1xen-vtpmmgr(7) Xen xen-vtpmmgr(7)
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6 xen-vtpmgr - Xen virtual TPM stubdomain
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9 Daniel De Graaf <dgdegra@tycho.nsa.gov>
10 Quan Xu <quan.xu@intel.com>
11 This document describes the operation and command line interface of
13 vtpmmgr-stubdom. See xen-vtpm(7) for details on the vTPM subsystem as a
14 whole.
15
17 The TPM Manager has three primary functions:
18 1. Securely store the encryption keys for vTPMs
20 2. Provide a single controlled path of access to the physical TPM
21 3. Provide evidence (via TPM Quotes) of the current configuration
22 When combined with a platform that provides a trusted method for
24 creating domains, the TPM Manager provides assurance that the private
25 keys in a vTPM are only available in specific trusted configurations.
26 The manager accepts commands from the vtpm-stubdom domains via the
28 mini-os TPM backend driver. The vTPM manager communicates directly with
29 hardware TPM using the mini-os tpm_tis driver.
30
32 The TPM Manager's data is secured by using the physical TPM's seal
33 operation, which allows data to be bound to specific PCRs. These PCRs
34 are populated in the physical TPM during the boot process, either by
35 the firmware/BIOS or by a dynamic launch environment such as TBOOT. In
36 order to provide assurance of the system's security, the PCRs used to
37 seal the TPM manager's data must contain measurements for domains used
38 to bootstrap the TPM Manager and vTPMs.
39 Because these measurements are based on hashes, they will change any
41 time that any component of the system is upgraded. Since it is not
42 possible to construct a list of all possible future good measurements,
43 the job of approving configurations is delegated to a third party,
44 referred to here as the system approval agent (SAA). The SAA is
45 identified by its public (RSA) signature key, which is used to sign
46 lists of valid configurations. A single TPM manager can support
47 multiple SAAs via the use of vTPM groups. Each group is associated with
48 a single SAA; this allows the creation of a multi-tenant environment
49 where tenants may not all choose to trust the same SAA.
50 Each vTPM is bound to a vTPM group at the time of its creation. Each
52 vTPM group has its own AIK in the physical TPM for quotes of the
53 hardware TPM state; when used with a conforming Privacy CA, this allows
54 each group on the system to form the basis of a distinct identity.
55
57 When the TPM Manager first boots up, it will create a stub vTPM group
58 along with entries for any vTPMs that communicate with it. This stub
59 group must be provisioned with an SAA and a boot configuration in order
60 to survive a reboot.
61 When a vTPM is connected to the TPM Manager using a UUID that is not
63 recognized, a slot will be created in group 0 for it. In the future,
64 this auto-creation may be restricted to specific UUIDs (such as the
65 all-zero UUID) to enforce the use of the TPM manager as the generator
66 of the UUID. The first vTPM to be connected is given administrative
67 privileges for the TPM Manager, and should be attached to dom0 or a
68 control domain in order to send provisioning commands.
69 Provisioning a vTPM group for the system requires the public key of the
71 SAA and privacy CA data used to certify the AIK (see the TPM spec for
72 details). Once the group is created, a signed list of boot measurements
73 can be installed. The initial group controls the ability to boot the
74 system as a whole, and cannot be deleted once provisioned.
75
77 Command line arguments are passed to the domain via the 'extra'
78 parameter in the VM config file. Each parameter is separated by white
79 space. For example:
80 extra="foo=bar baz"
82 Valid arguments:
84 owner_auth=<AUTHSPEC>
86 srk_auth=<AUTHSPEC>
87 Set the owner and SRK authdata for the TPM. If not specified, the
88 default is 160 zero bits (the well-known auth value). Valid values
89 of <AUTHSPEC> are:
90 well-known
92 Use the well known auth (default)
93 hash:<HASH>
95 Use the given 40-character ASCII hex string
96 text:<STR>
98 Use sha1 hash of <STR>.
99 tpmdriver=<DRIVER>
101 Choose the driver used for communication with the hardware TPM.
102 Values other than tpm_tis should only be used for testing.
103 The possible values of <DRIVER> are:
105 tpm_tis
107 Direct communication with a hardware TPM 1.2. The domain must
108 have access to TPM IO memory. (default)
109 tpmfront
111 Use the Xen tpmfront interface to talk to another domain which
112 provides access to the TPM.
