specs: Describe the TPM support in QEMU

QEMU TPM Device

===============

= Guest-side Hardware Interface =

The QEMU TPM emulation implements a TPM TIS hardware interface following the

Trusted Computing Group's specification "TCG PC Client Specific TPM Interface

Specification (TIS)", Specification Version 1.3, 21 March 2013. This

specification, or a later version of it, can be accessed from the following

URL:

https://trustedcomputinggroup.org/pc-client-work-group-pc-client-specific-tpm-interface-specification-tis/

The TIS interface makes a memory mapped IO region in the area 0xfed40000 -

0xfed44fff available to the guest operating system.

QEMU files related to TPM TIS interface:

 - hw/tpm/tpm_tis.c

 - hw/tpm/tpm_tis.h

= ACPI Interface =

The TPM device is defined with ACPI ID "PNP0C31". QEMU builds a SSDT and passes

it into the guest through the fw_cfg device. The device description contains

the base address of the TIS interface 0xfed40000 and the size of the MMIO area

(0x5000). In case a TPM2 is used by QEMU, a TPM2 ACPI table is also provided.

The device is described to be used in polling mode rather than interrupt mode

primarily because no unused IRQ could be found.

To support measurement logs to be written by the firmware, e.g. SeaBIOS, a TCPA

table is implemented. This table provides a 64kb buffer where the firmware can

write its log into. For TPM 2 only a more recent version of the TPM2 table

provides support for measurements logs and a TCPA table does not need to be

created.

The TCPA and TPM2 ACPI tables follow the Trusted Computing Group specification

"TCG ACPI Specification" Family "1.2" and "2.0", Level 00 Revision 00.37. This

specification, or a later version of it, can be accessed from the following

URL:

https://trustedcomputinggroup.org/tcg-acpi-specification/

QEMU files related to TPM ACPI tables:

 - hw/i386/acpi-build.c

 - include/hw/acpi/tpm.h

= TPM backend devices =

The TPM implementation is split into two parts, frontend and backend. The

frontend part is the hardware interface, such as the TPM TIS interface

described earlier, and the other part is the TPM backend interface. The backend

interfaces implement the interaction with a TPM device, which may be a physical

or an emulated device. The split between the front- and backend devices allows

a frontend to be connected with any available backend. This enables the TIS

interface to be used with the passthrough backend or the (future) swtpm backend.

QEMU files related to TPM backends:

 - backends/tpm.c

 - include/sysemu/tpm_backend.h

 - include/sysemu/tpm_backend_int.h

== The QEMU TPM passthrough device ==

In case QEMU is run on Linux as the host operating system it is possible to

make the hardware TPM device available to a single QEMU guest. In this case the

user must make sure that no other program is using the device, e.g., /dev/tpm0,

before trying to start QEMU with it.

The passthrough driver uses the host's TPM device for sending TPM commands

and receiving responses from. Besides that it accesses the TPM device's sysfs

entry for support of command cancellation. Since none of the state of a

hardware TPM can be migrated between hosts, virtual machine migration is

disabled when the TPM passthrough driver is used.

Since the host's TPM device will already be initialized by the host's firmware,

certain commands, e.g. TPM_Startup(), sent by the virtual firmware for device

initialization, will fail. In this case the firmware should not use the TPM.

Sharing the device with the host is generally not a recommended usage scenario

for a TPM device. The primary reason for this is that two operating systems can

then access the device's single set of resources, such as platform configuration

registers (PCRs). Applications or kernel security subsystems, such as the

Linux Integrity Measurement Architecture (IMA), are not expecting to share PCRs.

QEMU files related to the TPM passthrough device:

 - hw/tpm/tpm_passthrough.c

 - hw/tpm/tpm_util.c

 - hw/tpm/tpm_util.h

Command line to start QEMU with the TPM passthrough device using the host's

hardware TPM /dev/tpm0:

qemu-system-x86_64 -display sdl -enable-kvm \

  -m 1024 -boot d -bios bios-256k.bin -boot menu=on \

  -tpmdev passthrough,id=tpm0,path=/dev/tpm0 \

  -device tpm-tis,tpmdev=tpm0 test.img

The following commands should result in similar output inside the VM with a

Linux kernel that either has the TPM TIS driver built-in or available as a

module:

#> dmesg | grep -i tpm

[    0.711310] tpm_tis 00:06: 1.2 TPM (device=id 0x1, rev-id 1)

#> dmesg | grep TCPA

[    0.000000] ACPI: TCPA 0x0000000003FFD191C 000032 (v02 BOCHS  \

    BXPCTCPA 0000001 BXPC 00000001)

#> ls -l /dev/tpm*

crw-------. 1 root root 10, 224 Jul 11 10:11 /dev/tpm0

#> find /sys/devices/ | grep pcrs$ | xargs cat

PCR-00: 35 4E 3B CE 23 9F 38 59 ...

...

PCR-23: 00 00 00 00 00 00 00 00 ...