Bell-LaPadula (1).ppt

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C. Edward Chow
Confidentiality Policy
CS691 – Chapter 5 of Matt Bishop

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Goals of Confidentiality Policies
 Confidentiality Policies emphasize the protection of
confidentiality.
 Confidentiality policy also called information flow policy,
prevents unauthorized disclosure of information.
 Example: Privacy Act requires that certain personal data
be kept confidential. E.g., income tax return info only
available to IRS and legal authority with court order. It
limits the distribution of documents/info.

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Discretionary Access Control (DAC)
 Definition 4-13: Mechanism where a user can set access control to
allow or deny access to an object
 Also called Identity-based access control (IBAC) Section 4.4.
 It is a traditional access control techniques implemented by
traditional operating system such as Unix.
 Based on user identity and ownership
 Programs run by a user inherits all privileges granted to the
user.
 Programs is free to change access to the user’s objects
 Support only two major categories of users:
– Completely trusted admins
– Completely untrusted ordinary users

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Problems with DAC
 Each users has complete discretion over his objects.
 What is wrong with that?
 Difficult to enforce a system-wide security policy, e.g.
– A user can leak classified documents to a unclassified users.
– Other examples?
 Only based user’s identity and ownership, Ignoring security relevant info such as
 User’s role
 Function of the program
 Trustworthiness of the program
– Compromised program can change access to the user’s objects
– Compromised program inherit all the permissions granted to the users
(especially the root user)
 Sensitivity of the data
 Integrity of the data
 Only support coarse-grained privileges
 Unbounded privilege escalation
 Too simple classification of users (How about more than two categories of users?)

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Mandatory Access Control (MAC)
 Definition 4-14: Mechanism where system control
access to an object and a user cannot alter that access.
 Occasionally called rule-based access control?
 Defined by three major properties:
 Administratively-defined security policy
 Control over all subjects (process) and objects (files,
sockets, network interfaces)
 Decisions based on all security-relevant info
 MAC access decisions are based on labels that
contains security-relevant info.

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What Can MAC Offer?
 Supports a wide variety of categories of users in system.
 For example, Users with labels: (secret, {EUR, US}) (top secret,
{NUC, US}).
 Here security level is specified by the two-tuple: (clearance,
category)
 Strong separation of security domains
 System, application, and data integrity
 Ability to limit program privileges
 Confine the damage caused by flowed or malicious software
 Processing pipeline guarantees
 Authorization limits for legitimate users

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Mandatory and Discretionary
Access Control
 Bell-LaPadula model combines Mandatory and Discretionary
Access Controls.
 “S has discretionary read (write) access to O”
means that the access control matrix entry for S and O
corresponding to the discretionary access control component
contains a read (write) right.
A B C D O
Q
S read(D)
T
 If the mandatory controls not present, S would be able to read
(write) O.

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Bell-LaPadula Model
 Also called the multi-level model,
 Was proposed by Bell and LaPadula of MITRE for enforcing access
control in government and military applications.
 It corresponds to military-style classifications.
 In such applications, subjects and objects are often partitioned into
different security levels.
 A subject can only access objects at certain levels determined by
his security level.
 For instance, the following are two typical access specifications:
``Unclassified personnel cannot read data at confidential levels''
and ``Top-Secret data cannot be written into the files at unclassified
levels''

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Informal Description
 Simplest type of confidentiality classification is a set of
security clearances arranged in a linear (total) ordering.
 Clearances represent the security levels.
 The higher the clearance, the more sensitive the info.
 Basic confidential classification system:
individuals documents
Top Secret (TS) Tamara, Thomas Personnel Files
Secret (S) Sally, Samuel Electronic Mails
Confidential (C) Claire, Clarence Activity Log Files
Unclassified (UC)Ulaley, Ursula Telephone Lists

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Star Property (Preliminary Version)
 Let L(S)=ls be the security clearance of subject S.
 Let L(O)=lo be the security classification of object ).
 For all security classification li, i=0,…, k-1, li<li+1
 Simple Security Condition:
S can read O if and only if lo<=ls and
S has discretionary read access to O.
 *-Property (Star property):
S can write O if and only if ls<=lo and
S has discretionary write access to O.
 TS guy can not write documents lower than TS. 
Prevent classified information leak.
 But how can different groups communicate?

