects it would
 be denied otherwise. CAP_MAC_ADMIN allows a process to change
 Smack data, including rules and attributes.
 
 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
 /sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/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
 /sys/fs/smackfs/cipso2.
 
 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 /sys/fs/smackfs/direct and changed by writing to
 /sys/fs/smackfs/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 /sys/fs/smackfs/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 /sys/fs/smackfs/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 > /sys/fs/smackfs/netlabel
         echo 0.0.0.0/0 @      > /sys/fs/smackfs/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 > /sys/fs/smackfs/netlabel
         echo 192.168.0.0/16 -CIPSO > /sys/fs/smackfs/netlabel
         echo 0.0.0.0/0      @      > /sys/fs/smackfs/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.
 
   smackfstransmute=label:
         behaves exactly like smackfsroot except that it also
         sets the transmute flag on the root of the mount
 
 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 /sys/fs/smackfs/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 subject, the object, the rights requested, the action, the kernel function
 that triggered the event, plus other pairs depending on the type of event
 audited.
 
 Bringup Mode
 ------------
 
 Bringup mode provides logging features that can make application
 configuration and system bringup easier. Configure the kernel with
 CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup
 mode is enabled accesses that succeed due to rules marked with the "b"
 access mode will logged. When a new label is introduced for processes
 rules can be added aggressively, marked with the "b". The logging allows
 tracking of which rules actual get used for that label.
 
 Another feature of bringup mode is the "unconfined" option. Writing
 a label to /sys/fs/smackfs/unconfined makes subjects with that label
 able to access any object, and objects with that label accessible to
 all subjects. Any access that is granted because a label is unconfined
 is logged. This feature is dangerous, as files and directories may
 be created in places they couldn't if the policy were being enforced.

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