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+ "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
+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
+ 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 it's 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
+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 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.
+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
+ 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 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.
+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 or the "/" (slash) character.
+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.
+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.
+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
+ 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
+ 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
+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
+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.
+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.
+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.
+A process with CAP_MAC_OVERRIDE is privileged.
+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.
+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
+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
+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
+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.
+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
+ 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.
+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
+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
+ 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
+The socket attributes can be read using fgetxattr(2).
+A privileged process can set the Smack label of outgoing packets with
+ 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.
+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.