Mandatory Security Policies CS461/ECE422 Spring 2012

Reading Materials 13.1–13.4 in the text

Overview Remainder of MAC Bell-LaPadula Confidentiality Model Biba Integrity Model Lipner’s Integrity Model Clark-Wilson Integrity Model

Slide #5-4 MAC vs DAC Discretionary Access Control (DAC) – Normal users can change access control state directly assuming they have appropriate permissions – Access control implemented in standard OS’s, e.g., Unix, Linux, Windows – Access control is at the discretion of the user Mandatory Access Control (MAC) – Access decisions cannot be changed by normal rules – Generally enforced by system wide set of rules – Normal user cannot change access control schema “Strong” system security requires MAC – Normal users cannot be trusted

Slide #5-5 Confidentiality Policy Goal: prevent the unauthorized disclosure of information – Deals with information flow – Integrity incidental Multi-level security models are best-known examples – Bell-LaPadula Model basis for many, or most, of these

Security Levels Most basic example of security class Each subject and object has a security level The levels are completely ordered For example – Top secret > secret > confidential > restricted > unclassified The subject’s level is security clearance The object’s level is security classification

Security Level Example Security LevelSubjectObject Top SecretAliceNext Generation Designs SecretBobMarketing plans ConfidentialCarolLast Quarter Financial Earnings UnclassifiedDaveTelephone directory

Simple Security Property No Read Up Subject can only read an object of less or equal security level. Level(0) <= Level(S)

No write down *-Property A subject can only write into an object of greater or equal security level. Level(S) <= Level(O)

DAC in MAC ds-property A MAC system may also include a traditional discretionary access control check If *-property and simple security property checks pass, then also check the discretionary access rules

More Advanced Security Class Simple linear ordering not adequate for larger system Add set of categories to the security level to create a security label, – E.g., top secret:{project1, project2}. As clearance, subject is cleared to top secret only for project 1 and project 2 not project 3. Set of security labels forms a partial ordering or a lattice

Comparing Security Labels Replace < operator with dominates operator (A1, C1) dominates (A2, C2) iff A2 <= A1 and C2 subset of C1 Replace <= with dominate and simple security condition and *-property holds

Example Lattice of Categories CS461, CS411, CS463 CS461, CS411 CS461, CS463 CS411, CS463 CS461CS411CS463

Security Label Comparisons Susan Label = Secret:{461, 498} Igor Label = Secret:{461} Student label = Confidential:{461} Susan writes exam for CS461 – What label should it have, so Igor can help write? – What label should it have for student to read exam?

Adding Security Clearance Flexibility Define maximum and current level for subjects – maxlevel(S) dom curlevel(S) – In some systems, the min level is also defined *-property: allow write iff Level(O) dom curlevel(S) simple security property: – Allow read iff maxlevel(S) dom Level(O) – Raise curlevel(S) to join(Level(O),curlevel(S)) How does this ease the previous example?

Principle of Tranquility Raising object’s security level – Information once available so some subjects is no longer available – Usually assume information has already been accessed, so this does nothing Lowering object’s security level – The declassification problem – Essentially, a write down, violates *-property

Types of Tranquility Strong tranquility – The clearances of subjects, and the classification of objects, do not change during the lifetime of the system Weak tranquility – The clearances of subjects and the classifications of the objects change in accordance with a specified policy.

18 Biba Integrity Model Basis for all 3 models: Set of subjects S, objects O, integrity levels I, relation ≤  I  I holding when second dominates first min: I  I  I returns lesser of integrity levels i: S  O  I gives integrity level of entity r  S  O means s  S can read o  O w, x defined similarly Biba 77

19 Intuition for Integrity Levels The higher the level, the more confidence – That a program will execute correctly – That data is accurate and/or reliable Note relationship between integrity and trustworthiness Important point: integrity levels are not security levels

20 Information Transfer Path An information transfer path is a sequence of objects o 1,..., o n+1 and corresponding sequence of subjects s 1,..., s n such that s i r o i and s i w o i+1 for all i, 1 ≤ i ≤ n. Idea: information can flow from o 1 to o n+1 along this path by successive reads and writes O1S2O2S3O3

21 Strict Integrity Policy Dual of Bell-LaPadula model 1. s  S can read o  O iff i(s) ≤ i(o) 2. s  S can write to o  O iff i(o) ≤ i(s) 3. s 1  S can execute s 2  O iff i(s 2 ) ≤ i(s 1 ) Add compartments and discretionary controls to get full dual of Bell-LaPadula model Information can flow only down – no reads down, no writes up Term “ Biba Model ” refers to this

Notion of time Strict policy may be too strict O1 High Integrity O2 S1 Low Integrity read write Time

23 Low-Water-Mark Policy Idea: a subject ’ s integrity level changes with time – Tracks the lowest integrity level object it has read Rules 1.s  S can write to o  O if and only if i(o) ≤ i(s). 2.If s  S reads o  O, then i(s) = min(i(s), i(o)), where i(s) is the subject ’ s integrity level after the read. 3.s 1  S can execute s 2  S if and only if i(s 2 ) ≤ i(s 1 ).

