1xen-vtpm(7) Xen xen-vtpm(7)
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6 xen-vtpm - Xen virtual Trusted Platform Module (vTPM) subsystem
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9 Copyright (c) 2010-2012 United States Government, as represented by the
10 Secretary of Defense. All rights reserved. November 12 2012 Authors:
11 Matthew Fioravante (JHUAPL), Daniel De Graaf (NSA)
12 This document describes the virtual Trusted Platform Module (vTPM)
14 subsystem for Xen. The reader is assumed to have familiarity with
15 building and installing Xen, Linux, and a basic understanding of the
16 TPM and vTPM concepts.
17
19 The goal of this work is to provide a TPM functionality to a virtual
20 guest operating system (a DomU). This allows programs to interact with
21 a TPM in a virtual system the same way they interact with a TPM on the
22 physical system. Each guest gets its own unique, emulated, software
23 TPM. However, each of the vTPM's secrets (Keys, NVRAM, etc) are
24 managed by a vTPM Manager domain, which seals the secrets to the
25 Physical TPM. If the process of creating each of these domains
26 (manager, vTPM, and guest) is trusted, the vTPM subsystem extends the
27 chain of trust rooted in the hardware TPM to virtual machines in Xen.
28 Each major component of vTPM is implemented as a separate domain,
29 providing secure separation guaranteed by the hypervisor. The vTPM
30 domains are implemented in mini-os to reduce memory and processor
31 overhead.
32 This mini-os vTPM subsystem was built on top of the previous vTPM work
34 done by IBM and Intel corporation.
35
37 The architecture of vTPM is described below:
38 +------------------+
40 | Linux DomU | ...
41 | | ^ |
42 | v | |
43 | xen-tpmfront |
44 +------------------+
45 | ^
46 v |
47 +------------------+
48 | mini-os/tpmback |
49 | | ^ |
50 | v | |
51 | vtpm-stubdom | ...
52 | | ^ |
53 | v | |
54 | mini-os/tpmfront |
55 +------------------+
56 | ^
57 v |
58 +------------------+
59 | mini-os/tpmback |
60 | | ^ |
61 | v | |
62 | vtpmmgr-stubdom |
63 | | ^ |
64 | v | |
65 | mini-os/tpm_tis |
66 +------------------+
67 | ^
68 v |
69 +------------------+
70 | Hardware TPM |
71 +------------------+
72 Linux DomU
74 The Linux based guest that wants to use a vTPM. There many be more
75 than one of these.
76 xen-tpmfront.ko
78 Linux kernel virtual TPM frontend driver. This driver provides vTPM
79 access to a para-virtualized Linux based DomU.
80 mini-os/tpmback
82 Mini-os TPM backend driver. The Linux frontend driver connects to
83 this backend driver to facilitate communications between the Linux
84 DomU and its vTPM. This driver is also used by vtpmmgr-stubdom to
85 communicate with vtpm-stubdom.
86 vtpm-stubdom
88 A mini-os stub domain that implements a vTPM. There is a one to one
89 mapping between running vtpm-stubdom instances and logical vtpms on
90 the system. The vTPM Platform Configuration Registers (PCRs) are
91 all initialized to zero.
92 mini-os/tpmfront
94 Mini-os TPM frontend driver. The vTPM mini-os domain vtpm-stubdom
95 uses this driver to communicate with vtpmmgr-stubdom. This driver
96 could also be used separately to implement a mini-os domain that
97 wishes to use a vTPM of its own.
98 vtpmmgr-stubdom
100 A mini-os domain that implements the vTPM manager. There is only
101 one vTPM manager and it should be running during the entire
102 lifetime of the machine. This domain regulates access to the
103 physical TPM on the system and secures the persistent state of each
104 vTPM.
105 mini-os/tpm_tis
107 Mini-os TPM version 1.2 TPM Interface Specification (TIS) driver.
108 This driver used by vtpmmgr-stubdom to talk directly to the
109 hardware TPM. Communication is facilitated by mapping hardware
110 memory pages into vtpmmgr-stubdom.
111 Hardware TPM
113 The physical TPM that is soldered onto the motherboard.
114
116 Prerequisites:
117 You must have an x86 machine with a TPM on the motherboard. The only
118 extra software requirement for compiling vTPM is cmake. You must use
119 libxl to manage domains with vTPMs; 'xm' is deprecated and does not
120 support vTPMs.
121 Compiling the Xen tree:
123 Compile and install the Xen tree as usual; be sure that the vTPM
124 domains are enabled when you run configure.