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Basic Security Theorem
 Let  be a system with secure initial state 0
 Let T be the set of state transformations.
 If every element of T preserves the simple security
condition, preliminary version, and the *-property,
preliminary version,
Then every state i, i≥0, is secure.

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Categories and Need to Know Principle
 Expand the model by adding a set of categories.
 Each category describe a kind of information.
 These category arise from the “need to know” principle
 no subject should be able to read objects unless
reading them is necessary for that subject to perform its
function.
 Example: three categories: NUC, EUR, US.
 Each security level and category form a security level or
compartment.
 Subjects have clearance at (are cleared into, or are in) a
security level.
 Objects are at the level of (or are in) a security level.

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Security Lattice
 William may be cleared into level (SECRET, {EUR})
 George into level (TS, {NUC, US}).
 A document may be classified as (C, {EUR})
 Someone with clearance at (TS, {NUC, US}) will be
denied access to document with category EUR.
{NUC, EUR, US}
{NUC, EUR} {NUC, US} {EUR, US}
{NUC} {EUR} {US}


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Dominate (dom) Relation
 The security level (L, C) dominates the security level (L’,
C’) if and only if L’  L and C’  C
 Dom  dominate relation is false.
 Geroge is cleared into security level (S, {NUC, EUR})
 DocA is classified as (C, {NUC})
 DocB is classified as (S, {EUR, US})
 DocC is classified as (S, {EUR})
 George dom DocA
 George  dom DocB
 George dom DocC

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New Security Condition and *-Property
 Let C(S) be the category set of subject S.
 Let C(O) be the category set of object O.
 Simple Security Condition (not read up):
S can read O if and only if S dom O and
S has discretionary read access to O.
 *-Property (not write down):
S can write to O if and only if O dom S and
S has discretionary write access to O.
 Basic Security Theorem:
Let  be a system with secure initial state 0
Let T be the set of state transformations.
If every element of T preserves the simple security condition,
preliminary version, and the *-property, preliminary version,
Then every state i, i≥0, is secure.

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Allow Write Down?
 Bell-LaPadula allows higher-level subject to write into
lower level object that low level subject can read.
 A subject has a maximum security level and a current
security level. maximum security level must dominate
current security level.
 A subject may (effectively) decrease its security level
from the maximum in order to communicate with entities
at lower security levels.
 Colonel’s maximum security level is (S, {NUC, EUR}).
She changes her current security level to (S, {EUR}).
Now she can create document at Major is clearance
level (S, {EUR}).

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Data General B2 Unix System
 Data General B2 Unix (DG/UX) provides mandatory access
controls (MAC).
 The MAC label is a label identifying a particular compartment.
 The initial label (assigned at login time) is the label assigned to the
user in a database called Authorization and Authentication (A&A)
Database.
 When a process begins, it is assigned to MAC label of its parent
(whoever creates it).
 Objects are assigned labels at creation. The labels can be explicit
or implicit.
 The explicit label is stored as parts of the object’s attributes.
 The implicit label derives from the parent directory of the object.
 IMPL_HI: the least upper bound of all components in DG/UX lattice
has IMPL_HI as label.
 IMPL_LO: the greatest lower bound of all components in DG/UX
lattice has IMPL_LO as the label

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Three MAC Regions in
DG/UX MAC Lattice
Figure 5-3 The three MAC regions in the MAC lattice (modified
from the DG/UX Security Manual [257], p. 4-7, Figure 4-4). TCB
stands for "trusted computing base.“

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Accesses with MAC Labels
• Read up and write up from users to Admin Region
not allowed.
• Admin processes sanitize data sent to user
processes with MAC Labels in the user region.
• System programs are in the lowest region.
• No user can write to or alter them.
• Only programs with the same label as the directory
can create files in that directory.
• The above restriction will prevent
• compiling (need to access /tmp)
• mail delivery (need to access mail spool
directory)
• Solution multilevel directory.

