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"Good for you, you've decided to clean the elevator!"
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- The Elevator, from Dark Star
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Smack is the the Simplified Mandatory Access Control Kernel.
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Smack is a kernel based implementation of mandatory access
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control that includes simplicity in its primary design goals.
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Smack is not the only Mandatory Access Control scheme
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available for Linux. Those new to Mandatory Access Control
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are encouraged to compare Smack with the other mechanisms
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available to determine which is best suited to the problem
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Smack consists of three major components:
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- A start-up script and a few modified applications
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The kernel component of Smack is implemented as a Linux
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Security Modules (LSM) module. It requires netlabel and
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works best with file systems that support extended attributes,
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although xattr support is not strictly required.
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It is safe to run a Smack kernel under a "vanilla" distribution.
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Smack kernels use the CIPSO IP option. Some network
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configurations are intolerant of IP options and can impede
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access to systems that use them as Smack does.
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The startup script etc-init.d-smack should be installed
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in /etc/init.d/smack and should be invoked early in the
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start-up process. On Fedora rc5.d/S02smack is recommended.
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This script ensures that certain devices have the correct
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Smack attributes and loads the Smack configuration if
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any is defined. This script invokes two programs that
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ensure configuration data is properly formatted. These
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programs are /usr/sbin/smackload and /usr/sin/smackcipso.
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The system will run just fine without these programs,
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but it will be difficult to set access rules properly.
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A version of "ls" that provides a "-M" option to display
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Smack labels on long listing is available.
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A hacked version of sshd that allows network logins by users
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with specific Smack labels is available. This version does
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not work for scp. You must set the /etc/ssh/sshd_config
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UsePrivilegeSeparation no
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The format of /etc/smack/usr is:
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In keeping with the intent of Smack, configuration data is
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minimal and not strictly required. The most important
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configuration step is mounting the smackfs pseudo filesystem.
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Add this line to /etc/fstab:
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smackfs /smack smackfs smackfsdef=* 0 0
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and create the /smack directory for mounting.
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Smack uses extended attributes (xattrs) to store file labels.
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The command to set a Smack label on a file is:
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# attr -S -s SMACK64 -V "value" path
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NOTE: Smack labels are limited to 23 characters. The attr command
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does not enforce this restriction and can be used to set
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invalid Smack labels on files.
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If you don't do anything special all users will get the floor ("_")
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label when they log in. If you do want to log in via the hacked ssh
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at other labels use the attr command to set the smack value on the
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home directory and its contents.
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You can add access rules in /etc/smack/accesses. They take the form:
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subjectlabel objectlabel access
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access is a combination of the letters rwxa which specify the
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kind of access permitted a subject with subjectlabel on an
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object with objectlabel. If there is no rule no access is allowed.
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A process can see the smack label it is running with by
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reading /proc/self/attr/current. A privileged process can
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set the process smack by writing there.
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Look for additional programs on http://schaufler-ca.com
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From the Smack Whitepaper:
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The Simplified Mandatory Access Control Kernel
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casey@schaufler-ca.com
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Mandatory Access Control
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Computer systems employ a variety of schemes to constrain how information is
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shared among the people and services using the machine. Some of these schemes
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allow the program or user to decide what other programs or users are allowed
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access to pieces of data. These schemes are called discretionary access
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control mechanisms because the access control is specified at the discretion
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of the user. Other schemes do not leave the decision regarding what a user or
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program can access up to users or programs. These schemes are called mandatory
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access control mechanisms because you don't have a choice regarding the users
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or programs that have access to pieces of data.
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From the middle of the 1980's until the turn of the century Mandatory Access
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Control (MAC) was very closely associated with the Bell & LaPadula security
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model, a mathematical description of the United States Department of Defense
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policy for marking paper documents. MAC in this form enjoyed a following
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within the Capital Beltway and Scandinavian supercomputer centers but was
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often sited as failing to address general needs.
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Domain Type Enforcement
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Around the turn of the century Domain Type Enforcement (DTE) became popular.
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This scheme organizes users, programs, and data into domains that are
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protected from each other. This scheme has been widely deployed as a component
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of popular Linux distributions. The administrative overhead required to
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maintain this scheme and the detailed understanding of the whole system
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necessary to provide a secure domain mapping leads to the scheme being
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disabled or used in limited ways in the majority of cases.
