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Multilevel Security is a Mandatory Access Control (MAC) policy designed to control the flow of information between subjects based upon their trustworthiness. Subjects are assigned labels that represent their trustworthiness while objects are assigned labels that represent their sensitivity.
The simple and static nature of the Bell-La Padula rules provides the potential for a higher assurance of correctness as compared to more complex and dynamic security policy models. Trusted RUBIX fully integrates the Multilevel Security policy into its DBMS, labeling all DBMS objects and controlling access to all SQL operations. For more detailed information about Trusted RUBIX Multilevel Security please see the links below or see the Trusted RUBIX MLS White Paper. Trusted RUBIX labels its DBMS objects with the same MLS labels used by the underlying operating system.
The following diagram gives sample MLS labels in Solaris 10 Trusted Extensions (TX) and SELinux formats. In both cases the MLS label may be configured to have a user-friendly string representation. The Multilevel Security policy uses an ordering relationship between labels known as the dominance relationship. The intersection of the compartments of label1 and label2 must equal the compartments of label2. One label is said to strictly dominate the other if it dominates the other but is not equal to the other. Two labels are said to be incomparable if each label has at least one compartment that is not included in the other's set of compartments. The Multilevel Security policy is concerned with the flow of information from one subject to another. Within the Trusted RUBIX DBMS, all SQL operations are controlled to prevent improper information flows.
In addition to explicit information flow channels, such as selecting a row, Trusted RUBIX also eliminates some covert information flow channels. The following diagram shows the permitted and non-permitted information flows between a representative set of MLS labels.
The following diagram shows the MLS Security Lattice for a representative set of MLS labels. Trusted RUBIX uses the same MLS Security policy configuration, and thus security lattice, as the underlying operating system.
All operations of the Trusted RUBIX DBMS have been integrated with the MLS Security policy, including all SQL operations. The following diagram demonstrates a database configuration with multiple MLS multilevel DBMS objects. The following diagram shows the logical view a user would have of the database with an Unclassified, Confidential, and Secret clearance. The following diagram shows the results of two users, with differing MLS session labels, performing a SELECT operation on the same table. The MLS filtering of the table rows occurs within the RDBMS Kernel and below the SQL Engine, making it impossible that an improper row be returned for each SQL operation. The intent of Multilevel Security is to prevent unauthorized information flows to users who are not cleared to access the information.
Consider the following steps that may be performed within a non-secure DBMS to extract sensitive information.

When the low-level user receives an error message that says the row cannot be inserted because the unique key already exists; and, the low-level users verifies that no such key exists at a dominated level, then the low-level user knows the existence of the high-level key value.
This process may be incorporated into a computer program for quick retrieval of high-level information resulting in a high bandwidth covert channel.
Trusted RUBIX removes the potential for such illegal information flows through polyinstantiating DBMS objects that have unique names. Trusted RUBIX employs polyinstantiation to eliminate potential illegal information flows that may occur due to naming collision. In the case of primary key values, Trusted RUBIX creates a duplicate copy of the row containing the key value when the low-level user attempts an insert. The illegal information flow is achieved by a high-level user locking a low-level object for read.
Trusted RUBIX eliminates the covert channels provided through locking concurrency protocols by using a unique algorithm know as the Secure Multiversion Timestamp Ordering (Secure MVTO) concurrency control protocol.
The following diagram demonstrates how Trusted RUBIX and Secure MVTO remove illegal information flows caused by locking concurrency control algorithms. Trusted RUBIX fully integrates the MLS policy of the underlying secure operating system (OS) with that of the RDBMS. With MLS policy integration, the RDBMS session label and the OS session label will be the same for every user. Other RDBMS products that implement MLS policy enforcement may not integrate with the underlying secure operating system or may be designed to operate on top of a non-MLS operating system. The first of the following two diagrams demonstrates how a malicious user may illegally send information from Top Secret to Unclassified, by exploiting the lack of OS-RDBMS policy integration. In the first diagram, the malicious user Bob SELECT's Top Secret RDBMS data and redirects it to an Unclassified operating system file. The second diagram demonstrates how Trusted RUBIX prevents such illegal information flows by integrating the MLS policies.
In general information is allowed to flow from less trustworthy subjects to more trustworthy subjects.
In addition, Trusted RUBIX fully integrates with the Multilevel Security policy of the underlying operating system, providing coherent total-system security behavior. In general, information is allowed to flow from a source to a destination if the destination MLS label dominates (is higher than or equal to) the source MLS label. The security lattice is a graphical representation of the dominance relationship between all labels in the system. All DBMS objects (database, catalog, schema, view, table, index, row) are assigned an MLS label. For instance, databases contain catalogs, catalogs contain schemata, schemata contains tables, and tables contain rows. A Top Secret clearance would have all objects visible and present a view of the database that corresponds to the first diagram. One covert way that illegal information flows may occur within non-secure systems is by exploiting unique object naming collisions between security domains. Polyinstantiation is the process of using duplicate resources or objects to remove conflicts between concurrent accesses.
Therefore, even though a duplicate key value exists at a high-level, the low-level insert succeeds, thereby hiding the existence of the high-level key. Secure MVTO creates a new version of a row upon every update and uses transaction timestamps to ensure a user operates on the correct row version.

Additionally, the low-level user never requires a lock on the row, she simply creates a new version of the row alongside the original version. The string label representation and the set of valid label formats will also be the same for Trusted RUBIX and the secure operating system.
In these environments, illegal information flows may occur from the RDBMS, to the OS, and then to a user not cleared to access the information.
The second diagram demonstrates how Trusted RUBIX prevents this illegal information flow by integrating with the underlying secure operating system. Because there is no OS-RDBMS policy integration, Bob is able to connect to the RDBMS at Top Secret while remaining at Unclassified with respect to the operating system. Because Bob's Trusted RUBIX RDBMS and OS session labels are linked, he can only redirect the RDBMS data into a Top Secret operating system file.
In general, Bell-La Padula rules allows subjects to read objects at lower labels and update objects at their label.
The MLS label assigned to an object determines which operations may be performed upon the object. Two primary areas where Trusted RUBIX removes illegal covert channels are within the concurrency control algorithm and through the use of polyinstantiation. Each of these objects may contain child objects that have a sensitivity label that is different than the parent object.
Polyinstantiation will insure that naming collisions do not reveal the existence of higher level objects. If a user does not dominate a row, then the row is simply filtered from the result set (no error value is returned).
The value of a unique key within a row may easily be used to infer the existence of high-level key values by a low-level user not cleared to access the information.. When the table is subsequently selected, each user will receives the copy of the row with the highest, dominated label. Because the write lock requires a unique lock, it will be delayed or denied because of the existence of the high-level lock.
Additionally, the security administrator may have to maintain and configure two MLS policies, each with differing label structures and security lattices. Nancy then reads the Top Secret RDBMS data from the Unclassified operating system file, completing the illegal information flow.
For instance, a channel may not exist from a Top Secret user to an Unclassified user as this would allow an Unclassified user access to Top Secret information.
If a path exists from one node to a second node then the label associated with the first node strictly dominates the label associated with the second node. The low-level user can then create a program that exploits the existence (or non-existence) of the locking delay to receive information from the cooperating high-level user. Thus, the data remains protected by the same MLS policy in both the Trusted RUBIX RDBMS and operating system.

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