Exploiting Instruction Streams To Prevent Intrusion Milena Milenkovic

Outline Introduction Related Work Trusted Instruction Execution Framework The Framework Potential Conclusion

Introduction Most of today’s computers connected to Internet, Even more in the future, including embedded devices One of the major security problems: the execution of the unauthorized code A lot of applications may be vulnerable Attack examples: –buffer overflow (heap, stack) –format string attack –…

Introduction Stack-smashing

Introduction One solution: allow only execution of trusted instructions Trusted instructions – with a valid signature Previous research: limited number of instruction streams (dynamic basic blocks) Use basic block signatures: run-time verification of the last basic block in an instruction stream Signatures calculated during trusted installation

Related Work Two categories: –Static source code analysis –Dynamic detection/prevention Static code analysis: false alarms Dynamic –Secure Program Execution Framework (SPEF) –Tag “spurious” data –Monitoring program “behavior” (system calls, performance monitoring registers) –Split stack for data/addresses, or secure stack –Compilers, compiler and library patches

Related Work Most related – SPEF: a given program binary has numerous representations with the same functionality During installation, each instruction block is transformed; verified in the run-time –compute transformation-invariant hash of the instruction block –encrypt the hash value with the secret key –use encrypted value to select a transformation (e.g., a particular instruction order in case of instruction scheduling) 7.5%-17.1% increased execution time, Mediabench Also related: tamper-proof techniques, but different granularity

Trusted Instruction Execution Requirements: –prevent the execution of unauthorized code –the security features should not significantly increase the program execution time This project: proposes a framework that satisfies requirements

Trusted Instruction Execution Atomic code unit protected by its signature: a basic block Verify all basic blocks? It is enough to verify the signature of the last basic block (LBB) in the instruction stream, when LBB generated a cache miss Signatures on hard disk: encrypted

Architecture for Trusted Computing BBST L1I L1D MMU Datapath FPUs IF Control BBST_M Code Heap Stack BBST – Basic Block Signature Table BBST_M – Basic Block Signature Table (Memory) BBSVU – Basic Block Signature Verification Unit BBSVU

Signature Generation MISR (Multiple input signature register) Linear feedback coefficients – based on the processor secret key

Phases of the Security Mechanism Compilation and program installation –Signature table (BBST_M) is generated, encrypted and appended to the program binary Program loading in the memory –BBST_M is decrypted, loaded in the memory Program execution –Signature of each last basic block with cache miss is verified –If no match, generate trap to OS – kill process

Program Execution

The Framework Potential 32-bit MISR I-cache: 4 ways, 128 sets, 64B line BBST: 4 ways, 4B line, 128/256 sets LRU replacement Traces of SPEC CPU2000 benchmarks for Alpha architecture –F2B, M2B segments –10 integer (CINT), 11 floating-point benchmarks (CFP)

The Framework Potential

Also measured; –the number of instruction cache misses –the number of BBST accesses The number of BBST misses – very small Trusted instruction overhead will not significantly hurt performance

Conclusion Proposed a framework for trusted instruction execution, evaluated potential Promises to be faster than SPEF, with additional hardware resources and BBST appended to program binary Future work: –asses different BBST organizations –preload BBST? –simulate execution time –evaluate an alternative implementation: signature embedded in the code