I am trying to create a tpm2-based auto unlock sh script, but the script fails with file not found - sh

Question

I am trying to create a TPM-based unlock script using tpm2-tools with instructions from Tevora Secure boot tpm2. I have set up the key, loaded it with cryptsetup luksAddKey secret.bin, then tested it using tpm2_unlock -c 0x81000000 --auth pci:sha1:0,2,3,7 and returns the value of secret.bin. For extra measures, to make sure it works, I loaded secret.bin into "/etc/crypttab", ran # update-initramfs -u -k all, and rebooted. Upon reboot, the system unlocked.
I copied over the following code into "/etc/initramfs-tools/hooks/tpm2"
#!/bin/sh -e
if [ "$1" = "prereqs" ]; then exit 0; fi
. /usr/share/initramfs-tools/hook-functions
copy_exec /usr/local/bin/tpm2_unseal
copy_exec /usr/local/lib/libtss2-tcti-device.so
I appended my etc/crypttab from cryptname UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx none luks to cryptname UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx none luks,keyscript=/usr/local/bin/passphrase-from-tpm
I rewrote the following script because the tpm2-tools command was outdated, edited in the new command, and stored it in /usr/local/bin/passphrase-from-tpm:
#!/bin/sh
set -e
echo "Unlocking via TPM" >&2
export TPM2TOOLS_TCTI="device:/dev/tpm0"
/usr/local/bin/tpm2_unseal -c 0x81000000 --auth pcr:sha1:0,2,3,7
if [ $? -eq 0 ]; then
exit
fi
/lib/cryptsetup/askpass "Unlocking the disk fallback $CRYPTTAB_SOURCE ($CRYPTTAB_NAME)\nEnter passphrase: "
I ran # update-initramfs -u -k all then rebooted. In reboot, I get the following error: /lib/cryptsetup/scripts/passphrase-from-tpm: line 5: /usr/local/bin/tpm2_unseal: not found
I have tried many times to edit passphrase-from-tpm unsuccessfully, including:
Moving both passphrase-from-tpm into "/boot/efi/EFI/BOOT/" and referencing crypttab to that file
Modifying passphrase-from-tpm to use a relative file path to tpm_unseal
Before I figured out how to create a backup linux boot using:
objcopy \
--add-section .osrel=/etc/os-release --change-section-vma .osrel=0x20000 \
--add-section .cmdline=cmdline.txt --change-section-vma .cmdline=0x30000 \
--add-section .linux="/boot/vmlinuz" --change-section-vma .linux=0x40000 \
--add-section .initrd="/boot/initrd.img" --change-section-vma .initrd=0x3000000 \
/usr/lib/systemd/boot/efi/linuxx64.efi.stub /boot/EFI/BOOT/BOOT_RECX64.EFI
I would be locked out of the system completely because of the error and had to reinstall Ubuntu about 40 times. I have suffered a lot and want to quit but I am too stubborn to throw in the flag.

Answer 1

just copy tpm2_unseal to /usr/local/bin/

Answer 2

I'm trying to make a working setup following basically those instructions, plus a few others I have found. While not working 100% yet, check that both /etc/initramfs-tools/hooks/tpm2 and /usr/local/bin/passphrase-from-tpm are marked executable (sudo chmod ug+x $filename).
After making the initramfs, you can run the following to ensure that the TPM related files are actually in the image. Replace the path in the filename by whatever update-initramfs said it was generating:
$ lsinitramfs /boot/initrd.img-5.0.0-37-generic | egrep "(tpm|libtss)"
lib/cryptsetup/scripts/passphrase-from-tpm
lib/modules/5.0.0-37-generic/kernel/crypto/asymmetric_keys/tpm_key_parser.ko
lib/modules/5.0.0-37-generic/kernel/crypto/asymmetric_keys/asym_tpm.ko
lib/udev/rules.d/tpm-udev.rules
usr/local/lib/libtss2-sys.so.0
usr/local/lib/libtss2-mu.so.0
usr/local/lib/libtss2-sys.so.0.0.0
usr/local/lib/libtss2-tcti-device.so
usr/local/lib/libtss2-tcti-device.so.0
usr/local/lib/libtss2-tcti-device.so.0.0.0
usr/local/lib/libtss2-mu.so.0.0.0
usr/local/bin/tpm2_unseal
Additionally, I have modified /usr/local/bin/passphrase-from-tpm to the following:
#!/bin/sh
TPM_DEVICE=/dev/tpm0
TPM_REGISTER=0x81000001
TPM_SEAL_POLICY=sha256:0,2,4,7
export TPM2TOOLS_TCTI="device:$TPM_DEVICE"
if [ "$CRYPTTAB_TRIED" -eq 0 ]; then
echo "Unlocking via TPM" >&2
/usr/local/bin/tpm2_unseal -H $TPM_REGISTER -L $TPM_SEAL_POLICY
UNSEAL_STATUS=$?
echo "Unseal status $UNSEAL_STATUS" >&2
if [ $UNSEAL_STATUS -eq 0 ]; then
exit
fi
else
echo "TPM unlocking previously failed for $CRYPTTAB_SOURCE ($CRYPTTAB_NAME)" >&2
/lib/cryptsetup/askpass "Enter passphrase for $CRYPTTAB_SOURCE ($CRYPTTAB_NAME): "
fi
Note that the command line options to tpm2_unseal are for the 3.x versions of tpm2-tools. If you're using another version, you might need to update the options.
I pulled out various bits into variables at the top of the file. Modify TPM_REGISTER and TPM_SEAL_POLICY to match how you created the TPM object. set -e was removed since if any command failed, the whole script would exit, preventing the askpass fallback from ever running if tpm2_unseal failed.
Additionally, I noticed that if the script fails for some reason, systemd will attempt to run it again. If the secret in the TPM doesn't match the LUKS key, this will render the system unbootable, since the unseal succeeds, but unlocking fails, and systemd will run the script again.
Looking at the man page for crypttab, I discovered that one of the environment variables provided to the keyscript is CRYPTTAB_TRIED which is the number of tries it has attempted to unlock the volume. If CRYPTTAB_TRIED is 0, it'll attempt to use the TPM, as shown by this test (Running as non-root, so accessing the TPM device fails):
$ export CRYPTTAB_SOURCE=some_device
$ export CRYPTTAB_NAME=some_device_name
$ export CRYPTTAB_TRIED=0
$ ./passphrase-from-tpm
Unlocking via TPM
ERROR:tcti:src/tss2-tcti/tcti-device.c:440:Tss2_Tcti_Device_Init() Failed to open device file /dev/tpm0: Permission denied
ERROR: tcti init allocation routine failed for library: "device" options: "/dev/tpm0"
ERROR: Could not load tcti, got: "device"
Unseal status 1
When it tries running the script again, CRYPTTAB_TRIED will be greater than 0, making it display the password prompt instead:
$ export CRYPTTAB_TRIED=1
$ ./passphrase-from-tpm
TPM unlocking previously failed for some_device (some_device_name)
Enter passphrase for some_device (some_device_name):
Hopefully this is still of use to you, and helpful to anyone else trying to get the house of cards that is disk encryption with a TPM on Linux working.