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Multilevel Directory
 A directory with a set of subdirectories, one for each label.
 These hidden directories normally invisible to the user.
 When a process with label MAC_A creates a file in /tmp, it actually
create a file in hidden directory under /tmp with label MAC_A
 The parent directory of a file in /tmp is the hidden directory.
 A reference to the parent directory goes to the hidden directory.
 Process A with MAC_A creates /tmp/a. Process B with MAC_B
creates /tmp/a. Each of them performs
“cd /tmp/a; cd ..”
The system call stat(“.”, &stat_buffer) returns different inode
number for each process. It returns the inode number of the
respective hidden directory.
 Try “stat” command to display file and related status.
 DG/UX provides dg_mstat(“.”, &stat_buffer) to translate the current
working directory to the multilevel directory

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Mounting Unlabeled File System
 All files in that file system need to be labeled.
 Symbolic links aggravate this problem. Does the MAC label the target of the link
control, or does the MAC label the link itself? DG/UX uses a notion of inherited
labels (called implicit labels) to solve this problem.
 The following rules control the way objects are labeled.
1. Roots of file systems have explicit MAC labels. If a file system without labels is
mounted on a labeled file system, the root directory of the mounted file system
receives an explicit label equal to that of the mount point. However, the label of the
mount point, and of the underlying tree, is no longer visible, and so its label is
unchanged (and will become visible again when the file system is unmounted).
2. An object with an implicit MAC label inherits the label of its parent.
3. When a hard link to an object is created, that object must have an explicit label; if it
does not, the object's implicit label is converted to an explicit label. A corollary is that
moving a file to a different directory makes its label explicit.
4. If the label of a directory changes, any immediate children with implicit labels have
those labels converted to explicit labels before the parent directory's label is
changed.
5. When the system resolves a symbolic link, the label of the object is the label of the
target of the symbolic link. However, to resolve the link, the process needs access to
the symbolic link itself.

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Interesting Case with Hard Links
 Let /x/y/z: and /x/a/b be hard links to the same object. Suppose y has an explicit
label IMPL_HI and a an explicit label IMPL_B. Then the file object can be accessed
by a process at IMPL_HI as /x/y/z and by a process at IMPL_B as /x/alb.
Which label is correct? Two cases arise.
 Suppose the hard link is created while the file system is on a DG/UX B2 system.
Then the DG/UX system converts the target's implicit label to an explicit one (rule 3).
Thus, regardless of the path used to refer to the object, the label of the object will be
the same.
 Suppose the hard link exists when the file system is mounted on the DG/UX B2
system. In this case, the target had no file label when it was created, and one must
be added. If no objects on the paths to the target have explicit labels, the target will
have the same (implicit) label regardless of the path being used. But if any object on
any path to the target of the link acquires an explicit label, the target's label may
depend on which path is taken. To avoid this, the implicit labels of a directory's
children must be preserved when the directory's label is made explicit. Rule 4 does
this.
 Because symbolic links interpolate path names of files, rather than store Mode
numbers, computing the label of symbolic links is straightforward. If /x/y/z is a sym-
bolic link to /a/b/c, then the MAC label of c is computed in the usual way. However,
the symbolic link itself is a file, and so the process must also have access to the link
file z.