125 Compiling the LINUX dom0 kernel:
127 Because the TPM manager uses direct access to the physical TPM, it may
128 interfere with access to the TPM by dom0. The simplest solution for
129 this is to prevent dom0 from accessing the physical TPM by compiling
130 the kernel without a driver or blacklisting the module. If dom0 needs
131 a TPM but does not need to use it during the boot process (i.e. it is
132 not using IMA), a virtual TPM can be attached to dom0 after the system
133 is booted.
134 Access to the physical TPM may be required in order to manage the NVRAM
136 or to perform other advanced operations where the vTPM is insufficient.
137 In order to prevent interference, the TPM Manager and dom0 should use
138 different values for the TPM's locality; since Linux always uses
139 locality 0, using locality 2 for the TPM Manager is recommended. If
140 both Linux and the TPM Manager attempt to access the TPM at the same
141 time, the TPM device will return a busy status; some applications will
142 consider this a fatal error instead of retrying the command at a later
143 time. If a vTPM gets an error when loading its key, it will currently
144 generate a fresh vTPM image (with a new EK, SRK, and blank NVRAM).
145 Compiling the LINUX domU kernel:
147 The domU kernel used by domains with vtpms must include the
148 xen-tpmfront.ko driver. It can be built directly into the kernel or as
149 a module; however, some features such as IMA require the TPM to be
150 built in to the kernel.
151 CONFIG_TCG_TPM=y
153 CONFIG_TCG_XEN=y
154
156 Manager disk image setup:
157 The vTPM Manager requires a disk image to store its encrypted data. The
158 image does not require a filesystem and can live anywhere on the host
159 disk. The image is not large; the Xen 4.5 vtpmmgr is limited to using
160 the first 2MB of the image but can support more than 20,000 vTPMs.
161 Manager config file:
163 The vTPM Manager domain (vtpmmgr-stubdom) must be started like any
164 other Xen virtual machine and requires a config file. The manager
165 requires a disk image for storage and permission to access the hardware
166 memory pages for the TPM. The disk must be presented as "hda", and the
167 TPM memory pages are passed using the iomem configuration parameter.
168 The TPM TIS uses 5 pages of IO memory (one per locality) that start at
169 physical address 0xfed40000. By default, the TPM manager uses locality
170 0 (so only the page at 0xfed40 is needed); this can be changed on the
171 domain's command line. For full functionality in deep quotes, using
172 locality 2 is required to manipulate PCR 20-22.
173 Starting and stopping the manager:
175 The vTPM manager should be started at boot; you may wish to create an
176 init script to do this. If a domain builder is used, the TPM Manager
177 should be started by the domain builder to minimize the trusted
178 computing base for the vTPM manager's secrets.
179 Once initialization is complete you should see the following:
181 INFO[VTPM]: Waiting for commands from vTPM's:
183 The TPM Manager does not respond to shutdown requests; use the destroy
185 command to shut it down.
186
188 vTPM disk image setup:
189 The vTPM requires a disk image to store its persistent data (RSA keys,
190 NVRAM, etc). The image does not require a filesystem. The image does
191 not need to be large; 2 Mb should be sufficient.
192 vTPM config file:
194 The vTPM domain requires a configuration file like any other domain.
195 The vTPM requires a disk image for storage and a TPM frontend driver to
196 communicate with the manager. You are required to generate a uuid for
197 this vtpm, which is specified on the "vtpm=" line that describes its
198 connection to the vTPM Manager. The uuidgen application may be used to
199 generate a uuid, or one from the output of the "manage-vtpmmgr.pl
200 vtpm-add" command may be used to create a vTPM belonging to a specific
201 group.
202 If you wish to clear the vTPM data you can either recreate the disk
204 image or change the uuid.
205 Linux Guest config file:
207 The Linux guest config file needs to be modified to include the Linux
208 tpmfront driver. Add the following line:
209 vtpm=["backend=domu-vtpm"]
211 Currently only Linux guests are supported (PV or HVM with PV drivers).
213 While attaching a vTPM after a guest is booted (using xl vtpm-attach)
215 is supported, the attached vTPM will not have a record of the boot of
216 the attached guest. Furthermore, if the vTPM has been freshly created,
217 a malicious guest could then extend any values into PCRs, potentially
218 forging its boot configuration. Attaching a vTPM to a running domain
219 should only be used for trusted domains or when measurements have
220 already been sent to the vTPM from another source.
