QEMU TPM Device

Guest-side hardware interface

TIS 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. (see the TIS specification, or a later version of it).

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_common.c
  • hw/tpm/tpm_tis_isa.c
  • hw/tpm/tpm_tis_sysbus.c
  • hw/tpm/tpm_tis.h

Both an ISA device and a sysbus device are available. The former is used with pc/q35 machine while the latter can be instantiated in the Arm virt machine.

CRB interface

QEMU also implements a TPM CRB interface following the Trusted Computing Group’s specification “TCG PC Client Platform TPM Profile (PTP) Specification”, Family “2.0”, Level 00 Revision 01.03 v22, May 22, 2017. (see the CRB specification, or a later version of it)

The CRB interface makes a memory mapped IO region in the area 0xfed40000-0xfed40fff (1 locality) available to the guest operating system.

QEMU files related to TPM CRB interface:
  • hw/tpm/tpm_crb.c

SPAPR interface

pSeries (ppc64) machines offer a tpm-spapr device model.

QEMU files related to the SPAPR interface:
  • hw/tpm/tpm_spapr.c

fw_cfg interface

The bios/firmware may read the "etc/tpm/config" fw_cfg entry for configuring the guest appropriately.

The entry of 6 bytes has the following content, in little-endian:

#define TPM_VERSION_UNSPEC          0
#define TPM_VERSION_1_2             1
#define TPM_VERSION_2_0             2

#define TPM_PPI_VERSION_NONE        0
#define TPM_PPI_VERSION_1_30        1

struct FwCfgTPMConfig {
    uint32_t tpmppi_address;         /* PPI memory location */
    uint8_t tpm_version;             /* TPM version */
    uint8_t tpmppi_version;          /* PPI version */
};

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. (see the ACPI specification, or a later version of it)

ACPI PPI Interface

QEMU supports the Physical Presence Interface (PPI) for TPM 1.2 and TPM 2. This interface requires ACPI and firmware support. (see the PPI specification)

PPI enables a system administrator (root) to request a modification to the TPM upon reboot. The PPI specification defines the operation requests and the actions the firmware has to take. The system administrator passes the operation request number to the firmware through an ACPI interface which writes this number to a memory location that the firmware knows. Upon reboot, the firmware finds the number and sends commands to the TPM. The firmware writes the TPM result code and the operation request number to a memory location that ACPI can read from and pass the result on to the administrator.

The PPI specification defines a set of mandatory and optional operations for the firmware to implement. The ACPI interface also allows an administrator to list the supported operations. In QEMU the ACPI code is generated by QEMU, yet the firmware needs to implement support on a per-operations basis, and different firmwares may support a different subset. Therefore, QEMU introduces the virtual memory device for PPI where the firmware can indicate which operations it supports and ACPI can enable the ones that are supported and disable all others. This interface lies in main memory and has the following layout:

Field Length Offset Description
func 0x100 0x000 Firmware sets values for each supported operation. See defined values below.
ppin 0x1 0x100 SMI interrupt to use. Set by firmware. Not supported.
ppip 0x4 0x101 ACPI function index to pass to SMM code. Set by ACPI. Not supported.
pprp 0x4 0x105 Result of last executed operation. Set by firmware. See function index 5 for values.
pprq 0x4 0x109 Operation request number to execute. See ‘Physical Presence Interface Operation Summary’ tables in specs. Set by ACPI.
pprm 0x4 0x10d Operation request optional parameter. Values depend on operation. Set by ACPI.
lppr 0x4 0x111 Last executed operation request number. Copied from pprq field by firmware.
fret 0x4 0x115 Result code from SMM function. Not supported.
res1 0x40 0x119 Reserved for future use
next_step 0x1 0x159 Operation to execute after reboot by firmware. Used by firmware.
movv 0x1 0x15a Memory overwrite variable

The following values are supported for the func field. They correspond to the values used by ACPI function index 8.

Value Description
0 Operation is not implemented.
1 Operation is only accessible through firmware.
2 Operation is blocked for OS by firmware configuration.
3 Operation is allowed and physically present user required.
4 Operation is allowed and physically present user is not required.

The location of the table is given by the fw_cfg tpmppi_address field. The PPI memory region size is 0x400 (TPM_PPI_ADDR_SIZE) to leave enough room for future updates.

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 swtpm backend.

QEMU files related to TPM backends:
  • backends/tpm.c
  • include/sysemu/tpm.h
  • include/sysemu/tpm_backend.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:
  • backends/tpm/tpm_passthrough.c
  • backends/tpm/tpm_util.c
  • include/sysemu/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 -accel 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 ...

The QEMU TPM emulator device

The TPM emulator device uses an external TPM emulator called ‘swtpm’ for sending TPM commands to and receiving responses from. The swtpm program must have been started before trying to access it through the TPM emulator with QEMU.

The TPM emulator implements a command channel for transferring TPM commands and responses as well as a control channel over which control commands can be sent. (see the SWTPM protocol specification)

The control channel serves the purpose of resetting, initializing, and migrating the TPM state, among other things.

The swtpm program behaves like a hardware TPM and therefore needs to be initialized by the firmware running inside the QEMU virtual machine. One necessary step for initializing the device is to send the TPM_Startup command to it. SeaBIOS, for example, has been instrumented to initialize a TPM 1.2 or TPM 2 device using this command.