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Enable Flexible Write in DG/UX
 Provide a range of labels called MAC tuple.
 A range is a set of labels expressed by a lower bound and an upper
hound. A MAC tuple consists of up to three ranges (one for each of
the regions in Figure 5-3).
 Example: A system has two security levels. TS and S, the former
dominating the latter. The categories are COMP. NUC, and ASIA.
Examples of ranges are:
 [(S, { COMP } ), (TS, { COMP } )]
 [( S,  ), (TS, { COMP, NUC. ASIA } )]
 [( S, { ASIA } ), ( TS, { ASIA, NUC } )]
 The label ( TS, { COMP }) is in the first two ranges.
The label ( S, { NUC, ASIA } ) is in the last two ranges. However,
[( S, {ASIA} ), ( TS, { COMP, NUC} )]
is not a valid range because ( TS, {COMP. NUC } ) dom ( S, {
ASIA } ).

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Formal Model
 Let S be the set of subjects of a system and let O be the set of objects. Let
P be the set of rights r for read, a for write, w for read/write, and e for
empty.
 Let M be a set of possible access control matrices for the system. Let C be
the set of classifications (or clearances), let K be the set of categories, and
let L = C x K be the set of security levels. Finally, let F be the set of 3-tuples
(fs,fo,fc), where fs and, fc associate with each subject maximum and
current security levels, respectively, and, fo, associates with each object a
security level.
 The system objects may be organized as a set of hierarchies (trees and
single nodes).
 Let H represent the set of hierarchy functions h: OP(O).
P(O) is the power set of O, i.e., the set of all possible subsets of O.
 The hierarchy functions have two properties: Let oi, oj, ok O.
1. If oi oj, then h(oi)  h(oj) = .
2. There is no set { o1, o2, ..., ok }  O such that
for each i = 1, ..., k, oi+1  h(oi), and ok+1= o1.

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Formal Model: State, Request
 A state v  V of a system is a 4-tuple (b, m, f, h), where
 b  P(S x O x P) indicates which subjects have access to which objects, and
what those access rights are:
 m  M is the access control matrix for the current state;
 f  F is the 3-tuple indicating the current subject and object clearances and
categories; and
 h  H is the hierarchy of objects for the current state.
 The difference between b and m is that the rights in m may be unusable because of
differences in security levels; b contains the set of rights that may be exercised, and
m contains the set of discretionary rights.
 R denotes the set of requests for access. Four outcomes of each request are
possible:
 y for yes (allowed),
 n for no (not allowed),
 i for illegal request, and
 o for error (multiple outcomes are possible).
 D denotes the set of outcomes. The set W  R x D x V x V is the set of actions of
the system. This notation means that an entity issues a request in R, and a decision
in D occurs, moving the system from one state in V to another (possibly different)
state in V.

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Formal Model: History, System
 Let N be the set of positive integers. These integers represent
times. Let X = RN be a set whose elements x are sequences of
requests, let Y = DN be a set whose elements y are sequences of
decisions, and let Z = VN be a set whose elements z are sequences
of states. The ith components of x, y, and z are represented as xi, yi,
and zi. respectively.
 The interpretation is that for some t  N, the system is in state zt-1 
V, a subject makes request xt  R, the system makes a decision yt
 D, and as a result the system transitions into a (possibly new)
state zt  V
 A system is represented as an initial state and a sequence of
requests, decisions, and states.
 In formal terms, (R, D, W, z0)  X x Y x Z represents the system,
and z0 is the initial state of the system.
(x, y, z)  (R, D, W, z0) if and only if (xt, yt, zt, zt-1)  W for all t  N.
 (x, y, z) is an appearance of (R, D, W, z0) .

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Simple Security Condition, *-Property
 Definition 5-2. (s, o, p)  S x O x P satisfies the simple security
condition relative to f (written as ssc rel f) if and only if one of the
following holds:
a. p=e or p=a
b. p = r or p = w and fc(s) dom fo(o)
 Define b(s: p1, ..., pn) to be the set of all objects that s has p1, ..., pn
access to.
 b(s: p1, ..., pn)={ o | oO  [(s,o,p1)b ...(s,o,pn)b]}
 Definition 5-3. A state (h, m, f, h) satisfies the *-property if and only
if, for each s  S. the following hold:
a. b(s: a)    [ ob(s: a) [fo(o) dom fc(s)] ]
b. b(s: w)    [ ob(s: w) [fo(o) = fc(s)] ]
c. b(s: r)    [ ob(s: r) [fc(s) dom fo(o)] ]

