doc: ReSTify Smack.txt

=====

Smack

=====

    "Good for you, you've decided to clean the elevator!"

at hand.

Smack consists of three major components:

    - The kernel

    - Basic utilities, which are helpful but not required

    - Configuration data

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

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

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

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:

Add this line to ``/etc/fstab``::

    smackfs /sys/fs/smackfs smackfs defaults 0 0

The /sys/fs/smackfs directory is created by the kernel.

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

name space. A process must have ``CAP_MAC_ADMIN`` to change any of these

attributes.

The extended attributes that Smack uses are:

	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

	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:

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

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.

on ``/sys/fs/smackfs``.

access

	Provided for backward compatibility. The access2 interface

	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

	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:

	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

	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:

	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.

	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:

	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.

	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:

	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:

	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

	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:

	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.

	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:

	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

	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

	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.

	    ``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.

	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.

	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

	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:

you can add access rules in ``/etc/smack/accesses``. They take the form::

    subjectlabel objectlabel access

Look for additional programs on http://schaufler-ca.com

From the Smack Whitepaper:

The Simplified Mandatory Access Control Kernel

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

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

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

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 &

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.

  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.

  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

  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

  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

  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

  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.

(quote) and '"' (double-quote) characters.

Smack labels cannot begin with a '-'. This is reserved for special options.

There are some predefined labels:

There are some predefined labels::

	_ 	Pronounced "floor", a single underscore character.

	^ 	Pronounced "hat", a single circumflex character.

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

  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

  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

	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

mechanism for specifying rules allowing access between labels.

Access Rule Format

~~~~~~~~~~~~~~~~~~

The format of an access rule is:

The format of an access rule is::

	subject-label object-label access

Uppercase values for the specification letters are allowed as well.

Access mode specifications can be in any order. Examples of acceptable rules

are:

are::

	TopSecret Secret  rx

	Secret    Unclass R

	New       Old     rRrRr

	Closed    Off     -

Examples of unacceptable rules are:

Examples of unacceptable rules are::

	Top Secret Secret     rx

	Ace        Ace        r

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

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

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

/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

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

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

The label and category set are mapped to a Smack label as defined in

/etc/smack/cipso.

A Smack/CIPSO mapping has the form:

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:

examples of mappings::

	TopSecret 7

	TS:A,B    7 1 2

/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

  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.

  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 :

where you can add some exceptions in the form of::

	@IP1	   LABEL1 or

	@IP2/MASK  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 :

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 :

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 :

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

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

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).

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).

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);

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).

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.

program has appropriate privilege::

	rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);

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

  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

  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

  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

  smackfsfloor=label:

	specifies a label to which all labels set on the

	filesystem must have read access. Not yet enforced.

These mount options apply to all file system types.

Smack auditing

--------------

If you want Smack auditing of security events, you need to set CONFIG_AUDIT

in your kernel configuration.

By default, all denied events will be audited. You can change this behavior by

writing a single character to the /sys/fs/smackfs/logging file :

writing a single character to the /sys/fs/smackfs/logging file::

	0 : no logging

	1 : log denied (default)

	2 : log accepted

audited.

Bringup Mode

------------

Bringup mode provides logging features that can make application

configuration and system bringup easier. Configure the kernel with