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Controlling File Access with ACDs
ACDs are ordered lists of pairs (access permissions and user
specification) that specify access to objects. An ACD takes precedence
over certain other security features, such as lockwords and the file
security matrix.
Files located outside of MPE groups and HFS directories are automatically
assigned ACDs when they are created. By default, RACD (read ACD) is
assigned to all users and only the owner can access the file or
directory. The ACD can be modified using the ALTSEC command but the ACD
cannot be deleted.
When files are renamed to a group outside the original account, they are
automatically assigned ACDs. When a file located in an MPE group has its
group ID (GID) changed to the GID of another account, an ACD is
automatically assigned. The ACD can be modified using the ALTSEC command
but it cannot be deleted.
Access modes
ACD pairs control the access and manipulation of HFS directories and the
files within them. MPE/iX has enhanced ACDs to support four new ACD
access modes. The ACD access modes are as follows:
Permissions common to files and directories.
RACD Copy or read the ACD.
NONE Deny access.
File permissions.
R Read a file.
W Write to a file.
L Lock a file.
A Append to a file.
X Execute a file.
Directory permissions.
CD Create directory entries.
DD Delete directory entries.
RD Read directory entries.
TD Traverse directory entries.
User specifications.
The following new ACD user specifications are provided:
* $OWNER specifies users whose UID maches the UID of the object.
$OWNER enables file owners to voluntarily limit their access to an
object. For example, file owners can grant themselves read-only
access to a file to guard against accidentally modifying the file.
The $OWNER user specification is the only way for file owners to
limit their access to an object. If omitted, an owner has all
access to hierarchical files.
* $GROUP specifies users who GID matches the current group ID of the
object. $GROUP permits dynamic reference to the GID of an object.
This is useful because GIDs of files and directories can be
changed programmatically, using chown in the MPE/iX shell, or use
the :ALTFILE command. When the GID of a file is changed, it is
not necessary to modify an ACD to correct file sharing.
* $GROUP_MASK restricts the access granted by ACD entries other than
$OWNER and @.@. When an ACD contains a $GROUP_MASK entry, a user
is granted a specific access mode only if it is listed in the ACD
entry the user matches (in the form user.account, @.account, and
$GROUP) and in the $GROUP_MASK entry.
You can use traditional user specifications to describe individuals or
groups of users:
* username.accountname specifies a single user
* @.accountname specifies all users associated with the accountname
account.
Capabilities
SM and AM capability are checked before ACDs or the file access matrix.
Users with SM capability have unrestricted access to all file system
objects.
Users with SM capability can create files outside of the logon
account/group structure because they have implied CD access. Those
without SM capability can only create files in directories where they
explicitly have CD permission. Users must also have SF capability to
save files in directories and SAVE access to save files in an MPE group.
Account managers may not have complete access to all objects in their
account. Having AM capability enables a process to access file system
objects if the GID of an object (GID represented by an account name)
matches the GID (logon account) of the process. As a result, there may
be cases where the GID of a file or directory within an account has been
changed (programmatically, using chown() in the MPE/iX shell, or with the
:ALTFILE command) so that an account manager for that account cannot
access it. An account manager also may not have access to a file or
directory in the account if it was created by a user with a different
GID.
Lockwords
A file's creator can assign or remove a file lockword. Lockwords can
only be assigned to files, not to directories. Lockwords can only be
assigned to files in MPE groups.
All users are required to supply lockwords for files protected by active
lockwords. You can embed lockwords in MPE syntax file names or you can
type them in response to lockword prompting.
There is no way to specify a lockword using HFS syntax. Any attempt to
open a file with a lockword using HFS syntax results in a lockword
violation. The user is not prompted for the lockword.
Although system managers can assign ACDs to any file or directory in the
system, they must supply the lockword for any lockword-protected files
before they can assign an ACD. Once the file has an ACD, the ACD
supersedes the lockword.
Restricting Access to /tmp
Because any user can build files in /tmp, you can restrict access by
using the ALTSEC command.