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Discretionary Security Property, Action
 Definition 5-4. A state (b, m, f, h) satisfies the
discretionary security property (ds-property) if and only
if, for each triple (s, o, p)  b, p m[s, o].
 Definition 5-5. A system is secure if it satisfies the
simple security condition, the *-property, and the
discretionary security property
 Definition 5-6. (r, d, v, v')  R x D x V x V is an action of
(R, D, W, z0) if and only if there is an (x, y, z)  (R, D,
W, z0) and a t  N such that (r, d, v, v') = (xt, yt, zt, zt-1)
 An action is a request/decision pair that occurs during
the execution of the system.

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When the three properties hold
 Theorem 5-3. (R, D, W, z0) satisfies the simple security condition for any
secure state z0 if and only if, for every action (r, d, (b, m, f, h), (b', m', f', h')),
W satisfies the following:
a. Every (s, o, p)  b - b' satisfies ssc rel f.
b. Every (s, o, p)  b' that does not satisfy ssc rel f is not in b.
 Theorem 5-4. (R, D, W, z0) satisfies the *-property relative to S'  S for
any secure state z0 if and only if, for every action (r, d, (b, m, f, h), (b', m', f',
h')), W satisfies the following for every s  S':
a. Every (s, o, p)  b - b' satisfies the *-property with respect to S'.
b. Every (s, o, p)  b' that does not satisfy the *-property with respect to S'
is not in b.
 Theorem 5-5. (R, D, W, z0) satisfies the ds-property for any secure
state z0 if and only if, for every action (r, d, (b, m, f, h), (b', m', f', h')),
W satisfies the following:
a. Every (s, o, p)  b - b ' satisfies the ds-property.
b. Every (s, o, p)  b' that does not satisfy the ds-property is not in b.
 Theorem 5-6. Basic Security Theorem: (R, D. W, z0) is a secure system
if z0 is a secure state and W satisfies the conditions of Theorems 5-3, 5-4,
and 5-5.

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Rules of Transformation
 A rule is a function :R x VD x V Intuitively, a rule takes a state
and a request, and determines if the request meets the conditions
of the rule (the decision). If so, it moves the system to a (possibly
different) state.
 Definition 5-7. A rule p is ssc-preserving, if, for all (r, v)  R x V and
v satisfying ssc rel f, (r, v) = (d, v') means that v' satisfies ssc rel f'.
 Similar definitions hold for the property and the ds-property. If a rule
is sscpreserving, *-property-preserving, and ds-property-preserving,
the rule is said to be security-preserving.
 Definition 5-8. Let w = {1, ..., m } be a set of rules. For request r 
R, decision d  D, and states v, v'  V, (r, d, v, v')  W() if and only
if d  i and there is a unique integer i, 1 ≤ i ≤ m, such that i(r, v) =
(d, v' ).
 This definition says that if the request is legal and there is only one
rule that will change the state of the system from v to v', the
corresponding action is in W().

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When rule set preserves simple
security condition?
 Theorem 5-7. Let  be a set of ssc-preserving rules, and let z0 be a
state satisfying the simple security condition. Then (R, D, W, z0)
satisfies the simple security condition.
 When does adding a state preserve the simple security property?
 Theorem 5-8. Let v = (b, m, f, h) satisfy the simple security
condition. Let (s, o, p)  b, b' = b  {(s, o, p) }, and v' = (b', m, f, h).
Then v' satisfies the simple security condition if and only if either of
the following conditions is true.
a. Either p = e or p = a.
b. Either p = r or p = w, and fs(s) dom fo(o).
 Theorem 5-9. Let  be a set of *-property-preserving rules, and let
z0 be a state satisfying the *-property. Then (R, D, W, z0) satisfies
the *-property.