221 Using the vTPM in the guest:
223 If xen-tpmfront was compiled as a module, it must be loaded it in the
224 guest.
225 # modprobe xen-tpmfront
227 After the Linux domain boots and the xen-tpmfront driver is loaded, you
229 should see the following on the vtpm console:
230 Info: VTPM attached to Frontend X/Y
232 You can quickly test the vTPM by using the sysfs interface:
234 # cat /sys/devices/vtpm-0/pubek
236 # cat /sys/devices/vtpm-0/pcrs
237 If you have trousers and tpm_tools installed on the guest, the
239 tpm_version command should return the following:
240 The version command should return the following:
242 TPM 1.2 Version Info:
244 Chip Version: 1.2.0.7
245 Spec Level: 2
246 Errata Revision: 1
247 TPM Vendor ID: ETHZ
248 TPM Version: 01010000
249 Manufacturer Info: 4554485a
250 You should also see the command being sent to the vtpm console as well
252 as the vtpm saving its state. You should see the vtpm key being
253 encrypted and stored on the vtpmmgr console.
254 You may wish to write a script to start your vtpm and guest together
256 and to destroy the vtpm when the guest shuts down.
257
259 The vTPM currently starts up with all PCRs set to their default values
260 (all zeros for the lower 16). This means that any decisions about the
261 trustworthiness of the created domain must be made based on the
262 environment that created the vTPM and the domU; for example, a system
263 that only constructs images using a trusted configuration and guest
264 kernel be able to provide guarantees about the guests and any
265 measurements done that kernel (such as the IMA TCB log). Guests
266 wishing to use a custom kernel in such a secure environment are often
267 started using the pv-grub bootloader as the kernel, which then can load
268 the untrusted kernel without needing to parse an untrusted filesystem
269 and kernel in dom0. If the pv-grub stub domain succeeds in connecting
270 to a vTPM, it will extend the hash of the kernel that it boots into PCR
271 #4, and will extend the command line and initrd into PCR #5 before
272 booting so that a domU booted in this way can attest to its early boot
273 state.
274
276 See <xen-vtpmmgr(7)> for more details about how the manager domain
277 works, how to use it, and its command line parameters.
278
280 The vtpm-stubdom is a mini-OS domain that emulates a TPM for the guest
281 OS to use. It is a small wrapper around the Berlios TPM emulator
282 version 0.7.4. Commands are passed from the linux guest via the mini-
283 os TPM backend driver. vTPM data is encrypted and stored via a disk
284 image provided to the virtual machine. The key used to encrypt the data
285 along with a hash of the vTPM's data is sent to the vTPM manager for
286 secure storage and later retrieval. The vTPM domain communicates with
287 the manager using a mini-os tpm front/back device pair.
288
290 Command line arguments are passed to the domain via the 'extra'
291 parameter in the VM config file. Each parameter is separated by white
292 space. For example:
293 extra="foo=bar baz"
295 List of Arguments:
297 loglevel=<LOG>
298 Controls the amount of logging printed to the console. The
299 possible values for <LOG> are:
300 · error
302 · info (default)
304 · debug
306 clear
308 Start the Berlios emulator in "clear" mode. (default)
309 save
311 Start the Berlios emulator in "save" mode.
312 deactivated
314 Start the Berlios emulator in "deactivated" mode. See the Berlios
315 TPM emulator documentation for details about the startup mode. For
316 all normal use, always use clear which is the default. You should
317 not need to specify any of these.
318 maintcmds=<1|0>
320 Enable to disable the TPM maintenance commands. These commands are
321 used by tpm manufacturers and thus open a security hole. They are
322 disabled by default.
323 hwinitpcr=<PCRSPEC>
325 Initialize the virtual Platform Configuration Registers (PCRs) with
326 PCR values from the hardware TPM. Each pcr specified by <PCRSPEC>
327 will be initialized with the value of that same PCR in TPM once at
328 startup. By default all PCRs are zero initialized. Possible values
329 of <PCRSPEC> are:
330 · all: copy all pcrs
332 · none: copy no pcrs (default)
334 · <N>: copy pcr n
336 · <X-Y>: copy pcrs x to y (inclusive)
338 These can also be combined by comma separation, for example:
340 "hwinitpcrs=5,12-16" will copy pcrs 5, 12, 13, 14, 15, and 16.
341
343 Berlios TPM Emulator: <https://github.com/PeterHuewe/tpm-emulator>
3444.14.1 2021-03-18 xen-vtpm(7)