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Smack is a Mandatory Access Control mechanism designed to provide useful MAC
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while avoiding the pitfalls of its predecessors. The limitations of Bell &
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LaPadula are addressed by providing a scheme whereby access can be controlled
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according to the requirements of the system and its purpose rather than those
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imposed by an arcane government policy. The complexity of Domain Type
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Enforcement and avoided by defining access controls in terms of the access
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modes already in use.
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The jargon used to talk about Smack will be familiar to those who have dealt
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with other MAC systems and shouldn't be too difficult for the uninitiated to
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pick up. There are four terms that are used in a specific way and that are
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especially important:
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Subject: A subject is an active entity on the computer system.
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On Smack a subject is a task, which is in turn the basic unit
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Object: An object is a passive entity on the computer system.
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On Smack files of all types, IPC, and tasks can be objects.
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Access: Any attempt by a subject to put information into or get
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information from an object is an access.
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Label: Data that identifies the Mandatory Access Control
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characteristics of a subject or an object.
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These definitions are consistent with the traditional use in the security
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community. There are also some terms from Linux that are likely to crop up:
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Capability: A task that possesses a capability has permission to
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violate an aspect of the system security policy, as identified by
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the specific capability. A task that possesses one or more
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capabilities is a privileged task, whereas a task with no
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capabilities is an unprivileged task.
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Privilege: A task that is allowed to violate the system security
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policy is said to have privilege. As of this writing a task can
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have privilege either by possessing capabilities or by having an
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effective user of root.
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Smack is an extension to a Linux system. It enforces additional restrictions
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on what subjects can access which objects, based on the labels attached to
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each of the subject and the object.
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Smack labels are ASCII character strings, one to twenty-three characters in
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length. Single character labels using special characters, that being anything
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other than a letter or digit, are reserved for use by the Smack development
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team. Smack labels are unstructured, case sensitive, and the only operation
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ever performed on them is comparison for equality. Smack labels cannot
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contain unprintable characters, the "/" (slash), the "\" (backslash), the "'"
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(quote) and '"' (double-quote) characters.
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Smack labels cannot begin with a '-', which is reserved for special options.
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There are some predefined labels:
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_ Pronounced "floor", a single underscore character.
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^ Pronounced "hat", a single circumflex character.
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* Pronounced "star", a single asterisk character.
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? Pronounced "huh", a single question mark character.
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@ Pronounced "Internet", a single at sign character.
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Every task on a Smack system is assigned a label. System tasks, such as
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init(8) and systems daemons, are run with the floor ("_") label. User tasks
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are assigned labels according to the specification found in the
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/etc/smack/user configuration file.
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Smack uses the traditional access modes of Linux. These modes are read,
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execute, write, and occasionally append. There are a few cases where the
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access mode may not be obvious. These include:
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Signals: A signal is a write operation from the subject task to
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Internet Domain IPC: Transmission of a packet is considered a
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write operation from the source task to the destination task.
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Smack restricts access based on the label attached to a subject and the label
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attached to the object it is trying to access. The rules enforced are, in
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1. Any access requested by a task labeled "*" is denied.
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2. A read or execute access requested by a task labeled "^"
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3. A read or execute access requested on an object labeled "_"
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4. Any access requested on an object labeled "*" is permitted.
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5. Any access requested by a task on an object with the same
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6. Any access requested that is explicitly defined in the loaded
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rule set is permitted.
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7. Any other access is denied.
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With the isolation provided by Smack access separation is simple. There are
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many interesting cases where limited access by subjects to objects with
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different labels is desired. One example is the familiar spy model of
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sensitivity, where a scientist working on a highly classified project would be
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able to read documents of lower classifications and anything she writes will
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be "born" highly classified. To accommodate such schemes Smack includes a
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mechanism for specifying rules allowing access between labels.
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The format of an access rule is:
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subject-label object-label access
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Where subject-label is the Smack label of the task, object-label is the Smack
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label of the thing being accessed, and access is a string specifying the sort
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of access allowed. The Smack labels are limited to 23 characters. The access
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specification is searched for letters that describe access modes:
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a: indicates that append access should be granted.
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r: indicates that read access should be granted.
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w: indicates that write access should be granted.
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x: indicates that execute access should be granted.
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Uppercase values for the specification letters are allowed as well.
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Access mode specifications can be in any order. Examples of acceptable rules
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Examples of unacceptable rules are:
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Spaces are not allowed in labels. Since a subject always has access to files
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with the same label specifying a rule for that case is pointless. Only
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valid letters (rwxaRWXA) and the dash ('-') character are allowed in
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access specifications. The dash is a placeholder, so "a-r" is the same
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as "ar". A lone dash is used to specify that no access should be allowed.
