ConstraintStatements

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Revision as of 17:44, 29 November 2009

Constraint Statements

constrain Statement

The constrain statement allows further restriction on permissions for the specified object classes by using boolean expressions covering: source and target types, roles and users as described in the examples.

The statement definition is:

constrain class perm_set expression;


Where:

constrain The constrain keyword.
class One or more object classes. Multiple entries consist of a space separated list enclosed in braces ({}).
perm_set One or more permissions. Multiple entries consist of a space separated list enclosed in braces ({}).
expression The boolean expression of the constraint that is defined as follows:
( expression : expression )
not expression
expression and expression
expression or expression
u1 op u2
r1 role_op r2
t1 op t2
u1 op names
u2 op names
r1 op names
r2 op names
t1 op names
t2 op names
Where:

u1, r1, t1 = Source user, role, type u2, r2, t2 = Target user, role, type

and:

op : == | !=

role_op : == | != | eq | dom | domby | incomp

names : name | { name_list }

name_list : name | name_list name


The statement is valid in:

Monolithic Policy
Base Policy
Module Policy
Yes
Yes
No
Conditional Policy (if) Statement
optional Statement
require Statement
No
No
No


Examples:

These examples have been taken from the Reference Policy source ./policy/constraints file.

# This constrain statement is the SELinux process identity 
# change constraint taken from the Reference Policy source and
# contains multiple expressions.
#
# The overall constraint is on the process object class with the 
# transition permission, and is stating that a domain transition 
# is being constrained by the rules listed (u1 == u2 etc.), 
# however only the first two expressions are explained.
#
# The first expression u1 == u2 states that the source (u1) and
# target (u2) user identifiers must be equal for a process 
# transition to be allowed.
#
# However note that there are a number of or operators that can 
# override this first constraint.
#
# The second expression:
# ( t1 == can_change_process_identity and t2 == process_user_target )
# 
# states that if the source type (t1) is equal to any type
# associated to the can_change_process_identity attribute, and 
# the target type (t2) is equal to any type associated to the
# process_user_target attribute, then a process transition is
# allowed. 

# What this expression means in the 'standard' build Reference 
# Policy is that if the source domain is either cron_t, 
# firstboot_t, local_login_t, su_login_t, sshd_t or xdm_t (as 
# the can_change_process_identity attribute has these types 
# associated to it) and the target domain is sysadm_t (as that is
# the only type associated to the can_change_process_identity 
# attribute), then a domain transition is allowed.
#

# SELinux process identity change constraint:

constrain process transition ( 
u1 == u2 
or
( t1 == can_change_process_identity and t2 == process_user_target ) or
( t1 == cron_source_domain and ( t2 == cron_job_domain or u2 == system_u ))
or
( t1 == can_system_change and u2 == system_u )
or 
( t1 == process_uncond_exempt ) );
# This constrain statement is the SELinux file related object 
# identity change constraint taken from the Reference Policy 
# source and contains two expressions.
#
# The overall constraint is on the listed file related object 
# classes (dir, file etc.), covering the create, relabelto, and 
# relabelfrom permissions. It is stating that when any of the 
# object class listed are being created or relabeled, then they
# are subject to the constraint rules listed (u1 == u2 etc.).
#
# The first expression u1 == u2 states that the source (u1) and
# target (u2) user identifiers (within the security context) 
# must be equal when creating or relabeling any of the file 
# related objects listed.
#
# The second expression:
# or t1 == can_change_object_identity
# 
# states or if the source type (t1) is equal to any type
# associated to the can_change_object_identity attribute, then
# any of the object class listed can be created or relabeled.
#

# What this expression means in the 'standard' build 
# Reference Policy is that if the source domain (t1) matches a 
# type entry in the can_change_object_identity attribute, then
# any of the object class listed can be created or relabeled.
#

# SELinux file related object identity change constraint:

constrain { dir file lnk_file sock_file fifo_file chr_file 
blk_file } { create relabelto relabelfrom } 
(
u1 == u2
or t1 == can_change_object_identity
);


validatetrans Statement

Only file related object classes are currently supported by this statement and it is used to control the ability to change the objects security context.

Note there are no validatetrans statements specified within the Reference Policy source.

The statement definition is:

validatetrans class expression;


Where:

validatetrans The validatetrans keyword.
class One or more file related object classes. Multiple entries consist of a space separated list enclosed in braces ({}).
expression The boolean expression of the constraint that is defined as follows:
( expression : expression )
not expression
expression and expression
expression or expression
u1 op u2
r1 role_op r2
t1 op t2
u1 op names
u2 op names
r1 op names
r2 op names
t1 op names
t2 op names
u3 op names
r3 op names
t3 op names
Where:

u1, r1, t1 = Old user, role, type

u2, r2, t2 = New user, role, type

u3, r3, t3 = Process user, role, type

and:

op : == | !=

role_op : == | != | eq | dom | domby | incomp

names : name | { name_list }

name_list : name | name_list name


The statement is valid in:

Monolithic Policy
Base Policy
Module Policy
Yes
Yes
No
Conditional Policy (if) Statement
optional Statement
require Statement
No
No
No


Examples:

none yet
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