Software TPM in a Virtual Machine

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Software TPMin aVirtual Machine

• Jordan Jump

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Original Proposal

• Demonstrate a software-only TPM within a virtual
  machine
• Generally equivalent to real TPM, except doesnt
  contain correct certificate to link to CA.
• Guest OS within VM doesnt know not HW (excepting
  the certificate)

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Physical PC Diagram

• TPM typically located on Low Pin Count (LPC) bus
• Shares bus with other low rate peripherals

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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)

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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

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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

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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)

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The Goal

• Figure out how to link the TPM Emulator with
  unmodified Windows 7 running in a VM

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Virtual Machine Survey

• Xen
• KVM
• QEMU
• VMWare
• VirtualBox
• MS Virtual PC
• and many more
• What sets them apart??

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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 QEMUs device emulation.
• QEMU itself is slow because it doesnt
  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

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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

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VirtualBox

• Complicated
• Virtualizes using dynamic recompiler (from QEMU).
• Also disassembles and patches guest code so it
  doesnt have to redo recompilation
• Why do I care?

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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

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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

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Implementing in VirtualBox

• Struggled a lot!
• Few code comments
• Almost no documentation (some short, high-level
  docs that arent useful)
• Learned by reverse-engineering other drivers and
  search in files
• (seems simple now)

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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

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Other implementation details

• Windows wont 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,)
• )

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ImplementationScreenshots
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Opening tpm panel in Windows 7
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No SRTM
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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

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Thanks!

• jmjumps_at_iastate.edu

I like prerecorded presentations because I can
delete out most of my uhs and uhms
I dislike them because I cant gauge or engage
the class