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Applying Access Rules
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The developers of Linux rarely define new sorts of things, usually importing
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schemes and concepts from other systems. Most often, the other systems are
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variants of Unix. Unix has many endearing properties, but consistency of
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access control models is not one of them. Smack strives to treat accesses as
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uniformly as is sensible while keeping with the spirit of the underlying
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File system objects including files, directories, named pipes, symbolic links,
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and devices require access permissions that closely match those used by mode
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bit access. To open a file for reading read access is required on the file. To
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search a directory requires execute access. Creating a file with write access
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requires both read and write access on the containing directory. Deleting a
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file requires read and write access to the file and to the containing
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directory. It is possible that a user may be able to see that a file exists
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but not any of its attributes by the circumstance of having read access to the
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containing directory but not to the differently labeled file. This is an
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artifact of the file name being data in the directory, not a part of the file.
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IPC objects, message queues, semaphore sets, and memory segments exist in flat
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namespaces and access requests are only required to match the object in
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Process objects reflect tasks on the system and the Smack label used to access
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them is the same Smack label that the task would use for its own access
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attempts. Sending a signal via the kill() system call is a write operation
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from the signaler to the recipient. Debugging a process requires both reading
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and writing. Creating a new task is an internal operation that results in two
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tasks with identical Smack labels and requires no access checks.
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Sockets are data structures attached to processes and sending a packet from
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one process to another requires that the sender have write access to the
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receiver. The receiver is not required to have read access to the sender.
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The configuration file /etc/smack/accesses contains the rules to be set at
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system startup. The contents are written to the special file /smack/load.
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Rules can be written to /smack/load at any time and take effect immediately.
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For any pair of subject and object labels there can be only one rule, with the
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most recently specified overriding any earlier specification.
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The program smackload is provided to ensure data is formatted
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properly when written to /smack/load. This program reads lines
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subjectlabel objectlabel mode.
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The Smack label of a process can be read from /proc/<pid>/attr/current. A
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process can read its own Smack label from /proc/self/attr/current. A
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privileged process can change its own Smack label by writing to
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/proc/self/attr/current but not the label of another process.
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The Smack label of a filesystem object is stored as an extended attribute
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named SMACK64 on the file. This attribute is in the security namespace. It can
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only be changed by a process with privilege.
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A process with CAP_MAC_OVERRIDE is privileged.
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As mentioned before, Smack enforces access control on network protocol
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transmissions. Every packet sent by a Smack process is tagged with its Smack
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label. This is done by adding a CIPSO tag to the header of the IP packet. Each
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packet received is expected to have a CIPSO tag that identifies the label and
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if it lacks such a tag the network ambient label is assumed. Before the packet
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is delivered a check is made to determine that a subject with the label on the
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packet has write access to the receiving process and if that is not the case
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the packet is dropped.
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It is normally unnecessary to specify the CIPSO configuration. The default
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values used by the system handle all internal cases. Smack will compose CIPSO
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label values to match the Smack labels being used without administrative
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intervention. Unlabeled packets that come into the system will be given the
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Smack requires configuration in the case where packets from a system that is
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not smack that speaks CIPSO may be encountered. Usually this will be a Trusted
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Solaris system, but there are other, less widely deployed systems out there.
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CIPSO provides 3 important values, a Domain Of Interpretation (DOI), a level,
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and a category set with each packet. The DOI is intended to identify a group
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of systems that use compatible labeling schemes, and the DOI specified on the
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smack system must match that of the remote system or packets will be
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discarded. The DOI is 3 by default. The value can be read from /smack/doi and
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can be changed by writing to /smack/doi.
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The label and category set are mapped to a Smack label as defined in
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A Smack/CIPSO mapping has the form:
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smack level [category [category]*]
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Smack does not expect the level or category sets to be related in any
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particular way and does not assume or assign accesses based on them. Some
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examples of mappings:
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The ":" and "," characters are permitted in a Smack label but have no special
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The mapping of Smack labels to CIPSO values is defined by writing to
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/smack/cipso. Again, the format of data written to this special file
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is highly restrictive, so the program smackcipso is provided to
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ensure the writes are done properly. This program takes mappings
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on the standard input and sends them to /smack/cipso properly.
