tpmlsm

tpmlsm decides which programs may open the TPM on a Linux machine. You give it a list of binaries, it compiles that list in, and from then on the kernel refuses every other program that tries to open /dev/tpm0 or /dev/tpmrm0, also when it runs as root. Everything else on the machine works as before.

It's built on eBPF and BPF LSM. A binary is on the list by its path and the SHA-256 of its contents, so changing the file, or copying it somewhere else, takes it off the list.

Here tpm2_getrandom is on the list. Run as root, it can still read random bytes from the TPM, but cat can't open the TPM device:

$ sudo tpm2_getrandom --hex 8 -T device:/dev/tpmrm0
05fad09798a92bca
$ sudo cat /dev/tpm0
cat: /dev/tpm0: Operation not permitted

With -watch, tpmlsm stays in the foreground and logs every attempt to open the TPM, here the allowed tpm2_getrandom and the refused cat:

$ sudo ./tpmlsm -watch
2026/10/04 22:35:32 allow sha256=97e1fc0f22d92de63204eec74076a003d1ca820d1d6db3c8fde3227f1ca7f4fc /usr/bin/tpm2
2026/10/04 22:35:33 enforcing until reboot; pinned to /sys/fs/bpf/tpmlsm
2026/10/04 22:35:33 watching, Ctrl-C to stop (enforcement stays)
2026/10/04 22:35:34 ALLOW pid=1130 comm=tpm2_getrandom dev=252:65536
2026/10/04 22:35:34 DENY  pid=1132 comm=cat dev=10:224

tpmlsm was prototyped during eBPF and vTPM work for a customer, where the TPM protects the private keys used for mTLS. This repository is a reference implementation, published alongside the blog post A TPM bouncer in eBPF; it is not the code that runs there.

Running

Requirements

tpmlsm requires >= 5.18 kernel, for bpf_ima_file_hash. It has been tested on Ubuntu 24.04 with 6.8.0-146-generic.

The following kernel configuration is required.

You can use grep $OPTION /boot/config-$(uname -r) to validate whether an option is enabled.

BPF LSM also has to be in the active LSM list, which on Ubuntu it is not by default. Check with:

cat /sys/kernel/security/lsm

If bpf is missing, append it to that list and pass it as lsm= on the kernel command line. On Ubuntu:

echo 'GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT lsm=lockdown,capability,landlock,yama,apparmor,bpf ima_hash=sha256"' | sudo tee /etc/default/grub.d/99-tpmlsm.cfg
sudo update-grub && sudo reboot

tpmlsm refuses to start when BPF LSM is not active. IMA has to hash with SHA-256 (ima_hash=sha256, the default on Ubuntu); with any other algorithm every binary is denied.

Allowlist

Add every binary that may open the TPM to allowlist.txt as sha256sum prints it, the SHA-256 followed by the file's real path, and rebuild:

sha256sum "$(readlink -f /usr/bin/tpm2_getrandom)" >> allowlist.txt
make

tpmlsm refuses to start with an empty or malformed allowlist. Run sudo ./tpmlsm at every boot, before anything that uses the TPM, and add -watch to run it in the foreground and log every allowed and denied open. There's no command to remove it, so changing the list means building and shipping a new tpmlsm and rebooting. Root can still delete its pins; see Limitations.

Limitations

• Allowing a binary allows everything it can do. Allowing python3 allows every Python script, and on Ubuntu all tpm2_* commands are one tpm2 binary.
• Only the binary is checked, not the libraries it loads. Anyone who can start an allowed program can slip in code with LD_PRELOAD, and root also with /etc/ld.so.preload or a debugger. Prefer static binaries, such as Go built with CGO_ENABLED=0.
• Tested on ext4. On btrfs the file lookup never matches, so every TPM open is denied.
• Updating an allowed binary locks it out until you ship a new tpmlsm and reboot.
• Programs already running when tpmlsm loads are denied until they restart.
• Root can still switch tpmlsm off by deleting its pins, loading a kernel module or booting another kernel. Secure Boot and kernel lockdown (both modes work) make that harder. More hooks that refuse to delete the pins, unmount bpffs or detach the programs would close the rest while the machine runs, leaving a reboot as the only way. Attestation can then show a verifier whether tpmlsm loaded again after the reboot.

Developing

Dependencies

• Go >= 1.26
• LLVM/clang
• libbpf headers (libbpf-dev)
• bpftool (linux-tools)
• make

On Ubuntu:

sudo apt-get install -y clang llvm libbpf-dev golang-go linux-tools-$(uname -r) linux-tools-common make

Building

make

The generated BPF objects in cmd/tpmlsm are checked in, so make only needs Go. It builds for the machine you're on; from macOS, make GOOS=linux builds the Linux binary. To regenerate the objects after changing bpf/tpmlsm.c, on Linux:

make generate

License

The Go code is licensed under the BSD 3-Clause License. The BPF code in bpf/ is dual-licensed under the GPL-2.0-only and the BSD 3-Clause License; you can use the terms of either license, at your option.

Logo Credits

The bouncer gopher is based on the Go gopher designed by Renee French.