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Smack: Updates for Smack documentation 

Document the Smack bringup features. Update the proper location for
mounting smackfs from /smack to /sys/fs/smackfs. Fix some spelling errors.
Suggest the use of the load2 interface instead of the load interface.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
This commit is contained in:
Casey Schaufler 2015-03-31 09:49:40 -07:00
parent f43b65bad6
commit 18779b75e9
1 changed files with 79 additions and 50 deletions
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129
Documentation/security/Smack.txt
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@ -33,11 +33,18 @@ The current git repository for Smack user space is:
git://github.com/smack-team/smack.git
This should make and install on most modern distributions.
There are three commands included in smackutil:
There are five commands included in smackutil:
smackload - properly formats data for writing to /smack/load
smackcipso - properly formats data for writing to /smack/cipso
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
In keeping with the intent of Smack, configuration data is
minimal and not strictly required. The most important
@ -47,9 +54,9 @@ of this, but it can be manually as well.
Add this line to /etc/fstab:
smackfs /smack smackfs smackfsdef=* 0 0
smackfs /sys/fs/smackfs smackfs defaults 0 0
and create the /smack directory for mounting.
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
@ -92,13 +99,13 @@ 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
A process can see the Smack label it is running with by
reading /proc/self/attr/current. A process with CAP_MAC_ADMIN
can set the process smack by writing there.
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 usually
mounted on /smack.
in the smackfs filesystem. This pseudo-filesystem is mounted
on /sys/fs/smackfs.
access
This interface reports whether a subject with the specified
@ -206,23 +213,30 @@ onlycap
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.
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 overriden 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 overriden with CAP_SYS_PTRACE.
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.
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.
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
access is a combination of the letters rwxatb 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.
@ -318,8 +332,9 @@ 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
Smack labels are ASCII character strings. They can be up to 255 characters
long, but keeping them to twenty-three characters is recommended.
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
@ -335,10 +350,9 @@ There are some predefined labels:
? Pronounced "huh", a single question mark character.
@ Pronounced "web", 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.
Every task on a Smack system is assigned a label. The Smack label
of a process will usually be assigned by the system initialization
mechanism.
Access Rules
@ -393,6 +407,7 @@ describe access modes:
w: indicates that write access should be granted.
x: indicates that execute access should be granted.
t: indicates that the rule requests transmutation.
b: indicates that the rule should be reported for bring-up.
Uppercase values for the specification letters are allowed as well.
Access mode specifications can be in any order. Examples of acceptable rules
@ -402,6 +417,7 @@ are:
Secret Unclass R
Manager Game x
User HR w
Snap Crackle rwxatb
New Old rRrRr
Closed Off -
@ -413,7 +429,7 @@ Examples of unacceptable rules are:
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 (rwxatRWXAT) and the dash ('-') character are allowed in
valid letters (rwxatbRWXATB) 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.
@ -462,16 +478,11 @@ 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.
system startup. The contents are written to the special file
/sys/fs/smackfs/load2. Rules can be added 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.
Task Attribute
@ -488,7 +499,10 @@ only be changed by a process with privilege.
Privilege
A process with CAP_MAC_OVERRIDE is privileged.
A process with CAP_MAC_OVERRIDE or CAP_MAC_ADMIN is privileged.
CAP_MAC_OVERRIDE allows the process access to objects it would
be denied otherwise. CAP_MAC_ADMIN allows a process to change
Smack data, including rules and attributes.
Smack Networking
@ -510,14 +524,14 @@ 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
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.
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
/sys/fs/smackfs/doi and can be changed by writing to /sys/fs/smackfs/doi.
The label and category set are mapped to a Smack label as defined in
/etc/smack/cipso.
@ -539,15 +553,13 @@ 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.
/sys/fs/smackfs/cipso2.
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.
value can be read from /sys/fs/smackfs/direct and changed by writing to
/sys/fs/smackfs/direct.
Socket Attributes
@ -565,8 +577,8 @@ sockets.
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 :
unlabeled world. To do this there's a special file /sys/fs/smackfs/netlabel
where you can add some exceptions in the form of :
@IP1 LABEL1 or
@IP2/MASK LABEL2
@ -574,22 +586,22 @@ 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.
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 :
@ 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 127.0.0.1 -CIPSO > /smack/netlabel
echo 0.0.0.0/0 @ > /smack/netlabel
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 :
echo 127.0.0.1 -CIPSO > /smack/netlabel
echo 192.168.0.0/16 -CIPSO > /smack/netlabel
echo 0.0.0.0/0 @ > /smack/netlabel
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
@ -676,7 +688,7 @@ 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 :
writing a single character to the /sys/fs/smackfs/logging file :
0 : no logging
1 : log denied (default)
2 : log accepted
@ -686,3 +698,20 @@ Events are logged as 'key=value' pairs, for each event you at least will get
the subject, the object, the rights requested, the action, the kernel function
that triggered the event, plus other pairs depending on the type of event
audited.
Bringup Mode
Bringup mode provides logging features that can make application
configuration and system bringup easier. Configure the kernel with
CONFIG_SECURITY_SMACK_BRINGUP to enable these features. When bringup
mode is enabled accesses that succeed due to rules marked with the "b"
access mode will logged. When a new label is introduced for processes
rules can be added aggressively, marked with the "b". The logging allows
tracking of which rules actual get used for that label.
Another feature of bringup mode is the "unconfined" option. Writing
a label to /sys/fs/smackfs/unconfined makes subjects with that label
able to access any object, and objects with that label accessible to
all subjects. Any access that is granted because a label is unconfined
is logged. This feature is dangerous, as files and directories may
be created in places they couldn't if the policy were being enforced.