QEMU files related to the TPM emulator device:
  • backends/tpm/tpm_emulator.c
  • backends/tpm/tpm_util.c
  • include/sysemu/tpm_util.h

The following commands start the swtpm with a UnixIO control channel over a socket interface. They do not need to be run as root.

mkdir /tmp/mytpm1
swtpm socket --tpmstate dir=/tmp/mytpm1 \
  --ctrl type=unixio,path=/tmp/mytpm1/swtpm-sock \
  --log level=20

Command line to start QEMU with the TPM emulator device communicating with the swtpm (x86):

qemu-system-x86_64 -display sdl -accel kvm \
  -m 1024 -boot d -bios bios-256k.bin -boot menu=on \
  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
  -tpmdev emulator,id=tpm0,chardev=chrtpm \
  -device tpm-tis,tpmdev=tpm0 test.img

In case a pSeries machine is emulated, use the following command line:

qemu-system-ppc64 -display sdl -machine pseries,accel=kvm \
  -m 1024 -bios slof.bin -boot menu=on \
  -nodefaults -device VGA -device pci-ohci -device usb-kbd \
  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
  -tpmdev emulator,id=tpm0,chardev=chrtpm \
  -device tpm-spapr,tpmdev=tpm0 \
  -device spapr-vscsi,id=scsi0,reg=0x00002000 \
  -device virtio-blk-pci,scsi=off,bus=pci.0,addr=0x3,drive=drive-virtio-disk0,id=virtio-disk0 \
  -drive file=test.img,format=raw,if=none,id=drive-virtio-disk0

In case an Arm virt machine is emulated, use the following command line:

qemu-system-aarch64 -machine virt,gic-version=3,accel=kvm \
  -cpu host -m 4G \
  -nographic -no-acpi \
  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
  -tpmdev emulator,id=tpm0,chardev=chrtpm \
  -device tpm-tis-device,tpmdev=tpm0 \
  -device virtio-blk-pci,drive=drv0 \
  -drive format=qcow2,file=hda.qcow2,if=none,id=drv0 \
  -drive if=pflash,format=raw,file=flash0.img,readonly \
  -drive if=pflash,format=raw,file=flash1.img

In case SeaBIOS is used as firmware, it should show the TPM menu item after entering the menu with ‘ESC’.

Select boot device:
1. DVD/CD [ata1-0: QEMU DVD-ROM ATAPI-4 DVD/CD]
[...]
5. Legacy option rom

t. TPM Configuration

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 ...

Migration with the TPM emulator

The TPM emulator supports the following types of virtual machine migration:

  • VM save / restore (migration into a file)
  • Network migration
  • Snapshotting (migration into storage like QoW2 or QED)

The following command sequences can be used to test VM save / restore.

In a 1st terminal start an instance of a swtpm using the following command:

mkdir /tmp/mytpm1
swtpm socket --tpmstate dir=/tmp/mytpm1 \
  --ctrl type=unixio,path=/tmp/mytpm1/swtpm-sock \
  --log level=20 --tpm2

In a 2nd terminal start the VM:

qemu-system-x86_64 -display sdl -accel kvm \
  -m 1024 -boot d -bios bios-256k.bin -boot menu=on \
  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
  -tpmdev emulator,id=tpm0,chardev=chrtpm \
  -device tpm-tis,tpmdev=tpm0 \
  -monitor stdio \
  test.img

Verify that the attached TPM is working as expected using applications inside the VM.

To store the state of the VM use the following command in the QEMU monitor in the 2nd terminal:

(qemu) migrate "exec:cat > testvm.bin"
(qemu) quit

At this point a file called testvm.bin should exists and the swtpm and QEMU processes should have ended.

To test ‘VM restore’ you have to start the swtpm with the same parameters as before. If previously a TPM 2 [–tpm2] was saved, –tpm2 must now be passed again on the command line.

In the 1st terminal restart the swtpm with the same command line as before:

swtpm socket --tpmstate dir=/tmp/mytpm1 \
  --ctrl type=unixio,path=/tmp/mytpm1/swtpm-sock \
  --log level=20 --tpm2

In the 2nd terminal restore the state of the VM using the additional ‘-incoming’ option.

qemu-system-x86_64 -display sdl -accel kvm \
  -m 1024 -boot d -bios bios-256k.bin -boot menu=on \
  -chardev socket,id=chrtpm,path=/tmp/mytpm1/swtpm-sock \
  -tpmdev emulator,id=tpm0,chardev=chrtpm \
  -device tpm-tis,tpmdev=tpm0 \
  -incoming "exec:cat < testvm.bin" \
  test.img

Troubleshooting migration

There are several reasons why migration may fail. In case of problems, please ensure that the command lines adhere to the following rules and, if possible, that identical versions of QEMU and swtpm are used at all times.

VM save and restore:

  • QEMU command line parameters should be identical apart from the ‘-incoming’ option on VM restore
  • swtpm command line parameters should be identical

VM migration to ‘localhost’:

  • QEMU command line parameters should be identical apart from the ‘-incoming’ option on the destination side
  • swtpm command line parameters should point to two different directories on the source and destination swtpm (–tpmstate dir=…) (especially if different versions of libtpms were to be used on the same machine).

VM migration across the network:

  • QEMU command line parameters should be identical apart from the ‘-incoming’ option on the destination side
  • swtpm command line parameters should be identical
VM Snapshotting:
  • QEMU command line parameters should be identical
  • swtpm command line parameters should be identical

Besides that, migration failure reasons on the swtpm level may include the following:

  • the versions of the swtpm on the source and destination sides are incompatible
    • downgrading of TPM state may not be supported
    • the source and destination libtpms were compiled with different compile-time options and the destination side refuses to accept the state
  • different migration keys are used on the source and destination side and the destination side cannot decrypt the migrated state (swtpm … –migration-key … )