Information Flow Control - courses.cs.vt.educourses.cs.vt.edu/.../Presentations/Information-Flow.pdfInformation flow Long-term confinement of information to authorized receivers Controls

Fall, 2011 - Privacy&Security - Virginia Tech – Computer Science

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Information Flow Control

Language and System Level




Concept

Information flow Long-term confinement of information to authorized

receivers Controls how information moves among data handlers

and data storage units Applied at language, system, or application levels

Examples: Insure that “secret” data is only revealed to

individuals with a suitably high clearance level Guarantee that information available to a process

cannot leak to the network Certify that the outputs of a program only contain

information derived from specified inputs

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System Example

Guarantee that the anti-virus (AV) scanner cannot leak to the network any data found in its scan of user files

Possible leak methods Send data directly to a network connection Conspire with other processes (e.g, sendmail or httpd) Subvert another process and use its network access to send data Leave data in /tmp for other processes (e.g., the AV update daemon) to send Use other in/direct means of communication with the update daemon

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Denning Model

Flow model where N = {a,b,…} is a set of logical storage

objects P = {p,q,…} is a set of processes (active

objects) SC = {A.,B,…} is a set of security classes

• Disjoint classes of information• Each is bound to a security class

– Notation: a– may be static or dynamic (varies with content)

Class combining operator: a b N Flow relation: iff information in class

A is allowed to flow into class B

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Dorothy Denning




Example Security Classes

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public

top secret

confidential

secret(TS,[dip])

(S,[]}

(TS,[])(S,[mil]) (S,[dip])

(TS,[mil]) (S,[dip,mil])

(TS,[dip,mil])

Adapted from K. Rosen Discrete Mathematics and its Applications, 2003.




Class Combining Operations

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(TS,[dip])

(S,[]}

(TS,[]) (S,[mil]) (S,[dip])

(TS,[mil]) (S,[dip,mil])

(TS,[dip,mil])

least upper bound

greatest lower bound




Implicit/Explicit flows

In the statement: a=b+c; There is explicit flow from b to a and from c to a Here written as a b and ac

In the statement: if (a =0) {b = c;} There is an explicit flow from c to b (bc) There is an implicit flow from a to b (ba)

• Because testing the value of b before and after the statement can reveal the value of a

In the statement: if (c) {a=b+1;d=e+2;} explicit flows from b to a and from e to d (ab, ed) implicit flows from c to a and from c to d (ac, dc)

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Security Requirements

Elementary statement S: b a1,…,an is secure if ba1 ,…, ban are secure i.e., if a1 b ,…, an b i.e., if is allowed

Sequence S = S1; S2 Is secure if both S1 and S2 are secure

Conditional S = c: S1 ,…, Sn where Si updates bi is secure if bi c for i=1..n are secure i.e. if is allowed

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Static Binding

Access Control Process p can read from a only if ap Process p can write to b only if pb In general,

Data Mark Machine Associate a security class with the program counter For conditional structure c:S

• Push p onto the stack• Set p to p c• Execute S• On exit restore p from stack

For statement S that with ba1,…,an• Verify that

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⊕

⊕




Static Binding

Compiler-based For elementary statement S: f(a1,…,an)b

• verify that is allowed• Set S to b

For sequence S = S1;S2• Set S to S1 S2

For conditional structure S = c: S1,…,Sm• Set S to S1 … Sm

• Verify that c S

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Dynamic Binding

A pure dynamic binding is not practical Typical that some objects and most users have a

static security classDynamic Data Mark Machine Difficult to account for implicit flows, so… Compiler determines implicit flows and Inserts additional instructions to update class

associated with program counter accordingly Accounts for implicit flows even if flow not

executed

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HiStar : System Level Flow Control

Basic ideas Files and process are associated with a label whose taint

restricts the flow to lesser tainted components Many categories of taint each owned by its creator Selected components (e.g., wrap) can be given untainting

privileges

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Labels

Structure L = {c1l1, c2l2,…,cnln,ldefault} Each ci is a category and li is the taint level in that

category ldefault is the default level for unnamed categories L(c) = li if c=ci for some i and ldefault

otherwiseLevels

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Information Flow

General rule: information can flow from O1 to O2 only if O2 is at

least as tainted as O1 in every category Information cannot flow from O1 to O2 if O1 is more

tainted in some category than O2

Example Thread T with LT={1}, object O with LO={c3,1} LT(c)=1 < 3=LO(c) Flow is permitted from T to O (i.e., T can write to O) No flow permitted from O to T (i.e., T cannot

read/observe O)

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Example with Labels

User data labels set so that only owner can read (br3) and write (bw0)

Wrap program has ownership to read (br⋆) user data which it delegates to scanner

Wrap creates category v to (1) prevent the scanner from modifying User Data (since User Data has default level 1) and (2) prevent scanner from communicating with network

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Notation

Information flow Treatment of level ⋆ ⋆ should be high for reading, but low for writing Notation provides two ownership symbols

Used as L⋆ and L⍟; for example if L={a⋆, b⍟, 1} then L⍟ = {a⍟,b⍟,1} and L⋆ = {a⋆,b⋆,1}

Flow restriction: T can read/observe O only if T can write/modify O only if

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Kernel Object Types

Object structure objectID (unique, 61 bit) label (threads also have clearance label) quota metadata (64 bytes) flags

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Segment: variable-lengthbyte array




Design Rationale

Kernel interface The contents of object A can only affect object B if, for

every category c in which A is more tainted than B, a thread owning c takes part in the process. Provides end-to-end guarantee of which system

components can affect which others without need to understand component details

Application structure Organize applications so that key categories are

owned by small amounts of code Bulk of the system is not security critical

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Information Flow Control - courses.cs.vt.educourses.cs.vt.edu/.../Presentations/Information-Flow.pdfInformation flow Long-term confinement of information to authorized receivers Controls

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