24 Information Flow and Model If there is information transfer path from o 1  O to o n+1  O, enforcement of low-water-mark policy requires i(o n+1 ) ≤ i(o 1 ) for all n O1S2O2S3O3S2S3

25 Problems Subjects ’ integrity levels decrease as system runs – Soon no subject will be able to access objects at high integrity levels Alternative: change object levels rather than subject levels – Soon all objects will be at the lowest integrity level Crux of problem is model prevents indirect modification – Because subject levels lowered when subject reads from low-integrity object

26 Ring Policy Idea: subject integrity levels static Rules 1. s  S can write to o  O if and only if i(o) ≤ i(s). 2. Any subject can read any object. 3. s 1  S can execute s 2  S if and only if i(s 2 ) ≤ i(s 1 ). Eliminates indirect modification problem Does the information flow constraint hold?

27 Integrity Matrix Model Lipner proposed this as first realistic commercial model Combines Bell-LaPadula, Biba models to obtain model conforming to requirements Do it in two steps – Bell-LaPadula component first – Add in Biba component Lipner 82

28 Requirements of Integrity Policies 1.Users will not write their own programs, but will use existing production programs and databases. 2.Programmers will develop and test programs on a non-production system; if they need access to actual data, they will be given production data via a special process, but will use it on their development system. 3.A special process must be followed to install a program from the development system onto the production system. 4.The special process in requirement 3 must be controlled and audited. 5.The managers and auditors must have access to both the system state and the system logs that are generated. Lipner 82

29 Bell-LaPadula Clearances 2 security clearances/classifications – AM (Audit Manager): system audit, management functions – SL (System Low): any process can read at this level

30 Bell-LaPadula Categories 5 categories – D (Development): production programs in development but not yet in use – PC (Production Code): production processes, programs – PD (Production Data): data covered by integrity policy – SD (System Development): system programs in development but not yet in use – T (Software Tools): programs on production system not related to protected data

31 Users and Security Levels (SL, {D, PC, PD, SD, T}) and downgrade privilege System controllers (AM, { D, PC, PD, SD, T })System managers and auditors (SL, { SD, T })System programmers (SL, { D, T })Application developers (SL, { PC, PD })Ordinary users Security LevelSubjects

32 Objects and Classifications (AM, { appropriate })System and application logs (SL, { SD, T })System programs in modification (SL,  ) System programs (SL, { T })Software tools (SL, { PC, PD })Production data (SL, { PC })Production code (SL, { D, T })Development code/test data Security LevelObjects

Lipner Lattice SL, {PC, PD} S: Ordinary users O: Production data SL, {D, T} S: Developers O: Development code SL, {T} O: Software Tools SL, {PC} O: Production Code SL, {  } O: System programs SL, {SD,T} S: System programmers O: Tools in modification AM, {...} S: System Managers O: System Logs SL, {PC,PD,D,T,SD} S: System Controllers

34 Ideas Ordinary users can execute (read) production code but cannot alter it Ordinary users can alter and read production data System managers need access to all logs but cannot change levels of objects System controllers need to install code (hence downgrade capability) Logs are append only, so must dominate subjects writing them

35 Check Requirements 1.Users have no access to T, so cannot write their own programs 2.Applications programmers have no access to PD, so cannot access production data; if needed, it must be put into D, requiring the system controller to intervene 3.Installing a program requires downgrade procedure (from D to PC), so only system controllers can do it

36 More Requirements 4.Control: only system controllers can downgrade; audit: any such downgrading must be audited 5.System management and audit users are in AM and so have access to system state and logs

37 Problem Too inflexible – An application developer cannot run a program for repairing inconsistent or erroneous production database Application programmers are not given access to production data So add more …

38 Adding Biba 3 integrity classifications – ISP (System Program): for system programs – IO (Operational): production programs, development software – ISL (System Low): users get this on log in 2 integrity categories – ID (Development): development entities – IP (Production): production entities

39 Simplify Bell-LaPadula Reduce security categories to 3: – SP (Production): production code, data – SD (Development): same as D – SSD (System Development): same as old SD

