linux-kernel - QEMU launch failure with TPM passthrough

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Message-ID: <c5776544-235b-47bd-bab7-d00d436db893@quicinc.com>
Date: Tue, 3 Dec 2024 13:28:01 +0530
From: Yuvaraj Ranganathan <quic_yrangana@...cinc.com>
To: <peterhuewe@....de>, <jarkko@...nel.org>, <jgg@...pe.ca>,
        <linux-integrity@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: QEMU launch failure with TPM passthrough

Hi Everyone,

We are trying to launch a VM’s using qemu with TPM passthrough
(https://www.qemu.org/docs/master/specs/tpm.html) and
we are encountering the QEMU launch is failing on below experiments,

Problem 1:
qemu-system-aarch64 \
    -M virt -m 2G \
 -tpmdev passthrough,id=tpm0,path=/dev/tpmrm0 -device tpm-tis,tpmdev=tpm0 \
    -kernel /mnt/overlay/Image \
    -drive
file=/mnt/overlay/qcom-guestvm-image-qcs9100-ride-sx-20241118050754.rootfs.ext4,if=virtio,format=raw
\
    -append "root=/dev/vda" \
    -cpu host --enable-kvm -smp 4 -nographic


qemu-system-aarch64: -tpmdev passthrough,id=tpm0,path=/dev/tpmrm0:
tpm_passthrough: Could not guess TPM cancel path


Problem 2: (experimented based on open source discussions)
qemu-system-aarch64 \
    -M virt -m 2G \
 -tpmdev
passthrough,id=tpm0,path=/dev/tpmrm0,cancel-path=/sys/class/tpm/tpm0/device/cancel
-device tpm-tis,tpmdev=tpm0 \
    -kernel /mnt/overlay/Image \
    -drive
file=/mnt/overlay/qcom-guestvm-image-qcs9100-ride-sx-20241118050754.rootfs.ext4,if=virtio,format=raw
\
    -append "root=/dev/vda" \
    -cpu host --enable-kvm -smp 4 -nographic

qemu-system-aarch64: -tpmdev
passthrough,id=tpm0,path=/dev/tpmrm0,cancel-path=/sys/class/tpm/tpm0/device/cancel:
tpm_passthrough: Could not open TPM cancel path: No such file or directory

On further checking, it is due to /sys/class/tpm/tpm0/device/cancel
attribute is not created by TPM driver.

Temporary Solution explored:

We made the change in TPM driver to use tpm1_dev_group irrespective of
TPM version.
tpm2_dev_group is creating only tpm_version_major attribute while
tpm1_dev_group is creating multiple attributes including cancel + tpm2
attributes.
With this change, we are able to launch VM’s successfully using QEMU
command.

TPM HW version: 2.0
QEMU emulator version: 6.2.0

This seems to be basic issue with QEMU + TPM 2.0 HW.
Could you please help us know are we missing any procedure in the QEMU
steps ?

If the procedure is good, I shall go ahead to post an upstream patch
with the fix. Need your comments here!!!

Thanks,
Yuvaraj.