113 The following options only apply to the tpm_tis driver:
115 tpmiomem=<ADDR>
117 The base address of the hardware memory pages of the TPM. The
118 default is 0xfed40000, as defined by the TCG's PC Client spec.
119 tpmirq=<IRQ>
121 The irq of the hardware TPM if using interrupts. A value of "probe"
122 can be set to probe for the irq. A value of 0 disables interrupts
123 and uses polling (default 0).
124 tpmlocality=<LOC>
126 Attempt to use locality <LOC> of the hardware TPM. For full
127 functionality of the TPM Manager, this should be set to "2".
128
130 While the TPM Manager has the ability to check the hash of the vTPM
131 requesting a key, there is currently no trusted method to inform the
132 TPM Manager of the hash of each new domain. Because of this, the TPM
133 Manager trusts the UUID key in Xenstore to identify a vTPM in a trusted
134 manner. The XSM policy may be used to strengthen this assumption if
135 the creation of vTPM-labeled domains is more constrained (for example,
136 only permitted to a domain builder service): the only grants mapped by
137 the TPM Manager should belong to vTPM domains, so restricting the
138 ability to map other domain's granted pages will prevent other domains
139 from directly requesting keys from the TPM Manager. The TPM Manager
140 uses the hash of the XSM label of the attached vTPM as the kernel hash,
141 so vTPMs with distinct labels may be further partitioned using vTPM
142 groups.
143 A domain with direct access to the hardware TPM will be able to decrypt
145 the TPM Manager's disk image if the haredware TPM's PCR values are in a
146 permitted configuration. To protect the TPM Manager's data, the list
147 of permitted configurations should be chosen to include PCRs that
148 measure the hypervisor, domain 0, the TPM Manager, and other critical
149 configuration such as the XSM policy. If the TPM Manager is configured
150 to use locality 2 as recommended, it is safe to permit the hardware
151 domain to access locality 0 (the default in Linux), although concurrent
152 use of the TPM should be avoided as it can result in unexpected busy
153 errors from the TPM driver. The ability to access locality 2 of the
154 TPM should be enforced using IO memory labeling in the XSM policy; the
155 physical address 0xFED42xxx is always locality 2 for TPMs using the TIS
156 driver.
157
159 There is no direct upgrade supported from previous versions of the
160 vtpmmgr domain due to changes in the on-disk format and the method used
161 to seal data. If a vTPM domain supports migration, this feature should
162 be used to migrate the vTPM's data; however, the vTPM packaged with Xen
163 does not yet support migration.
164 If adding migration support to the vTPM is not desired, a simpler
166 migration domain usable only for local migration can be constructed.
167 The migration process would look like the following:
168 1. Start the old vtpmmgr
170 2. Start the vTPM migration domain
171 3. Attach the vTPM migration domain's vtpm/0 device to the old vtpmmgr
172 4. Migration domain executes vtpmmgr_LoadHashKey on vtpm/0
173 5. Start the new vtpmmgr, possibly shutting down the old one first
174 6. Attach the vTPM migration domain's vtpm/1 device to the new vtpmmgr
175 7. Migration domain executes vtpmmgr_SaveHashKey on vtpm/1
176 This requires the migration domain to be added to the list of valid
178 vTPM kernel hashes. In the current version of the vtpmmgr domain, this
179 is the hash of the XSM label, not the kernel.
180
182 Manager disk image setup:
183 The vTPM Manager requires a disk image to store its encrypted data. The
184 image does not require a filesystem and can live anywhere on the host
185 disk. The image is not large; the Xen 4.5 vtpmmgr is limited to using
186 the first 2MB of the image but can support more than 20,000 vTPMs.
187 dd if=/dev/zero of=/home/vtpm2/vmgr bs=16M count=1
189 Manager config file:
191 The vTPM Manager domain (vtpmmgr-stubdom) must be started like any
192 other Xen virtual machine and requires a config file. The manager
193 requires a disk image for storage and permission to access the hardware
194 memory pages for the TPM. The disk must be presented as "hda", and the
195 TPM memory pages are passed using the iomem configuration parameter.
196 The TPM TIS uses 5 pages of IO memory (one per locality) that start at
197 physical address 0xfed40000. By default, the TPM manager uses locality
198 0 (so only the page at 0xfed40 is needed).
