ConstraintStatements

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[edit] Constraint Statements

[edit] constrain

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
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
);

[edit] validatetrans

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
if Statement
optional Statement
require Statement
No
No
No


Example:

validatetrans { file } { t1 == unconfined_t );

[edit] mlsconstrain

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

The statement definition is:

mlsconstrain class perm_set expression;

Where:

mlsconstrain The mlsconstrain 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_mls_op r2
t1 op t2
l1 role_mls_op l2
l1 role_mls_op h2
h1 role_mls_op l2
h1 role_mls_op h2
l1 role_mls_op h1
l2 role_mls_op h2
u1 op names
u2 op names
r1 op names
r2 op names
t1 op names
t2 op names
Where:
   u1, r1, t1, l1, h1 = Source user, role, type, low level, high level
   u2, r2, t2, l2, h2 = Target user, role, type, low level, high level

and:

   op : == | !=
   role_mls_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
if Statement
optional Statement
require Statement
No
No
No


Example:

This example has been taken from the Reference Policy source ./policy/mls constraints file. These are built into the policy at build time and add constraints to many of the object classes.

# The MLS Reference Policy mlsconstrain statement for searching 
# directories that comprises of multiple expressions. Only the
# first two expressions are explained.
#
# Expression 1 ( l1 dom l2 ) reads as follows:
# The dir object class search permission is allowed if the
# source low security level is dominated by the targets 
# low security level.
# OR 
# Expression 2 (( t1 == mlsfilereadtoclr ) and ( h1 dom l2 )) 
# reads as follows:
# If the source type is equal to a type associated to the 
# mlsfilereadtoclr attribute and the source high security
# level is dominated by the targets low security level, 
# then search permission is allowed on the dir object class.

mlsconstrain dir search
(( l1 dom l2 ) or
    (( t1 == mlsfilereadtoclr ) and ( h1 dom l2 )) or
    ( t1 == mlsfileread ) or
    ( t2 == mlstrustedobject ));

[edit] mlsvalidatetrans

The mlsvalidatetrans is the MLS equivalent of the validatetrans statement and is only used for file related object classes where it is used to control the ability to change the objects security context.

The statement definition is:

mlsvalidatetrans class expression;

Where:

mlsvalidatetrans The mlsvalidatetrans keyword.
class One or more file type 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
and (expression and expression
or expression or expression
u1 op u2
r1 role_mls_op r2
t1 op t2
l1 role_mls_op l2
l1 role_mls_op h2
h1 role_mls_op l2
h1 role_mls_op h2
l1 role_mls_op h1
l2 role_mls_op h2
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, l1, h1 = Old user, role, type, low level, high level
   u2, r2, t2, l2, h2 = New user, role, type, low level, high level
   u3, r3, t3, l3, h3 = Process user, role, type, low level, high level

and:

   op : == | !=
   role_mls_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
if Statement
optional Statement
require Statement
No
No
No


Example:

This example has been taken from the Reference Policy source ./policy/mls file.

# The MLS Reference Policy mlsvalidatetrans statement for 
# managing the file upgrade/downgrade rules that comprises of
# multiple expressions. Only the first two expressions are 
# explained.
#
# Expression 1: ( l1 eq l2 ) reads as follows:
#   For a file related object to change security context, its 
#   current (old) low security level must be equal to the new 
#   objects low security level.
#
# The second part of the expression:
#    or (( t3 == mlsfileupgrade ) and ( l1 domby l2 ))
# reads as follows:
#   or the process type must equal a type associated to the 
#   mlsfileupgrade attribute and its current (old) low security 
#   level must be dominated by the new objects low security level.
# 
mlsvalidatetrans { dir file lnk_file chr_file blk_file sock_file fifo_file }
((( l1 eq l2 ) or
    (( t3 == mlsfileupgrade ) and ( l1 domby l2 )) or
    (( t3 == mlsfiledowngrade ) and ( l1 dom l2 )) or
    (( t3 == mlsfiledowngrade ) and ( l1 incomp l2 ))) and (( h1 eq h2 ) or
    (( t3 == mlsfileupgrade ) and ( h1 domby h2 )) or
    (( t3 == mlsfiledowngrade ) and ( h1 dom h2 )) or
    (( t3 == mlsfiledowngrade ) and ( h1 incomp h2 ))));


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