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e2683957 DDG |
1 | Virtual TPM interface for Xen |
2 | ||
3 | Authors: Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA) | |
4 | ||
5 | This document describes the virtual Trusted Platform Module (vTPM) subsystem for | |
6 | Xen. The reader is assumed to have familiarity with building and installing Xen, | |
7 | Linux, and a basic understanding of the TPM and vTPM concepts. | |
8 | ||
9 | INTRODUCTION | |
10 | ||
11 | The goal of this work is to provide a TPM functionality to a virtual guest | |
12 | operating system (in Xen terms, a DomU). This allows programs to interact with | |
13 | a TPM in a virtual system the same way they interact with a TPM on the physical | |
14 | system. Each guest gets its own unique, emulated, software TPM. However, each | |
15 | of the vTPM's secrets (Keys, NVRAM, etc) are managed by a vTPM Manager domain, | |
16 | which seals the secrets to the Physical TPM. If the process of creating each of | |
17 | these domains (manager, vTPM, and guest) is trusted, the vTPM subsystem extends | |
18 | the chain of trust rooted in the hardware TPM to virtual machines in Xen. Each | |
19 | major component of vTPM is implemented as a separate domain, providing secure | |
20 | separation guaranteed by the hypervisor. The vTPM domains are implemented in | |
21 | mini-os to reduce memory and processor overhead. | |
22 | ||
23 | This mini-os vTPM subsystem was built on top of the previous vTPM work done by | |
24 | IBM and Intel corporation. | |
25 | ||
26 | ||
27 | DESIGN OVERVIEW | |
28 | --------------- | |
29 | ||
30 | The 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 | ||
101 | INTEGRATION WITH XEN | |
102 | -------------------- | |
103 | ||
104 | Support for the vTPM driver was added in Xen using the libxl toolstack in Xen | |
105 | 4.3. See the Xen documentation (docs/misc/vtpm.txt) for details on setting up | |
106 | the vTPM and vTPM Manager stub domains. Once the stub domains are running, a | |
107 | vTPM device is set up in the same manner as a disk or network device in the | |
108 | domain's configuration file. | |
109 | ||
110 | In order to use features such as IMA that require a TPM to be loaded prior to | |
111 | the initrd, the xen-tpmfront driver must be compiled in to the kernel. If not | |
112 | using such features, the driver can be compiled as a module and will be loaded | |
113 | as usual. |