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In addition to explicit mappings Smack supports direct CIPSO mappings. One
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CIPSO level is used to indicate that the category set passed in the packet is
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in fact an encoding of the Smack label. The level used is 250 by default. The
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value can be read from /smack/direct and changed by writing to /smack/direct.
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There are two attributes that are associated with sockets. These attributes
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can only be set by privileged tasks, but any task can read them for their own
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SMACK64IPIN: The Smack label of the task object. A privileged
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program that will enforce policy may set this to the star label.
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SMACK64IPOUT: The Smack label transmitted with outgoing packets.
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A privileged program may set this to match the label of another
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task with which it hopes to communicate.
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Smack Netlabel Exceptions
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You will often find that your labeled application has to talk to the outside,
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unlabeled world. To do this there's a special file /smack/netlabel where you can
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add some exceptions in the form of :
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It means that your application will have unlabeled access to @IP1 if it has
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write access on LABEL1, and access to the subnet @IP2/MASK if it has write
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Entries in the /smack/netlabel file are matched by longest mask first, like in
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classless IPv4 routing.
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A special label '@' and an option '-CIPSO' can be used there :
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@ means Internet, any application with any label has access to it
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-CIPSO means standard CIPSO networking
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If you don't know what CIPSO is and don't plan to use it, you can just do :
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echo 127.0.0.1 -CIPSO > /smack/netlabel
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echo 0.0.0.0/0 @ > /smack/netlabel
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If you use CIPSO on your 192.168.0.0/16 local network and need also unlabeled
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Internet access, you can have :
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echo 127.0.0.1 -CIPSO > /smack/netlabel
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echo 192.168.0.0/16 -CIPSO > /smack/netlabel
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echo 0.0.0.0/0 @ > /smack/netlabel
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Writing Applications for Smack
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There are three sorts of applications that will run on a Smack system. How an
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application interacts with Smack will determine what it will have to do to
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work properly under Smack.
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Smack Ignorant Applications
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By far the majority of applications have no reason whatever to care about the
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unique properties of Smack. Since invoking a program has no impact on the
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Smack label associated with the process the only concern likely to arise is
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whether the process has execute access to the program.
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Smack Relevant Applications
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Some programs can be improved by teaching them about Smack, but do not make
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any security decisions themselves. The utility ls(1) is one example of such a
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Smack Enforcing Applications
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These are special programs that not only know about Smack, but participate in
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the enforcement of system policy. In most cases these are the programs that
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set up user sessions. There are also network services that provide information
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to processes running with various labels.
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File System Interfaces
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Smack maintains labels on file system objects using extended attributes. The
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Smack label of a file, directory, or other file system object can be obtained
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len = getxattr("/", "security.SMACK64", value, sizeof (value));
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will put the Smack label of the root directory into value. A privileged
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process can set the Smack label of a file system object with setxattr(2).
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len = strlen("Rubble");
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rc = setxattr("/foo", "security.SMACK64", "Rubble", len, 0);
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will set the Smack label of /foo to "Rubble" if the program has appropriate
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The socket attributes can be read using fgetxattr(2).
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A privileged process can set the Smack label of outgoing packets with
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len = strlen("Rubble");
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rc = fsetxattr(fd, "security.SMACK64IPOUT", "Rubble", len, 0);
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will set the Smack label "Rubble" on packets going out from the socket if the
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program has appropriate privilege.
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rc = fsetxattr(fd, "security.SMACK64IPIN, "*", strlen("*"), 0);
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will set the Smack label "*" as the object label against which incoming
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packets will be checked if the program has appropriate privilege.
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Smack supports some mount options:
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smackfsdef=label: specifies the label to give files that lack
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the Smack label extended attribute.
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smackfsroot=label: specifies the label to assign the root of the
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file system if it lacks the Smack extended attribute.
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smackfshat=label: specifies a label that must have read access to
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all labels set on the filesystem. Not yet enforced.
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smackfsfloor=label: specifies a label to which all labels set on the
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filesystem must have read access. Not yet enforced.
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These mount options apply to all file system types.
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If you want Smack auditing of security events, you need to set CONFIG_AUDIT
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in your kernel configuration.
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By default, all denied events will be audited. You can change this behavior by
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writing a single character to the /smack/logging file :
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1 : log denied (default)
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3 : log denied & accepted
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Events are logged as 'key=value' pairs, for each event you at least will get
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the subjet, the object, the rights requested, the action, the kernel function
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that triggered the event, plus other pairs depending on the type of event
added
removed
Documentation/security/Smack.txt
