Ubuntu Manpage: compiled.rules - compiled fapolicyd rules to determine access rights fapolicyd.rules

NAME

       compiled.rules  -  compiled  fapolicyd  rules  to  determine  access  rights fapolicyd.rules - deprecated
       fapolicyd rules to determine access rights

DESCRIPTION

compiled.rules is a file that is compiled by fagenrules which contains the rules that fapolicyd uses to
make decisions about access rights. The rules follow a simple format of:

decision perm subject : object

They are evaluated from top to bottom with the first rule to match being used for the access control
decision. The colon is mandatory to separate subject and object since they share keywords.

Decision
The decision is either allow, deny, allow_audit, deny_audit, allow_syslog, deny_syslog, allow_log, or
deny_log. If the rule triggers, this is the access decision that fapolicyd will tell the kernel. If the
decision is one of the audit variety, then the decision will trigger a FANOTIFY audit event with all
relevant information. You must have at least one audit rule loaded to generate an audit event. If the
decision is one of the syslog variety, then the decision will trigger writing an event into syslog. If
the decision is of one the log variety, then it will create an audit event and a syslog event. Regardless
of the notification, any rule with a deny in the keyword will deny access and any with an allow in the
keyword will allow access.

Perm
Perm describes what kind permission is being asked for. The permission is either open, execute, or any.
If none are given, then open is assumed.

Subject
The subject is the process that is performing actions on system resources. The fields in the rule that
describe the subject are written in a name=value format. There can be one or more subject fields. Each
field is and'ed with others to decide if a rule triggers. The name values can be any of the following:

all This matches against any subject. When used, this must be the only subject in the
rule.

auid This is the login uid that the audit system assigns users when they log in to the
system. Daemons have a value of -1. The given value may be numeric or the account
name.

uid This is the user id that the program is running under. The given value may be numeric
or the account name.

gid This is the group id that the program is running under. The given value may be numeric
or the group name.

sessionid This is the numeric session id that the audit system assigns to users when they log
in. Daemons have a value of -1.

pid This is the numeric process id that a program has.

ppid This is the numeric process id of the program's parent. Note that programs that are
orphaned or started directly from systemd have a ppid value of 1. Kernel threads have
a ppid value of 2.

trust This is a boolean describing whether it is required for the subject to be in the trust
database or not. A value of 1 means its required while 0 means its not. Trust checking
is extended by the integrity setting in fapolicyd.conf. When trust is used on the
subject, it could be a daemon. If that daemon gets updated on disk, the trustdb will
be updated to the new SHA256 hash. If the integrity setting is not none, the running
daemon is not likely to be trusted unless it gets restarted. The default rules are not
written in a way that this would happen. But this needs to be highlighted as it may
not be obvious when writing a new rule.

comm This is the shortened command name. When an interpreter starts a program, it usually
renames the program to the script rather than the interpreter.

exe This is the full path to the executable. Globbing is not supported. You may also use
the special keyword untrusted to match on the subject not being listed in the rpm
database.

dir If you wish to match a directory, then use this by giving the full path to the
directory. Its recommended to end with the / to ensure it matches a directory. There
are 3 keywords that dir supports: execdirs, systemdirs, untrusted.

execdirs The execdirs option will match against the following list of directories:

/usr/ /bin/ /sbin/ /lib/ /lib64/ /usr/libexec/

systemdirs The systemdirs option will match against the same list as execdirs but
also includes /etc/.

untrusted The untrusted option will look up the current executable's full path in
the rpm database to see if the executable is known to the system. The rule
will trigger if the file in question is not in the trust database. This
option is deprecated in favor of using obj_trust with execute permission
when writing rules.

ftype This option takes the mime type of a file as an argument. If you wish to check the
mime type of a file while writing rules, run the following command:

fapolicyd-cli --ftype /path-to-file

device This option will match against the device that the executable resides on. To use it,
start with /dev/ and add the target device name.

pattern There are various ways that an attacker may try to execute code that may reveal itself
in the pattern of file accesses made during program startup. This rule can take one of
several options depending on which access patterns is wished to be blocked. Fapolicyd
is able to detect these different access patterns and provide the access decision as
soon as it identifies the pattern. The pattern type can be any of:

normal This matches against any ELF program that is dynamically linked.

ld_so This matches against access patterns that indicate that the program is
being started directly by the runtime linker.

ld_preload This matches against access patterns that indicate that the program is
being started with either LD_PRELOAD or LD_AUDIT present in the
environment. Note that even without this rule, you have protection against
LD_PRELOAD of unknown binaries when the rules are written such that trust
is used to determine if a library should be opened. In that case, the
preloaded library would be denied but the application will still execute.
This rule makes it so that even trusted libraries can be denied and the
application will not execute.

static This matches against ELF files that are statically linked.

