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The Guide to Writing SELinux Policy
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4. Attributes: the attrib.te file

This section will discuss attributes, which are a way of grouping sets of types. The attrib.te file will be briefly examined. This file is located in your policy source directory, and contains attribute declarations for domains and types. Editing this file is not very common, however if you wanted to add a new attribute, you would edit it. The comments at the top of attrib.te state "a type attribute can be used to identify a set of types with a similar property. Each type can have any number of attributes, and each attribute can be associated with any number of types." Domains attributes are groupings of domains, just as attributes are groupings of types.

The domain attribute identifies every type that can be assigned to a process. This attribute refers to all processes that could be run, such as ps, top, inetd and so forth.

The privuser attribute identifies every domain that can change its SELinux user identity. Note that we are talking about SE Linux user identity and not the standard Unix uid. Running the command grep ^type.*privuser policy.conf shows that the domains which can change their identity include sysadm_su_t, initrc_su_t, staff_su_t, run_init_t, local_login_t, remote_login_t, sshd_t, sshd_extern_t and xdm_t.

The privrole attribute identifies every domain that can change its SELinux role. A domain can spawn processes that have a different role. Take newrole for example. The point of newrole is to change to another role, so the privrole attribute needs to be assigned to newrole_t in order for this to happen. privrole only allows changing to other user roles. priv_system_role allows you to change to system_r.

The privowner attribute identifies every domain that can assign a different SELinux user identity to a file, or that can create a file with an identity that's not the same as the process identity. Using passwd_t as an example, the passwd_t process has the identity of the user running it, and it wants to relabel /etc/shadow with the system_u identity, thereby requiring privowner.

The userpty_type attribute identifies all non-administrative devpts types such as user_devpts_t and staff_devpts_t . For instance, if I run the command ls --context /dev/pts on my system, I will see

crw-------  faye     staff    faye:object_r:staff_devpts_t     0
Here, /dev/pts/0 has the type staff_devpts_t .

The sysadmfile attribute identifies all types assigned to files that should be completely accessible to administrators. Note that shadow_t is not by default accessible to administrators. It can be accessed by something like, for instance, setfiles, depending on what you're trying to do.

The fs_type attribute identifies all types assigned to filesystems (not limited to persistent filesystems). security_t applies to the /selinux filesystem in the new SE Linux.

The ptyfile attribute identifies all types assigned to ptys. The explanation for the ttyfile attribute also applies here, but with pty's. Running ls --context `tty` in an xterm would show the type of the pty device you are attached to, for e.g.

faye@kaos:/etc/selinux$ ls --context `tty`
crw-------  faye     faye     faye:object_r:user_devpts_t /dev/pts/1
If I then switch to sysadm_r and run the same command, I see
faye@kaos:/etc/selinux$ newrole -r sysadm_r
Authenticating faye.
faye@kaos:/etc/selinux$ id
uid=1000(faye) gid=1000(faye) groups=1000(faye),20(dialout),25(floppy),29(audio),30(dip) \
faye@kaos:/etc/selinux$ ls --context `tty`
crw-------  faye     faye     faye:object_r:sysadm_devpts_t /dev/pts/1
Note that my pty is now labelled with the type sysadm_devpts_t.

The login_contexts attribute identifies the files used to define default contexts for login types (e.g., login, cron). Default context for login types are contained in the file /etc/security/default-contexts.

The Guide to Writing SELinux Policy
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  Published with kind permission of Faye Coker Design by Interspire