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Properties
 Theorem 5-10. Let v= (b, m, f, h) satisfy the *-property. Let (s, o, p)  b, b' = b  {
(s, o, p) }, and v' = (b', m, f, h). Then v' satisfies the *-property if and only if one of
the following conditions holds.
a. p = a and fo(o) dom fc(s)
b. p = w and. fo(o) = fc(s)
c. p = r and fc(s) dom fo(o)
 Theorem 5-11. Let  be a set of ds-property-preserving rules, and let z0 be a state
satisfying the ds-property. Then (R, D, W, z0) satisfies the ds-property.
 Theorem 5-12. Let v = (b, m,,f; h) satisfy the ds-property. Let (s, o, p)  b, b' = b  {
(s, o. p) }, and v' = (b', m, f, h).
Then v' satisfies the ds-property if and only if p  m[s, o].
 Theorem 5-13. Let  he a rule and (r, v) = (d, v'), where v= (b, m, f, h) and v' = (b',
m', f', h'). Then:
a. If b' b, f'=,f, and v satisfies the simple security condition, then v‘ satisfies
the simple security condition.
b. If b'  h, f' =f, and v satisfies the *-property, then v' satisfies the *-property.
c. If b'  h, , m[s, o]  m' [s, o] for all s  S and o  O, and v satisfies the ds-
property, then v' satisfies the ds-property.

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Multics Example (Model Instantiation)
 The Multics system [68, 788 has I 1 rules affecting the rights on the system. These rules are
divided into five groups. Let the set Q contain the set of request operations (such as get, give,
and so forth). Then:
1. R(1) = Q x S x O x M. This is the set of requests to request and release access. The rules are get-
read, get-append, get-execute, get-write, and release-read/execute/write/append. These rules
differ in the conditions necessary for the subject to be able to request the desired right. The rule
get-read is discussed in more detail in Section 5.2.4.1.
2. R(2) = S x Q x S x O x M. This is the set of requests to give access to and remove access from a
different subject. The rules are give-read/execute/write/append and rescind-
read/execute/write/append. Again, the rules differ in the conditions needed to acquire and delete
the rights, but within each rule, the right being added or removed does not affect the conditions.
Whether the right is being added or deleted does affect them. The rule give-
read/execute/write/append is discussed in more detail in Section 5.2.4.2.
3. R(3) = Q x S x O x L. This is the set of requests to create and reclassify objects. It contains the
create-object and change-object-security-level rules. The object's security level is either assigned
(create-object) or changed (change-object-security-Ievel ).
4. R(4) = S x O. This is the set of requests to remove objects. It contains only the rule delete-object-
group, which deletes an object and all objects beneath it in the hierarchy.
5. R(5) = S x L. This is the set of requests to change a subject's security level. It contains only the rule
change-subject-current-security-level, which changes a subject's current security level (not the
maximum security level).
 Then, the set of requests R = R(1)  R(2)  R(3)  R(4)  R(5)
 The Multics system includes the notion of trusted users. The system does not enforce the *-
property for this set of subjects ST S, however, members of ST are trusted not to violate that
property.
 For each rule , define () as the domain of the request (that is, whether or not the components
of the request form a valid operand for the rule).

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The get-read Rule
 The get-read rule enables a subject s to request the right to read an object o.
Represent this request as r = (get, s, o, r)  R(1) , and let the current state of the
system be v= (b, m, f, h). Then get-read is the rule 1(r, v):
if (r  (1)) then 1(r, v)=(i, v);
else if ( fs(s) dom fo(o) and [s  ST or fc(s) dom fo(o)] and r  m[s, o])
then 1(r, v)=(y, (b  { (s, o, r) }, m, f, h));
else 1(r, v)=(n, v);
 The first if tests the parameters of the request: if any of them are incorrect, the
decision is "illegal" and the system state remains unchanged.
 The second if checks three conditions. The simple security property for the
maximum security level of the subject and the classification of the object must hold.
Either the subject making the request must be trusted, or the simple security
property must hold for the current security level of the subject (this allows trusted
subjects to read information from objects above their current security levels but at or
below their maximum security levels; they are trusted not to reveal the information
inappropriately). Finally, the discretionary security property must hold. If these three
conditions hold, so does the Basic Security Theorem. The decision is "yes" and the
system state is updated to reflect the new access. Otherwise, the decision is "no"
and the system state remains unchanged.

