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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
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
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
From the Smack Whitepaper:
The Simplified Mandatory Access Control Kernel
Casey Schaufler
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.
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.
Smack labels are ASCII character strings, one to twenty-three characters in
length. Single character labels using special characters, that being anything
other than a letter or digit, are reserved for use by the Smack development
team. Smack labels are unstructured, case sensitive, and the only operation
ever performed on them is comparison for equality. Smack labels cannot
contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
(quote) and '"' (double-quote) characters.
Smack labels cannot begin with a '-', which is reserved for special options.
There are some predefined labels:
_ Pronounced "floor", a single underscore character.
^ Pronounced "hat", a single circumflex character.
* Pronounced "star", a single asterisk character.
? Pronounced "huh", a single question mark character.
@ Pronounced "Internet", a single at sign character.
Every task on a Smack system is assigned a label. System tasks, such as
init(8) and systems daemons, are run with the floor ("_") label. User tasks
are assigned labels according to the specification found in the
/etc/smack/user configuration file.
Access Rules
Smack uses the traditional access modes of Linux. These modes are read,
execute, write, and occasionally append. There are a few cases where the
access mode may not be obvious. These include:
Signals: A signal is a write operation from the subject task to
the object task.
Internet Domain IPC: Transmission of a packet is considered a
write operation from the source task to the destination task.
Smack restricts access based on the label attached to a subject and the label
attached to the object it is trying to access. The rules enforced are, in
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.
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.
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
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.
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
SMACK64IPIN: The Smack label of the task object. A privileged
program that will enforce policy may set this to the star label.
SMACK64IPOUT: The Smack label transmitted with outgoing packets.
A privileged program may set this to match the label of another
task with which it hopes to communicate.
Smack Netlabel Exceptions
You will often find that your labeled application has to talk to the outside,
unlabeled world. To do this there's a special file /smack/netlabel where you can
add some exceptions in the form of :
@IP1 LABEL1 or
It means that your application will have unlabeled access to @IP1 if it has
write access on LABEL1, and access to the subnet @IP2/MASK if it has write
access on LABEL2.
Entries in the /smack/netlabel file are matched by longest mask first, like in
classless IPv4 routing.
A special label '@' and an option '-CIPSO' can be used there :
@ means Internet, any application with any label has access to it
-CIPSO means standard CIPSO networking
If you don't know what CIPSO is and don't plan to use it, you can just do :
echo -CIPSO > /smack/netlabel
echo @ > /smack/netlabel
If you use CIPSO on your local network and need also unlabeled
Internet access, you can have :
echo -CIPSO > /smack/netlabel
echo -CIPSO > /smack/netlabel
echo @ > /smack/netlabel
Writing Applications for Smack
There are three sorts of applications that will run on a Smack system. How an
application interacts with Smack will determine what it will have to do to
work properly under Smack.
Smack Ignorant Applications
By far the majority of applications have no reason whatever to care about the
unique properties of Smack. Since invoking a program has no impact on the
Smack label associated with the process the only concern likely to arise is
whether the process has execute access to the program.
Smack Relevant Applications
Some programs can be improved by teaching them about Smack, but do not make
any security decisions themselves. The utility ls(1) is one example of such a
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
Socket Interfaces
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.
Smack auditing
If you want Smack auditing of security events, you need to set CONFIG_AUDIT
in your kernel configuration.
By default, all denied events will be audited. You can change this behavior by
writing a single character to the /smack/logging file :
0 : no logging
1 : log denied (default)
2 : log accepted
3 : log denied & accepted
Events are logged as 'key=value' pairs, for each event you at least will get
the subjet, the object, the rights requested, the action, the kernel function
that triggered the event, plus other pairs depending on the type of event