specs: Describe the TPM support in QEMU
This patch adds a description of the current TPM support in QEMU to the specs. Several public specs are referenced via their landing page on the trustedcomputinggroup.org website. Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
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docs/specs/tpm.txt
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docs/specs/tpm.txt
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QEMU TPM Device
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===============
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= Guest-side Hardware Interface =
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The QEMU TPM emulation implements a TPM TIS hardware interface following the
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Trusted Computing Group's specification "TCG PC Client Specific TPM Interface
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Specification (TIS)", Specification Version 1.3, 21 March 2013. This
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specification, or a later version of it, can be accessed from the following
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URL:
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https://trustedcomputinggroup.org/pc-client-work-group-pc-client-specific-tpm-interface-specification-tis/
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The TIS interface makes a memory mapped IO region in the area 0xfed40000 -
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0xfed44fff available to the guest operating system.
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QEMU files related to TPM TIS interface:
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- hw/tpm/tpm_tis.c
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- hw/tpm/tpm_tis.h
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= ACPI Interface =
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The TPM device is defined with ACPI ID "PNP0C31". QEMU builds a SSDT and passes
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it into the guest through the fw_cfg device. The device description contains
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the base address of the TIS interface 0xfed40000 and the size of the MMIO area
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(0x5000). In case a TPM2 is used by QEMU, a TPM2 ACPI table is also provided.
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The device is described to be used in polling mode rather than interrupt mode
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primarily because no unused IRQ could be found.
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To support measurement logs to be written by the firmware, e.g. SeaBIOS, a TCPA
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table is implemented. This table provides a 64kb buffer where the firmware can
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write its log into. For TPM 2 only a more recent version of the TPM2 table
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provides support for measurements logs and a TCPA table does not need to be
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created.
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The TCPA and TPM2 ACPI tables follow the Trusted Computing Group specification
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"TCG ACPI Specification" Family "1.2" and "2.0", Level 00 Revision 00.37. This
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specification, or a later version of it, can be accessed from the following
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URL:
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https://trustedcomputinggroup.org/tcg-acpi-specification/
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QEMU files related to TPM ACPI tables:
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- hw/i386/acpi-build.c
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- include/hw/acpi/tpm.h
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= TPM backend devices =
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The TPM implementation is split into two parts, frontend and backend. The
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frontend part is the hardware interface, such as the TPM TIS interface
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described earlier, and the other part is the TPM backend interface. The backend
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interfaces implement the interaction with a TPM device, which may be a physical
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or an emulated device. The split between the front- and backend devices allows
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a frontend to be connected with any available backend. This enables the TIS
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interface to be used with the passthrough backend or the (future) swtpm backend.
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QEMU files related to TPM backends:
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- backends/tpm.c
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- include/sysemu/tpm_backend.h
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- include/sysemu/tpm_backend_int.h
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== The QEMU TPM passthrough device ==
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In case QEMU is run on Linux as the host operating system it is possible to
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make the hardware TPM device available to a single QEMU guest. In this case the
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user must make sure that no other program is using the device, e.g., /dev/tpm0,
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before trying to start QEMU with it.
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The passthrough driver uses the host's TPM device for sending TPM commands
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and receiving responses from. Besides that it accesses the TPM device's sysfs
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entry for support of command cancellation. Since none of the state of a
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hardware TPM can be migrated between hosts, virtual machine migration is
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disabled when the TPM passthrough driver is used.
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Since the host's TPM device will already be initialized by the host's firmware,
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certain commands, e.g. TPM_Startup(), sent by the virtual firmware for device
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initialization, will fail. In this case the firmware should not use the TPM.
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Sharing the device with the host is generally not a recommended usage scenario
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for a TPM device. The primary reason for this is that two operating systems can
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then access the device's single set of resources, such as platform configuration
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registers (PCRs). Applications or kernel security subsystems, such as the
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Linux Integrity Measurement Architecture (IMA), are not expecting to share PCRs.
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QEMU files related to the TPM passthrough device:
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- hw/tpm/tpm_passthrough.c
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- hw/tpm/tpm_util.c
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- hw/tpm/tpm_util.h
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Command line to start QEMU with the TPM passthrough device using the host's
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hardware TPM /dev/tpm0:
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qemu-system-x86_64 -display sdl -enable-kvm \
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-m 1024 -boot d -bios bios-256k.bin -boot menu=on \
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-tpmdev passthrough,id=tpm0,path=/dev/tpm0 \
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-device tpm-tis,tpmdev=tpm0 test.img
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The following commands should result in similar output inside the VM with a
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Linux kernel that either has the TPM TIS driver built-in or available as a
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module:
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#> dmesg | grep -i tpm
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[ 0.711310] tpm_tis 00:06: 1.2 TPM (device=id 0x1, rev-id 1)
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#> dmesg | grep TCPA
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[ 0.000000] ACPI: TCPA 0x0000000003FFD191C 000032 (v02 BOCHS \
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BXPCTCPA 0000001 BXPC 00000001)
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#> ls -l /dev/tpm*
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crw-------. 1 root root 10, 224 Jul 11 10:11 /dev/tpm0
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#> find /sys/devices/ | grep pcrs$ | xargs cat
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PCR-00: 35 4E 3B CE 23 9F 38 59 ...
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...
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PCR-23: 00 00 00 00 00 00 00 00 ...
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