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The give-read Rule
 The give-read rule enables a subject s to give subject s2 the (discretionary) right to read an object
o. Conceptually, a subject can give another subject read access to an object if the giver can alter
(write to) the parent of the object. If the parent is the root of the hierarchy containing the object, or
if the object itself is the root of the hierarchy, the subject must be specially authorized to grant
access.
 Some terms simplify the definitions and proofs. Define root(o) as the root object of the hierarchy
h containing o, and define parent(o) as the parent of o in h. If the subject is specially authorized
to grant access to the object in the situation just mentioned, the predicate canallow(s, o, v) is
true. Finally, define m  m[s, o]r as the access control matrix m with the right r added to entry
m[s, o].
 Represent the give-read request as r = (s1, give, s2, o, r)  R(2), and let the current state of the
system be v = (b, m, f, h). Then, give-read is the rule 6(r, v):
if (r  (6)) then 6(r, v) = (i, v);
else if ( [ o  root(o) and parent(o)  root(o) and parent(o)  b(s1: w)] or
[ parent(o) = root(o) and canallow(s1, o, v) ] or
[ o = root(o) and canallow(s1, root(o), v) ])
then 6(r, v) = (y, (b, m  m[s2, o]r, f, h));
else 6(r, v) = (n, v);
 The first if tests the parameters of the request; if any of them are incorrect, the decision is "illegal"
and the system state remains unchanged. The second if checks several conditions. If neither the
object nor its parent is the root of the hierarchy containing the object, then s1 must have write
rights to the parent. If the object or its parent is the root of the hierarchy, then s1 must have
special permission to give s2 the read right to o. The decision is "yes" and the access control
matrix is updated to reflect the new access. Otherwise, the decision is "no" and the system state
remains unchanged.

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Tranquility
 The principle of tranquility states that subjects and objects may not change their
security levels once they have been instantiated.
 Suppose that security levels of objects can be changed, and consider the effects on
a system with one category and two security clearances, HIGH and LOW. If an
object's security classification is raised from LOW to HIGH, then any subjects
cleared to only LOW can no longer read that object. Similarly, if an object's
classification is dropped from HIGH to LOW, any subject can now read that object.
 Both situations violate fundamental restrictions.
 Raising the classification of an object means that information that was available is no
longer available; lowering the classification means that information previously
considered restricted is now available to all.
 Raising the classification of an object is not considered a problem. The model does
not define how to determine the appropriate classification of information. It merely
describes how to manipulate an object containing the information once that object
has been assigned a classification.
 declassification problem. Because this makes information available to subjects who
did not have access to it before, it is in effect a "write down" that violates the
 *-property. The typical solution is to define a set of trusted entities or subjects that
will remove all sensitive information from the HIGH object before its classification is
changed to LOW.

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Strong/Weak Tranquility
 Definition 5-9. The principle of strong tranquility states that security levels do not
change during the lifetime of the system.
 Strong tranquility eliminates the need for trusted declassifiers, because no
declassification can occur. Moreover, no raising of security levels can occur. This
eliminates the problems discussed above. However, strong tranquility is also
inflexible and in practice is usually too strong a requirement.
 Definition 5-10. The principle of weak tranquility states that security levels do not
change in a way that violates the rules of a given security policy.
 Weak tranquility moderates the restriction to allow harmless changes of security
levels. It is more flexible, because it allows changes, but it disallows any violations of
the security policy (in the context of the Bell-LaPadula Model, the simple security
condition and *-property).
 EXAMPLE: In the Data General DG/UX system, only the security administrator, a
trusted user, can change MAC labels on objects. In general, when a user wishes to
assume a new MAC label, that user must initiate a new session; the MAC labels of
processes cannot be changed. However, a user may be designated as able to
change a process label within a specified range. This makes the system more
amenable to commercial environments.

