SELinux Permissions Management and Creation

SELinux Permission Management and Creation

As stated in other articles, there are two main concepts that exist in MAC architectures, subjects and objects, both of which are intrinsic to understanding the aggregate concept of SELinux architecture. Objects are simply assets that exist on the system, such as files. For each of these objects, there are permission sets that are associated with them that control the access and actions that subjects have in relation to those specific objects.

In SELinux, objects are grouped into the concept of object classes, and all objects that exist in an object class are of a type. Just like in OOP programming in the .NET framework that SharePoint builds on, an object class allows the instantiation of objects, and each instantiation represents an individual object. For administrators of SharePoint, it is easiest to think of this in terms of the file system as opposed to that of a programmatic object model, where you might have a folder of related files, which is the object class, for each file in that folder that you can individually instantiate, is an object. For access to the various objects, there are rules that are applied when access to the object is demanded to subjects, and these rules are constructed by the overall security policy. Although, it is important to understand that the concept of object classes extends to networking assets such as sockets, any type of communication that would in turn expose resources that exist at the OS kernel level. Ergo, the important thing to grasp is that for each object, it is typically grouped into a relevant object class, and for each of the object classes there are rules that are applied which build into the parent concept of SELinux security policies.

Permissions to objects are in an easy to read and create syntax. First we have to define whether the process is allowed or denied, the subject that the rule applies too, the object class file, and the permission rules that will generate the rules that define the access control in the object / subject relationship (this relates to the concept of type enforcement access control). With type enforcement access control, you define the source, target, object, and the permissions, which we can see translate to the above concept nicely.

Making new object classes in SELinux is fairly straightforward, and similar to how one constructs classes within .NET compliant languages. As stated previously, it is important to realize that several of the OOP rules that we see within such languages as C# still exists in SELinux since the concept of classes and objects is somewhat, if not, identical.

To declare a new object class names (which is known as a class declaration statement), for example declaring an object class sharepointsecurity, we can simply execute:

Class sharepointsecurity

We can see that the declaring a new class for consumption entails using the keyword class and then stating what the class name is, keeping in mind that the class name must be of ASCII characters and numerical standards. Unlike C# though, there is no terminating character to the class creation statement. But like C# and .NET OOP programming practice in general, there are individual namespaces that are made for relevant classes.

Defining SELinux security policies means building the structures that will build the permission sets related to the object classes. The object classes are typically already defined by the file system, however in order to construct and apply the relevant security policies to the hybrid SharePoint environment it is necessary to research and analyze the object classes that exist within the environment.

Now we have the object classes that are relevant to the policies that we are construct in our environment, either specific or not specific to our SharePoint implementation. Now that we have objects and objects classes, it is necessary for one to declare the permissions that are related to those objects by defining the sets. Similar to group and individual groups within Active Directory within a Windows Based environment (i.e. you can compile an AD group in windows and bind it to an object, or declare a domain account that has access to that object), SELinux offers two main methods of binding relevant permissions to an object within the system. The first of these is known as common permissions.

Common permissions can easily be thought of as associating permissions with a group in relation to an object class.  When creating common permissions, we are creating permissions that are associated instead with a more than one class, as opposed to specific permissions which are bound to a specific class. Declaring common permissions is a relatively simple class and resembles class declaration in C#. The structure is easy to follow, we declare that we are stating common permissions, declaring the set name, and then declaring the object classes that the permissions apply to. The important thing to grasp with common permissions is just that they apply to more than one object class.

Common myexamplepermissionset








In the above example we are declaring the common permission name by using the common keyword and stating what the common permission set name is. The common name can be ASCII characters, numbers, dashes, or periods, and the permission set can be ASCII characters, numbers, dashes, or periods as well.

Now that we have seen that we can declare a permission set that is common, they have to be bound to a specific object class. The way that this is done is by using an access vector statement. We must associate it with an object class because although we have declared the permission set it is simply an asset that can be associated with an object class, however this is not implied. Meaning, once we associate the common permission set with the object class is when we truly see the effect that they can have, until then they simply exist and have no real effect on the overall hybrid SharePoint environment.

To associate the permission set with the object class is a relatively simple task, we have to declare the object class, and then define the permission set that we wish to bind to the object class. To create an access vector statement that would cause the sharepointsecurity object class to have class myexamplepermissionset permission would look like the following:

Class  sharepointsecurity {  myexamplepermissionset }

We are basically defining the object class and the permission set that we wish to bind, in essence this looks very similar to the C# syntax, the only difference is that there is no terminating character present.

The concept of inheritance is well known is C#, and it is no different in SELinux. Indeed, permission sets are also subject to the concept of inheritance.

Class sharepointsecurity

Inherits myexamplepermissionset





If we instigate the following example of inheritance, we are easily associating the shared permission set myexamplepermissionset with the object class sharepointsecurity, along with the custom permission set that we are defining inbetween the brackets. So, we can inherit an already defined permission set, and extend the permission set with any custom permissions that we wish to define by placing them in the ending statement parameters. This doesn’t mean that you have to extend an object class when inheriting a custom permission set, you can easily just inherit a permission set to an object class after its creation by excluding the statement as defined between the brackets. In the above example, we could modify it to look like:

Class sharepointsecurity inherits myexamplepermissionset

In this, we are simply inheriting the permission of myexamplepermissionset and not including any other custom permission such as execute or mkdir.


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