Documentation/Smack.txt - kernel/msm - Gitiles



     "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 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

 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 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.

 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.

 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.


	"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 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

	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 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.

	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.

	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.