199 Add:
201 extra="tpm2=1"
203 extra option to launch vtpmmgr-stubdom domain on TPM 2.0, and ignore it
205 on TPM 1.x. for example:
206 kernel="/usr/lib/xen/boot/vtpmmgr-stubdom.gz"
208 memory=128
209 disk=["file:/home/vtpm2/vmgr,hda,w"]
210 name="vtpmmgr"
211 iomem=["fed40,5"]
212 extra="tpm2=1"
213 Key Hierarchy
215 +------------------+
216 | vTPM's secrets | ...
217 +------------------+
218 | ^
219 | |(Bind / Unbind)
220 - - - - - -v |- - - - - - - - TPM 2.0
221 +------------------+
222 | SK +
223 +------------------+
224 | ^
225 v |
226 +------------------+
227 | SRK |
228 +------------------+
229 | ^
230 v |
231 +------------------+
232 | TPM 2.0 Storage |
233 | Primary Seed |
234 +------------------+
235 Now the secrets for the vTPMs are only being bound to the presence of
237 thephysical TPM 2.0. Since using PCRs to seal the data can be an
238 important security feature that users of the vtpmmgr rely on. I will
239 replace TPM2_Bind/TPM2_Unbind with TPM2_Seal/TPM2_Unseal to provide as
240 much security as it did for TPM 1.2 in later series of patch.
241 Design Overview
243 The architecture of vTPM subsystem on TPM 2.0 is described below:
244 +------------------+
246 | Linux DomU | ...
247 | | ^ |
248 | v | |
249 | xen-tpmfront |
250 +------------------+
251 | ^
252 v |
253 +------------------+
254 | mini-os/tpmback |
255 | | ^ |
256 | v | |
257 | vtpm-stubdom | ...
258 | | ^ |
259 | v | |
260 | mini-os/tpmfront |
261 +------------------+
262 | ^
263 v |
264 +------------------+
265 | mini-os/tpmback |
266 | | ^ |
267 | v | |
268 | vtpmmgr-stubdom |
269 | | ^ |
270 | v | |
271 | mini-os/tpm2_tis |
272 +------------------+
273 | ^
274 v |
275 +------------------+
276 | Hardware TPM 2.0 |
277 +------------------+
278 Linux DomU
280 The Linux based guest that wants to use a vTPM. There many be more
281 than one of these.
282 xen-tpmfront.ko
284 Linux kernel virtual TPM frontend driver. This driver provides vTPM
285 access to a para-virtualized Linux based DomU.
286 mini-os/tpmback
288 Mini-os TPM backend driver. The Linux frontend driver connects to
289 this backend driver to facilitate communications between the Linux
290 DomU and its vTPM. This driver is also used by vtpmmgr-stubdom to
291 communicate with vtpm-stubdom.
292 vtpm-stubdom
294 A mini-os stub domain that implements a vTPM. There is a one to one
295 mapping between running vtpm-stubdom instances and logical vtpms on
296 the system. The vTPM Platform Configuration Registers (PCRs) are
297 all initialized to zero.
298 mini-os/tpmfront
300 Mini-os TPM frontend driver. The vTPM mini-os domain vtpm-stubdom
301 uses this driver to communicate with vtpmmgr-stubdom. This driver
302 could also be used separately to implement a mini-os domain that
303 wishes to use a vTPM of its own.
304 vtpmmgr-stubdom
306 A mini-os domain that implements the vTPM manager. There is only
307 one vTPM manager and it should be running during the entire
308 lifetime of the machine. This domain regulates access to the
309 physical TPM on the system and secures the persistent state of each
310 vTPM.
311 mini-os/tpm2_tis
313 Mini-os TPM version 2.0 TPM Interface Specification (TIS) driver.
314 This driver used by vtpmmgr-stubdom to talk directly to the
315 hardware TPM 2.0. Communication is facilitated by mapping hardware
316 memory pages into vtpmmgr-stubdom.
317 Hardware TPM 2.0
319 The physical TPM 2.0 that is soldered onto the motherboard.
320 Noted:
322 functionality for a virtual guest operating system (a DomU) is
323 still TPM 1.2.
3244.14.2 2021-05-04 xen-vtpmmgr(7)