40 Users and Levels (ISL, { IP }) (ISP, { IP, ID}) (ISL,  ) (ISL, { ID }) (ISL, { IP }) Integrity Level (SL, { SP })Repair (SL, { SP, SD, SSD }) and downgrade privilege System controllers (AM, { SP, SD, SSD })System managers and auditors (SL, { SSD })System programmers (SL, { SD })Application developers (SL, { SP })Ordinary users Security LevelSubjects

41 Objects and Classifications (ISL, { IP }) (ISL,  ) (ISL, { ID }) (ISP, { IP, ID }) (IO, { ID }) (ISL, { IP }) (IO, { IP }) (ISL, { ID } ) Integrity Level (SL, {SP})Repair (AM, { appropriate })System and application logs (SL, { SSD })System programs in modification (SL,  ) System programs (SL,  ) Software tools (SL, { SP })Production data (SL, { SP })Production code (SL, { SD })Development code/test data Security LevelObjects

42 Ideas Security clearances of subjects same as without integrity levels Ordinary users need to modify production data, so ordinary users must have write access to integrity category IP Ordinary users must be able to write production data but not production code; integrity classes allow this – Note writing constraints removed from security classes

43 Clark-Wilson Integrity Model Integrity defined by a set of constraints – Data in a consistent or valid state when it satisfies these Example: Bank – D today ’ s deposits, W withdrawals, YB yesterday ’ s balance, TB today ’ s balance – Integrity constraint: TB = D + YB –W Well-formed transaction move system from one consistent state to another Issue: who examines, certifies transactions done correctly? Clark, Wilson 87

44 Entities CDIs: constrained data items – Data subject to integrity controls UDIs: unconstrained data items – Data not subject to integrity controls IVPs: integrity verification procedures – Procedures that test the CDIs conform to the integrity constraints TPs: transaction procedures – Procedures that take the system from one valid state to another

45 Certification Rule 1 All CDI Any IVP CR1 CR1When any IVP is run, it must ensure all CDIs are in a valid state

46 CR2 CDI set TP CR2 CR2For some associated set of CDIs, a TP must transform those CDIs in a valid state into a (possibly different) valid state  Defines relation certified that associates a set of CDIs with a particular TP  Example: TP balance, CDIs accounts, in bank example

47 CR1 and ER1 CDI set TP ER1 ER1The system must maintain the certified relations and must ensure that only TPs certified to run on a CDI manipulate that CDI.

48 Other Rules User CDI Set Log (CDI) TP ER3 ER2/CR3 CR4 ER2The system must associate a user with each TP and set of CDIs. The TP may access those CDIs on behalf of the associated user. The TP cannot access that CDI on behalf of a user not associated with that TP and CDI.  System must maintain, enforce certified relation  System must also restrict access based on user ID (allowed relation)

49 Other Rules User CDI Set Log (CDI) TP ER3 ER2/CR3 CR4 CR3The allowed relations must meet the requirements imposed by the principle of separation of duty.

50 Other Rules User CDI Set Log (CDI) TP ER3 ER2/CR3 CR4 ER3The system must authenticate each user attempting to execute a TP  Type of authentication undefined, and depends on the instantiation  Authentication not required before use of the system, but is required before manipulation of CDIs (requires using TPs)

51 Other Rules User CDI Set Log (CDI) TP ER3 ER2/CR3 CR4 CR4All TPs must append enough information to reconstruct the operation to an append-only CDI.  This CDI is the log  Auditor needs to be able to determine what happened during reviews of transactions

52 Handling Untrusted Input CR5Any TP that takes as input a UDI may perform only valid transformations, or no transformations, for all possible values of the UDI. The transformation either rejects the UDI or transforms it into a CDI. UDI CDI TP

53 Separation of Duty In Model ER4Only the certifier of a TP may change the list of entities associated with that TP. No certifier of a TP, or of an entity associated with that TP, may ever have execute permission with respect to that entity. – Enforces separation of duty with respect to certified and allowed relations User1 User2 TP ExecCert User1  User2 = ø

54 Comparison With Requirements 1.Users can ’ t certify TPs, so CR5 and ER4 enforce this 2.Procedural, so model doesn ’ t directly cover it; but special process corresponds to using TP No technical controls can prevent programmer from developing program on production system; usual control is to delete software tools 3.TP does the installation, trusted personnel do certification

55 Comparison With Requirements 4.CR4 provides logging; ER3 authenticates trusted personnel doing installation; CR5, ER4 control installation procedure New program UDI before certification, CDI (and TP) after 5.Log is CDI, so appropriate TP can provide managers, auditors access Access to state handled similarly

56 Comparison to Biba Biba – No notion of certification rules; trusted subjects ensure actions obey rules – Untrusted data examined before being made trusted Clark-Wilson – Explicit requirements that actions must meet – Trusted entity must certify method to upgrade untrusted data (and not certify the data itself)