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Controversy Over Bell-LaPadula Modoel
 1985 McLean define a †-property which is not secure (allow write down) and show that the basic
theorem is not correct.
 Definition 5-11. A state (b, m, f, h) satisfies the †-property if and only if, for each subject s c S,
the following conditions hold:
a. b(s: a)    [ ob(s: a) [fc(s) dom fo(o) ] ]
b. b(s: w)    [ ob(s: w) [fc(s) = fo(o) ] ]
c. b(s: r)    [ ob(s: r) [fc(s) dom fo(o) ] ]
 McLean then proved the analogue to Theorem 5-4:
 Theorem 5-16. (R, D, W, z0) satisfies the †-property relative to S'  S for any secure state z0 if
and only if, for every action (r, d, (b, m, f, h), (b', m', f', h')), W satisfies the following conditions for
every s  S
a. Every (s, o, p)  b - b' satisfies the †-property with respect to S
b. Every (s, o, p)  b' that does not satisfy the †-property with respect to S' is not in b.
 From this theorem, and from Theorems 5-3 and 5-5, the analogue to the Basic Security Theorem
follows.
 Theorem 5-17. Basic Security Theorem: (R, D, W, z0) is a secure system if and only if zt is a
secure state and W satisfies the conditions of Theorems 5-3, 5-16, and 5-5.
 But the system (R, D, W, z0) is clearly not secure.
 Bell-LaPadula argue that their model assumes the transition introduces no changes that violate
security.

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McClean’s System Z
 In 1987, McClean presented System Z where system transitions
can alter any system component, including b, f, m, and h, as long
as the new state does not violate security. He demonstrated system
satisfies the model but is not a confidentiality security policy.
 Bell [64] responded by exploring the fundamental nature of
modeling. Newtonia math cannot explain planet movement while
Einstein’s theory of general relativity can.
 Bell-LaPadula Model is a tool for demonstrating certain properties
of rules. Whether the properties of System Z is desirable is an
issue the model cannot answer.
 Bell-LaPadula Model enforces the principle of strong tranquility.
 System Z deals with the case of weak tranquility (security level can
change).

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Problem with Traditional MAC
 Poor support for
 Data and application integrity (Clark Wilson Integrity model;
Chinese Wall security policy)
 Separation of duty
 Least privilege requirement
 Require special trusted subject that act outside of the access
control model (e.g., lower security level to write down)
 Fail to tightly control the relationship between subject and the code
it executes. This limits:
 Limit protection based on function and trustworthiness of the
code.
 Correctly manage permissions required for execution
 Minimize the likelihood of malicious code execution

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History Security-Enhanced Linux
(SELinux)
 National Security Agency (NSA) and Secure Computing Corporation (SCC)
provide strong MAC.
 Flexible support for security policies (no single MAC policy can satisfy
everyone’s security requirements)
 Cleanly separate the security policy logic from enforcing mechanism
 Developed DTMach, DTOS (Mach-based prototype)
 Apply formal method to validate the security properties of the
architecture (High Assurance)
 Work with Univ. Utah Flux Research Group
 integrate the architecture to Fluke research operating system
 Result: Flask architecture support dynamic security policies.
 NSA create SELinux integrate Flash architecture to Linux OS.
 NAI implements control on procfs and devpts fiel ssytems
 MITRE/SCC contribute application security policies, modified utility
programs

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SELinux
 Support
 Separation policies:
– Enforce legal restriction on data
– Establish well-defined user roles
– Restrict access to classified data
 Containment policies for
– Restrict web server access to only authorized data
– Minimize damage caused by virues/malicious code
 Integrity policies that protect unauthorized modifications to data
and applications
 Invocation policies that guarantee data is processed as required.

Bell-LaPadula (1).ppt

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