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Minimal TCB Code Execution

Minimal TCB Code Execution. Jonathan McCune, Bryan Parno , Adrian Perrig, Michael Reiter, and Arvind Seshadri Carnegie Mellon University. May 22, 2007. Trusted Computing Base (TCB). …. …. App 1. App. App 1. App. S. S. OS. OS. Shim. DMA Devices . DMA Devices . CPU, RAM

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Minimal TCB Code Execution

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  1. Minimal TCB Code Execution Jonathan McCune, Bryan Parno, Adrian Perrig, Michael Reiter, and Arvind Seshadri Carnegie Mellon University May 22, 2007

  2. Trusted Computing Base (TCB) … … App 1 App App 1 App S S OS OS Shim DMA Devices DMA Devices CPU, RAM TPM, Chipset CPU, RAM TPM, Chipset (Network, Disk, USB, etc.) (Network, Disk, USB, etc.)

  3. Contributions • Isolate security-sensitive code execution from all other code and devices • Attest to security-sensitive code and its arguments and nothing else • Convince a remote party that security-sensitive code was protected • Add < 250 LoC to the software TCB S Software TCB < 250 LoC Shim

  4. TPM Background • The Trusted Platform Module (TPM) is a dedicated security chip • It can provide an attestation to remote parties • Platform Configuration Registers (PCRs) summarize the computer’s software state • TPM provides a signature over PCR values • TPM spec v1.2 includes dynamic PCRs • Values can be reset without a reboot

  5. Late Launch Background • Supported by new commodity CPUs • SVM for AMD • TXT (formerly LaGrande) for Intel • Designed to launch a VMM without a reboot • Hardware-based protections ensure launch integrity • New CPU instruction (SKINIT/SENTER) accepts a memory region as input and atomically: • Resets dynamic PCRs • Disables interrupts • Extends a measurement of the region into PCR 17 • Begins executing at the start of the memory region

  6. Adversary Capabilities … App 1 App • Run arbitrary code with maximum privileges • Subvert any DMA-enabled device • E.g., network cards, USB devices, hard drives • Perform limited hardware attacks • E.g., power cycle the machine • Excludes physically monitoring/modifying CPU-to-RAM communication OS S Shim DMA Devices CPU, RAM TPM, Chipset (Network, Disk, USB, etc.)

  7. Architecture Overview • Core technique • Pause current execution environment • Execute security-sensitive code with hardware-enforced isolation • Resume previous execution • Extensions • Preserve state securely across invocations • Attest only to code execution and protection • Establish secure communication with remote parties

  8. App RAM OS Module S Shim SKINIT Reset Execution Flow App OS Outputs Inputs 0 0 0 Module Module S Shim TPM … PCRs: CPU K-1

  9. S Shim Attestation TPM PCRs: Inputs … Outputs K-1 TPM … PCRs: K-1

  10. App App 1 App 2 App 3 App 4 App 5 … S OS TPM PCRs: 0 0 0 Inputs What code are you running? S … Shim Outputs Inputs Outputs K-1 S ( ) Shim Sign , K-1 ( ) Sign , K-1 Attestation Versus

  11. Potential Applications • Server applications • Password authentication, SSL keys, Certificate Authority (CA), etc. • Verifiable distributed computing • SETI@Home, Folding@Home, distcc, etc. • Client-side applications • Secure password entry

  12. Ongoing Work • Extracting security-sensitive code from existing applications • Containing malicious or malfunctioning security-sensitive code • Coping with slow security-sensitive code • Creating a trusted path to the user

  13. Related Work • Secure coprocessors • Dyad [Yee 1994], IBM 4758 [JiSmiMi 2001] • System-wide attestation • Secure Boot[ArFaSm 1997],IMA [SaZhJaDo 2004],Enforcer [MaSmWiStBa 2004] • VMM-based isolation • BIND[ShPeDo2005],AppCores [SiPuHaHe 2006],Trustworthy Kiosks [GaCáBeSaDoZh 2006],Proxos [TaLiLi 2006]

  14. Conclusions • Explore how far an application’s TCB can be minimized • Isolate security-sensitive code execution • Provide fine-grained attestations • Allow application writers to focus on the security of their own code

  15. Thank you! parno@cmu.edu

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