Software TPM in a Virtual Machine

1. Software TPMin aVirtual Machine Jordan Jump Go to next slide to begin the presentation … Cpre681 - Semester Project - Jordan Jump

2. Original Proposal • Demonstrate a software-only TPM within a virtual machine • Generally equivalent to real TPM, except doesn’t contain correct certificate to link to CA. • Guest OS within VM doesn’t know not HW (excepting the certificate) Cpre681 - Semester Project - Jordan Jump

3. Physical PC Diagram • TPM typically located on Low Pin Count (LPC) bus • Shares bus with other low rate peripherals Cpre681 - Semester Project - Jordan Jump

4. Accessing a Physical TPM v1.1 • x86 has two address spaces • Typical memory addressing • I/O addressing • I/O addresses accessed via OUT and IN instructions • Used to access LPC bus • Commonly used addresses, but NOT standardized (the set 0x4E, 0x4F, 0x400, 0x401 is common) Cpre681 - Semester Project - Jordan Jump

5. Accessing a Physical TPM v1.2 • TCG PC Client Specific TPM Interface Specification (TIS) • Defines and mandates a Memory Mapped (MMIO) interface • Physical addresses 0xFED40000 – 0xFED44FFF • One device driver to rule them all • The reason why … Cpre681 - Semester Project - Jordan Jump

6. Windows TPM Architecture • Windows TPM architecture only supports 1.2 TPMs • Windows Vista and Windows 7 device drivers builtin • Sure would be nice to use that… Cpre681 - Semester Project - Jordan Jump

7. Software TPM • TPM Emulator by Mario Strasser • Open source • De facto standard for projects using SW TPMs • Implements 100% TPM commands • Accessible via TDDL • Runs as daemon (no device driver necessary) Cpre681 - Semester Project - Jordan Jump

8. The Goal • Figure out how to link the TPM Emulator with unmodified Windows 7 running in a VM Cpre681 - Semester Project - Jordan Jump

9. Virtual Machine Survey Xen KVM QEMU VMWare VirtualBox MS Virtual PC … and many more What sets them apart?? Cpre681 - Semester Project - Jordan Jump

10. Virtual Machine Survey • … Not much • VMWare and VirtualPC not Open Source, so not viable option • Xen, KVM, QEMU, VirtualBox all support Windows (some require VT-x) • In fact, all borrow from QEMU’s device emulation. • QEMU itself is slow because it doesn’t paravirtualize • side project to accelerate QEMU abandoned because VirtualBox does it better. • Chose VirtualBox • Rather arbitrary choice; Xen might have been OK • Xen and KVM were first and foremost for paravirtualization Cpre681 - Semester Project - Jordan Jump

11. VirtualBox • Developed by Innotek, purchased by Sun, purchased by Oracle • Provides Open Source Edition (OSE) • No USB support • No builtin remote desktop server • … otherwise the same Cpre681 - Semester Project - Jordan Jump

12. VirtualBox • Complicated • Virtualizes using dynamic recompiler (from QEMU). • Also disassembles and patches guest code so it doesn’t have to redo recompilation • Why do I care? Cpre681 - Semester Project - Jordan Jump

13. TPM to VM Interface • Dynamic recompiler enables the green box • Guest VM writes or reads to TPM memory mapped address • Recompiler replaces write/read with hook to my code • My code processes the write/read and, if a read, provides a value back • Missing piece was to emulate the TPM MMIO interface to act as gatekeeper between Windows VM and Software TPM Cpre681 - Semester Project - Jordan Jump

14. TPM MMIO Interface • TIS provides implementation details • Localities allow multiple accessors • Only 1 can access at a time • Locality 4 highest priority; cannot be accessed by software • Locality 0/Legacy for SRTM • Windows (BitLocker) only uses Locality 0 • Inputs/Outputs read 1 byte at a time Cpre681 - Semester Project - Jordan Jump

15. Implementing in VirtualBox • Struggled a lot! • Few code comments • Almost no documentation (some short, high-level docs that aren’t useful) • Learned by reverse-engineering other drivers and ‘search in files’ • (seems simple now) Cpre681 - Semester Project - Jordan Jump

16. Implementing in VirtualBox • On startup, register memory address range with VirtualBox and provide callbacks • Memory Write/Read by VM triggers write/read callback • My code handles TIS protocol items such as locking, signaling, buffering, etc. • Once a full command has been received, my code calls SW TPM via host TDDL and retrieves result • My code handles TIS protocol to send back response Cpre681 - Semester Project - Jordan Jump

17. Other implementation details • Windows won’t read at memory address unless registered in ACPI • Entered ACPI source language (ASL) listed in TCG PC Client Specific Implementation Specification For Conventional BIOS Device (TPM) { Name (_HID, EISAID(“PNP0C31”)) Name (_CRS, ResourceTemplate() { Memory32Fixed (ReadWrite, 0xFED40000, 0x5000,) }) } Cpre681 - Semester Project - Jordan Jump

18. ImplementationScreenshots Cpre681 - Semester Project - Jordan Jump

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20. Opening tpm panel in Windows 7 Cpre681 - Semester Project - Jordan Jump

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22. No SRTM Cpre681 - Semester Project - Jordan Jump

23. Future Work • Modify VirtualBox BIOS to support SRT • Support hashing function for locality 4 • Multiple SW TPM instances for multiple VMs • Start/Stop SW TPM instance when starting/stopping VM • More robust misbehavior checking • Modify SW TPM to allow locality to be specified via TDDL Cpre681 - Semester Project - Jordan Jump

24. Thanks! jmjumps@iastate.edu “I like prerecorded presentations because I can delete out most of my ‘uhs’ and ‘uhms’” “I dislike them because I can’t gauge or engage the class” Cpre681 - Semester Project - Jordan Jump