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1Virtual TPM interface for Xen
2
3Authors: Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA)
4
5This document describes the virtual Trusted Platform Module (vTPM) subsystem for
6Xen. The reader is assumed to have familiarity with building and installing Xen,
7Linux, and a basic understanding of the TPM and vTPM concepts.
8
9INTRODUCTION
10
11The goal of this work is to provide a TPM functionality to a virtual guest
12operating system (in Xen terms, a DomU).  This allows programs to interact with
13a TPM in a virtual system the same way they interact with a TPM on the physical
14system.  Each guest gets its own unique, emulated, software TPM.  However, each
15of the vTPM's secrets (Keys, NVRAM, etc) are managed by a vTPM Manager domain,
16which seals the secrets to the Physical TPM.  If the process of creating each of
17these domains (manager, vTPM, and guest) is trusted, the vTPM subsystem extends
18the chain of trust rooted in the hardware TPM to virtual machines in Xen. Each
19major component of vTPM is implemented as a separate domain, providing secure
20separation guaranteed by the hypervisor. The vTPM domains are implemented in
21mini-os to reduce memory and processor overhead.
22
23This mini-os vTPM subsystem was built on top of the previous vTPM work done by
24IBM and Intel corporation.
25
26
27DESIGN OVERVIEW
28---------------
29
30The architecture of vTPM is described below:
31
32+------------------+
33|    Linux DomU    | ...
34|       |  ^       |
35|       v  |       |
36|   xen-tpmfront   |
37+------------------+
38        |  ^
39        v  |
40+------------------+
41| mini-os/tpmback  |
42|       |  ^       |
43|       v  |       |
44|  vtpm-stubdom    | ...
45|       |  ^       |
46|       v  |       |
47| mini-os/tpmfront |
48+------------------+
49        |  ^
50        v  |
51+------------------+
52| mini-os/tpmback  |
53|       |  ^       |
54|       v  |       |
55| vtpmmgr-stubdom  |
56|       |  ^       |
57|       v  |       |
58| mini-os/tpm_tis  |
59+------------------+
60        |  ^
61        v  |
62+------------------+
63|   Hardware TPM   |
64+------------------+
65
66 * Linux DomU: The Linux based guest that wants to use a vTPM. There may be
67	       more than one of these.
68
69 * xen-tpmfront.ko: Linux kernel virtual TPM frontend driver. This driver
70                    provides vTPM access to a Linux-based DomU.
71
72 * mini-os/tpmback: Mini-os TPM backend driver. The Linux frontend driver
73		    connects to this backend driver to facilitate communications
74		    between the Linux DomU and its vTPM. This driver is also
75		    used by vtpmmgr-stubdom to communicate with vtpm-stubdom.
76
77 * vtpm-stubdom: A mini-os stub domain that implements a vTPM. There is a
78		 one to one mapping between running vtpm-stubdom instances and
79                 logical vtpms on the system. The vTPM Platform Configuration
80                 Registers (PCRs) are normally all initialized to zero.
81
82 * mini-os/tpmfront: Mini-os TPM frontend driver. The vTPM mini-os domain
83		     vtpm-stubdom uses this driver to communicate with
84		     vtpmmgr-stubdom. This driver is also used in mini-os
85		     domains such as pv-grub that talk to the vTPM domain.
86
87 * vtpmmgr-stubdom: A mini-os domain that implements the vTPM manager. There is
88		    only one vTPM manager and it should be running during the
89		    entire lifetime of the machine.  This domain regulates
90		    access to the physical TPM on the system and secures the
91		    persistent state of each vTPM.
92
93 * mini-os/tpm_tis: Mini-os TPM version 1.2 TPM Interface Specification (TIS)
94                    driver. This driver used by vtpmmgr-stubdom to talk directly to
95                    the hardware TPM. Communication is facilitated by mapping
96                    hardware memory pages into vtpmmgr-stubdom.
97
98 * Hardware TPM: The physical TPM that is soldered onto the motherboard.
99
100
101INTEGRATION WITH XEN
102--------------------
103
104Support for the vTPM driver was added in Xen using the libxl toolstack in Xen
1054.3.  See the Xen documentation (docs/misc/vtpm.txt) for details on setting up
106the vTPM and vTPM Manager stub domains.  Once the stub domains are running, a
107vTPM device is set up in the same manner as a disk or network device in the
108domain's configuration file.
109
110In order to use features such as IMA that require a TPM to be loaded prior to
111the initrd, the xen-tpmfront driver must be compiled in to the kernel.  If not
112using such features, the driver can be compiled as a module and will be loaded
113as usual.
114