114 lines
4.3 KiB
Plaintext
114 lines
4.3 KiB
Plaintext
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Virtual TPM interface for Xen
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Authors: Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA)
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This document describes the virtual Trusted Platform Module (vTPM) subsystem for
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Xen. The reader is assumed to have familiarity with building and installing Xen,
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Linux, and a basic understanding of the TPM and vTPM concepts.
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INTRODUCTION
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The goal of this work is to provide a TPM functionality to a virtual guest
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operating system (in Xen terms, a DomU). This allows programs to interact with
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a TPM in a virtual system the same way they interact with a TPM on the physical
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system. Each guest gets its own unique, emulated, software TPM. However, each
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of the vTPM's secrets (Keys, NVRAM, etc) are managed by a vTPM Manager domain,
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which seals the secrets to the Physical TPM. If the process of creating each of
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these domains (manager, vTPM, and guest) is trusted, the vTPM subsystem extends
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the chain of trust rooted in the hardware TPM to virtual machines in Xen. Each
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major component of vTPM is implemented as a separate domain, providing secure
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separation guaranteed by the hypervisor. The vTPM domains are implemented in
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mini-os to reduce memory and processor overhead.
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This mini-os vTPM subsystem was built on top of the previous vTPM work done by
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IBM and Intel corporation.
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DESIGN OVERVIEW
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---------------
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The architecture of vTPM is described below:
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+------------------+
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| Linux DomU | ...
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| v | |
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| xen-tpmfront |
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+------------------+
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| ^
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v |
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+------------------+
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| mini-os/tpmback |
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| v | |
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| vtpm-stubdom | ...
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| v | |
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| mini-os/tpmfront |
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+------------------+
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| ^
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v |
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+------------------+
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| mini-os/tpmback |
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| | ^ |
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| v | |
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| vtpmmgr-stubdom |
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| v | |
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| mini-os/tpm_tis |
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+------------------+
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| ^
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v |
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+------------------+
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| Hardware TPM |
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+------------------+
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* Linux DomU: The Linux based guest that wants to use a vTPM. There may be
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more than one of these.
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* xen-tpmfront.ko: Linux kernel virtual TPM frontend driver. This driver
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provides vTPM access to a Linux-based DomU.
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* mini-os/tpmback: Mini-os TPM backend driver. The Linux frontend driver
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connects to this backend driver to facilitate communications
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between the Linux DomU and its vTPM. This driver is also
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used by vtpmmgr-stubdom to communicate with vtpm-stubdom.
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* vtpm-stubdom: A mini-os stub domain that implements a vTPM. There is a
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one to one mapping between running vtpm-stubdom instances and
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logical vtpms on the system. The vTPM Platform Configuration
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Registers (PCRs) are normally all initialized to zero.
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* mini-os/tpmfront: Mini-os TPM frontend driver. The vTPM mini-os domain
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vtpm-stubdom uses this driver to communicate with
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vtpmmgr-stubdom. This driver is also used in mini-os
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domains such as pv-grub that talk to the vTPM domain.
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* vtpmmgr-stubdom: A mini-os domain that implements the vTPM manager. There is
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only one vTPM manager and it should be running during the
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entire lifetime of the machine. This domain regulates
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access to the physical TPM on the system and secures the
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persistent state of each vTPM.
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* mini-os/tpm_tis: Mini-os TPM version 1.2 TPM Interface Specification (TIS)
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driver. This driver used by vtpmmgr-stubdom to talk directly to
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the hardware TPM. Communication is facilitated by mapping
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hardware memory pages into vtpmmgr-stubdom.
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* Hardware TPM: The physical TPM that is soldered onto the motherboard.
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INTEGRATION WITH XEN
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--------------------
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Support for the vTPM driver was added in Xen using the libxl toolstack in Xen
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4.3. See the Xen documentation (docs/misc/vtpm.txt) for details on setting up
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the vTPM and vTPM Manager stub domains. Once the stub domains are running, a
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vTPM device is set up in the same manner as a disk or network device in the
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domain's configuration file.
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In order to use features such as IMA that require a TPM to be loaded prior to
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the initrd, the xen-tpmfront driver must be compiled in to the kernel. If not
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using such features, the driver can be compiled as a module and will be loaded
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as usual.
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