Enabling Trusted Software Integrity

Enabling Trusted Software Integrity Darko Kirovski Microsoft Research Milenko Drinić Miodrag Potkonjak Computer Science Department University of California, Los Angeles

Problem Description

Buffer Overrun • Goal • Explore improperly implemented I/O • Divert execution to attack code • Simplest variant – Stack smashing • “Smashing The Stack For Fun And Profit” by Aleph One (aleph1@underground.org), Phrack 49, 1996. • Numerous variants explore different vulnerabilities • Tutorials on the Web with bug descriptions • setuid() – Chen, Wagner, Dean, 2002.

What Can Be Done? • StackGuard – Cowan et al., 1998 • Dummy value next to return address • Bounds checking for all pointers – Jones, Kelly, 1995 • Slow in pointer-intensive software • Static analysis – Wagner, 2000 • Verify all buffers – promising idea • Too many false alarms • Need to be resolved manually

Intrusion Prevention • Current approaches • Intrusion detection • PREVENT rather than DETECT is easier • Intrusion prevention system • Adversary must solve a computationally difficult task to run programs in high priority • Two types of binaries • Ordinary • Touched with a security wand • Run-time verification

Outline • How the system works? • Software installation • Example of constraint embedding • Run-time verification • How to break the system? • Effect on performance

Software Installation • Installer is on-chip or on an EPROM with verified contents • Single process • I/O – memory mapped • Interrupts disabled • Used registers, memory overwritten • ~ BOOT on PCs GOAL: embed constraintsw/o revealing CPUID.

Example: Instruction Scheduling

How the Bitstream Reorders Ops?

Constraint Embedding Techniques • Entropy of program representation is high • Reduce entropy w/ constraints for 50+ bits with preserved performance • Exact entropy reduction unique for each CPUID • Constraint types • Requirements • High entropy • Functional transparency • Transformation invariance • Effective implementation • Low performance overhead • Examples • Instruction rescheduling • Register assignment • Basic block reordering • Conditional branch selection • Filling unused opcode fields • Toggling signs of operands

Run-time Code Verification • ARM instruction set and simulated system • 50 cycles • 20K gates • HW support? Cache line

How to Break the System? • Cryptographically secure keyed MAC • Hard to extract CPUID from working-copies • Hard to create an I-block with CPUID constraints satisfied w/o the CPUID • Patch low entropy instruction blocks • I-block with low entropy? Example: • I-block = one instruction and all other NOPS • Hardware must detect I-blocks with low entropy • Count and limit domain cardinality • Done during domain ordering • Patch I-blocks from working copies • Difficult? Hard to evaluate w/o a lot of software

Simulated w/ ARMulator • ARM instruction set • MediaBench suite Performance • Embedded bits of entropy • Performance effect • 13-25% overhead • 7-17% with a cache that logs TI-hashes

Summary • Intrusion prevention • On-line software verification for authenticity • Keyed message authentication code • Stored as footer • Stored as constraints • 50% decrease in code size overhead • Public and trusted execution mode • Relatively hi/lo performance overhead • No hardware acceleration • 20% - sets back Moore’s Law 4.5 months 

Enabling Trusted Software Integrity