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1 Information Security – Theory vs. Reality 0368-4474-01, Winter 2011 Lecture 7: Information flow control Eran Tromer Slides credit: Max Krohn, MIT Ian Goldberg and Urs Hengartner, University of Waterloo
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Information Flow Control An information flow policy describes authorized paths along which information can flow For example, Bell-La Padula describes a lattice-based information flow policy
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Example Lattice (“Top Secret”, {“Soviet Union”, “East Germany”}), (“Unclassified”, ) (“Top Secret”, {“Soviet Union”}) (“Secret”, {“Soviet Union”}) (“Secret”, {“East Germany”}) (“Secret”, {“Soviet Union”, “East Germany”})
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Lattices Dominance relationship ≥ defined in military security model is transitive and antisymmetric Therefore, it defines a lattice For two levels a and b, neither a ≥ b nor b ≥ a might hold However, for every a and b, there is a lowest upper bound u for which u ≥ a and u ≥ b, and a greatest lower bound l for which a ≥ l and b ≥ l There are also two elements U and L that dominate/are dominated by all levels In example, U = (“Top Secret”, {“Soviet Union”, “East Germany”}) L = (“Unclassified”, )
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Information Flow Control Input and output variables of program each have a (lattice-based) security classification S() associated with them For each program statement, compiler verifies whether information flow is secure For example, x = y + z is secure only if S( x ) ≥ lub(S( y ), S( z )), where lub() is lowest upper bound Program is secure if each of its statements is secure
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Implicit information flow Conditional branches carry information about the condition: secret bool a; public bool b; … b = a; if (a) { b = 0; } else { b = 1; } Possible solution: assign label to program counter and include it in every operation. (Too conservative!)
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Issues Label creep Example: false flows Example: branches Declassification example: encryption
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Implementation approaches 8 Language-based IFC (JIF & others) – Compile-time tracking for Java, Haskell – Static analysis – Provable security – Label creep (false positives) Dynamic analysis – Language / bytecode / machine code – Tainting – False positives, false negatives – Hybrid solutions OS-based DIFC (Asbestos, HiStar, Flume) – Run-time tracking enforced by a trusted kernel Works with any language, compiled or scripting, closed or open
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9 Distributed Information Flow Control DB Bob’s Data Alice’s Data Gateway Alice’s Data 1. Data tracking 2. Isolated declassification
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Information Flow Control (IFC) Goal: track which secrets a process has seen Mechanism: each process gets a secrecy label – Label summarizes which categories of data a process is assumed to have seen. – Examples: { “Financial Reports” } { “HR Documents” } { “Financial Reports” and “HR Documents” } “tag” “label”
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Tags + Labels Process p tag_t HR = create_tag(); S p = {} D p = {} D p = { HR } Universe of Tags: Finance Legal SecretProjects change_label({Finance}); S p = { Finance }S p = { Finance, HR } HR change_label({Finance,HR}); change_label({Finance}); change_label({}); DIFC: Declassification in action. Same as Step 1. Any process can add any tag to its label. DIFC Rule: A process can create a new tag; gets ability to declassify it.
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DIFC OS 12 KERNEL pq declassi fier Alice’s Data S q = {} S p = {a} S q = {a} S d = {a} D d = {a}
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Communication Rule Process qProcess p S q = { HR, Finance } S p = { HR } p can send to q iff S p S q
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Question: What interface should the kernel expose to applications? – Easier question than “is my OS implementation secure” – Easy to get it wrong! 14
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Approach 1: “Floating Labels” [IX,Asbestos] 15 KERNEL pq S q = {} S p = {a} S q = {a}
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Floaters Leaks Data 16 attacker Alice’s Data S = {a} b0b0 b1b1 S = {} 1001 Leak file S = {} 0000 S = {} 1001 S = {a} b2b2 S = {}S = {a} b3b3 S = {}
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Approach 2: “Set Your Own” [HiStar/Flume] 17 KERNEL pq S q = {} O q = {a + } S p = {a} O p = {a -, a + } S q = {a} O q = {a + } Rule: S p S q necessary precondition for send
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Case study: Flume Goal: User-level implementation – apt-get install flume Approach: – System Call Delegation [Ostia by Garfinkel et al, 2003] – Use Linux 2.6 (or OpenBSD 3.9)
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System Call Delegation Web App glibc Linux Kernel Layoff Plans open(“/hr/LayoffPlans”, O_RDONLY);
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System Call Delegation Web App Flume Libc Linux Kernel Layoff Plans open(“/hr/LayoffPlans”, O_RDONLY); Flume Reference Monitor Web App
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System Calls IPC: read, write, select Process management: fork, exit, getpid DIFC: create tag, change label, fetch label, send capabilities, receive capabilities 21
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How To Prove Security Property: Noninterference 22
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Noninterference [Goguen & Meseguer ’82] 23 p q S p = {a} S q = {} Experiment #1: p’ q S p’ = {a} S q = {} Experiment #2: = “HIGH”“LOW”
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How To Prove Security (cont.) Property: Noninterference Process Algebra model for a DIFC OS – Communicating Sequential Processes (CSP) Proof that the model fits the definition 24
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Proving noninterference Formalism (Communicating Sequential Formalisms) Consider any possible system (with arbitrary user applications) Induction over all possible sequences of moves a system can make (i.e., traces) At each step, case-by-case analysis of all system calls in the interface. – Prove no “interference” 25
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Results on Flume Allows adoption of Unix software? – 1,000 LOC launcher/declassifier – 1,000 out of 100,000 LOC in MoinMoin changed – Python interpreter, Apache, unchanged Solves security vulnerabilities? – Without our knowing, we inherited two ACL bypass bugs from MoinMoin – Both are not exploitable in Flume’s MoinMoin Performance? – Performs within a factor of 2 of the original on read and write benchmarks Example App: MoinMoin Wiki
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Flume Limitations Bigger TCB than HiStar / Asbestos – Linux stack (Kernel + glibc + linker) – Reference monitor (~22 kLOC) Covert channels via disk quotas Confined processes like MoinMoin don’t get full POSIX API. – spawn() instead of fork() & exec() – flume_pipe() instead of pipe()
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DIFC vs. Traditional MAC 28 Traditional MACDIFC Military SystemsWeb-based Systems, Desktops Superior Contraining Users Developers Protecting Themselves from Bugs Centralized Declassification Policy Application-Specific Declassification Policies
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What are we trusting? Proof of noninterference Platform – Hardware – Virtual machine manager – Kernel Reference monitor Declassifier – Human decisions Covert channels Physical access User authentication 29
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Practical barriers to deployment of Information Flow Control systems Programming model confuses programmers Lack of support: – Applications – Libraries – Operating systems People don’t care about security until it's too late 30
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Quantitative information flow control Quantify how much information (#bits) is leaking from (secret) inputs to outputs Approach: dynamic analysis (extended tainting) followed by flow graph analysis Example: battleship online game 31
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