Jefferson’s Polygraph Polygraphing Processes: N‑Variant Systems for Secretless Security David Evans UVa/CMU Genesis Project DARPA SRS PIs Meeting 12 July 2005 Jefferson's Polygraph (Copying Machine), courtesy the University of Virginia. Image credit: Thomas Jefferson Foundation/Edward Owen. http://www.monticello.org/gallery/innovations/polygraph.html Hoover’s Polygraph

Motivating Observation Previous diversity approaches (including ours) rely on keeping secrets Keeping secrets is hard [Shacham, et al., CCS 2004] [Sovarel, et al., USENIX Security 2005] Can we use diversity effectively without needing any secrets? DARPA SRS Genesis Project

DARPA SRS Genesis Project N-Variant Systems Construct a system that requires attacker to “simultaneously” compromise multiple variants Variations designed to make this impossible for certain attack classes Provides security without needing secrets Framework for proving resistance to classes of attack DARPA SRS Genesis Project

N-Version N-Variant Programming System [Avizienis & Chen, 1977] Multiple teams of programmers implement same spec Voter compares results and selects most common No guarantees: teams may make same mistake Transformer automatically produces diverse variants Monitor compares results and detects attack Guarantees: variants behave differently on particular input classes DARPA SRS Genesis Project

DARPA SRS Genesis Project 2-Variant System Input (Possibly Malicious) Server Variant 1 Monitor Output Polygrapher DARPA SRS Genesis Project

DARPA SRS Genesis Project N-Variant Framework Variant 1 Monitor Poly- grapher Polygrapher Replicate “same” input to all variants Monitor Delay effects until all variants finish successfully Detect failure of one variant: “Crash”: other variants may have been compromised Need to recover to known valid states Set of Variants Must be disjoint with respect to attack requirement An attack input that succeeds against one variant, must cause some other variant to fail detectably DARPA SRS Genesis Project

Establishing Disjoint Variants Normal Equivalence Property Under normal inputs, the variants stay in equivalent states: A0(S0)  A1(S1) Detection Property Any attack that compromises one variant causes another variant to exhibit detection behavior (e.g., crash) DARPA SRS Genesis Project

Example: Memory Partitioning Variation Variant 0: addresses all start with 0 Variant 1: addresses all start with 1 Normal Equivalence Map addresses to same address space Broken if code depends on absolute addresses Detection Property Any absolute load/store is invalid on one of the variants DARPA SRS Genesis Project

Instruction Set Partitioning JMP JMP CALL CALL JO JO JNO JNO JB JB JNB JNB JZ JZ JNZ JNZ … Variant A Variant B DARPA SRS Genesis Project

Instruction Set Tagging Variation: add an extra bit to all opcodes Variation 0: tag bit is a 0 Variation 1: tag bit is a 1 At run-time check and remove tag using Strata Normal Equivalence: Remove the tag bits Detection Property Any (tagged) opcode is invalid on one variant Injected code (identical on both) cannot run on both DARPA SRS Genesis Project

DARPA SRS Genesis Project Composing Variations Must preserve normal equivalence property Detect memory attack Detect direct code injection 1 Memory Space 1 1 Instruction Tags P1 P2 P3 DARPA SRS Genesis Project

DARPA SRS Genesis Project Implementations Two prototypes: Linux Kernel Modification Divert Sockets Ad hoc establishment of normal equivalence Transformation used to create variants Run-time checking for equivalent behavior at security-critical events DARPA SRS Genesis Project

Kernel Implementation Modify process table to record variants Create new fork routine to launch variants Intercept system calls: Check parameters match for all variants Make call once Send same result to all Low overhead, lack of isolation DARPA SRS Genesis Project

Divert Sockets Implementation Process intercepts traffic (nvpd) Uses divert sockets to send copies to isolated variants (can be on different machines) Waits until all variants respond to request before returning to client Adjusts TCP sequence numbers to each variant appears to have normal connection DARPA SRS Genesis Project

Divert Sockets 3-Variant System P1 Polygrapher Input from Client P2 Output to Client Monitor P3 nvpd Server DARPA SRS Genesis Project

DARPA SRS Genesis Project Results Implemented 3-Variant system Address space partitioning Instruction set tagging Thwarts any attack that: Depends on referencing an absolute address Depends on executing directly injected code Latency Overhead (apache) http https 4 machines 54x (10.8 ms) 2.1x (4778 ms) 1 machine 89x (17.8 ms) 2.3x (5271 ms) DARPA SRS Genesis Project

DARPA SRS Genesis Project Open Problems Non-determinism, persistent state Formally establishing normal equivalence Statically + dynamically Variations to prevent larger classes of attacks File naming, scheduling, protocol, configuration, etc. Limited by need to preserve (unspecified) application semantics DARPA SRS Genesis Project

N-Variant Systems Summary Use artificial diversity in a controlled way Framework requires attacker to compromise multiple variants “simultaneously” Create variations that make this impossible (for important attack classes) Opens promise of system security proofs that do not require any assumptions about keeping secrets DARPA SRS Genesis Project

DARPA SRS Genesis Project Credits Ben Cox Jack Davidson David Evans Adrian Filipi Jason Hiser Wei Hu John Knight Anh Nguyen‑Tuong Jonathan Rowanhill DARPA SRS Genesis Project