Object
The object is the file that the subject is interacting with. The fields in the rule that describe the
object are written in a name=value format. There can be one or more object fields. Each field is and'ed
with others to decide if a rule triggers. The name values can be any of the following:

all This matches against any obbject. When used, this must be the only object in the rule.

path This is the full path to the file that will be accessed. Globbing is not supported.
You may also use the special keyword untrusted to match on the object not being listed
in the rpm database.

dir If you wish to match on access to any file in a directory, then use this by giving the
full path to the directory. Its recommended to end with the / to ensure it matches a
directory. There are 3 keywords that dir supports: execdirs, systemdirs, untrusted.
See the dir option under Subject for an explanation of these keywords.

device This option will match against the device that the file being accessed resides on. To
use it, start with /dev/ and add the target device name.

ftype This option matches against the mime type of the file being accessed. See ftype under
Subject for more information on determining the mime type.

trust This is a boolean describing whether it is required for the object to be in the trust
database or not. A value of 1 means its required while 0 means its not. Trust checking
is extended by the integrity setting in fapolicyd.conf.

sha256hash This option matches against the sha256 hash of the file being accessed. The hash in
the rules should be all lowercase letters and do NOT start with 0x. Lowercase is the
default output of sha256sum.

SETS

       Set is a named group of values of the same type.  Fapolicyd  internally  distinguishes  between  INT  and
       STRING  set  types.  You can define your own set and use it as a value for a specific rule attribute. The
       definition is in key=value syntax and starts with a set name. The set name has to start with '%' and  the
       rest  is  alphanumeric  or  '_'.  The value is a comma separated list. The set type is inherited from the
       first item in the list. If that can be turned into number then whole list is expected to  carry  numbers.
       One  can  use these sets as a value for subject and object attributes. It is also possible to use a plain
       list as an attribute value without previous definition. The assigned set has to match the attribute type.
       It is not possible set groups for TRUST and PATTERN attributes.

   SETS EXAMPLES
       # definition
       %python=/usr/bin/python2.7,/usr/bin/python3.6
       allow exe=%python : all trust=1
       #
       # definition
       # number set
       %uuids=0,1000
       allow uid=%uuids : all

NOTES

When writing rules, you should keep them focused to one goal and store them in one file. These rule files
are kept in the /etc/fapolicyd/rules.d directory. During daemon startup, fagenrules will run and compile
all these component files into one master file, compiled.rules. See the fagenrules man page for more
information.

When you are writing a rule for the execute permission, remember that the file to be executed is an
object. For example, you type ssh into the shell. The shell calls execve on /usr/bin/ssh. At that
instant in time, ssh is the object that bash is working on. However, if you are blocking execution from a
specific program, then you would normally state the program on the subject side and use all for the
object side.

If you are writing rules that use patterns, just select any as the permission to be clear that this
applies to anything. In reality, pattern matching ignores the permission but the suggestion is for
documentation purposes.

Some interpreters do not immediately read all lines of input. Rather, they read content as needed until
they get to end of file. This means that if they do stuff like networking or sleeping or anything that
takes time, someone with the privileges to modify the file can add to it after the file's integrity has
been checked. This is not unique to fapolicyd, it's simply how things work. Make sure that trusted file
permissions are not excessive so that no unexpected file content modifications can occur.

EXAMPLES

       The following rules illustrate the rule syntax.

       deny_audit perm=open exe=/usr/bin/wget : dir=/tmp
       allow perm=open exe=/usr/bin/python3.7 : ftype=text/x-python trust=1
       deny_audit perm=any pattern ld_so : all
       deny perm=any all : all

AUTHOR