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Add a guide on setting up TPM unlocking
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# TPM Unlocking
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WIP
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This will explain how to set up TPM (Trusted Platform Module) based automatic unlocking of your encrypted partition(s).
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Encryption usually requires that you manually type the password in each time you boot. This can however be pretty
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annoying (especially if you use a long password, like I do). This guide aims to fix this problem, without compromising
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security.
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Once finished, this will basically store another decryption key(s) to your encrypted partition(s) in the TPM module.
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During boot, while in initrd, we will request this decryption key from TPM, which will only do so under certain
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conditions, to ensure safety.
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The guide assumes you have already a working NixOS system, that uses LUKS encryption. You will also need to set up
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secure-boot, as described in [SECURE_BOOT](./03_SECURE_BOOT.md). This is a requirement, as while it is possible to set
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up TPM unlocking without it, doing so is incredibly insecure, and might lead to unauthorized users getting TPM to
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release your decryption keys.
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> [!WARNING]
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> This solution will be mostly safe, however, it is technically possible to hook up wires to the motherboard, to listen
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> to the communication coming from the TPM chip. In that case, the attacker would be able to observe the key as it gets
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> released by the chip. They could then take out your SSD/HDD, and mount it on their machine, using these obtained keys
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> to decrypt the contents. See: <https://astralvx.com/stealing-the-bitlocker-key-from-a-tpm/>
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>
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> If you can't afford to be vulnerable to this type of attack, you can still follow through with this, however instead
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> of the TPM seamlessly releasing the decryption password, you can require a password to be entered, without which TPM
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> won't release the decryption password.
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>
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> This can be useful if you use a very long encryption passwords, and you want to be able to enter a shorter passphrase,
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> or a PIN code instead (TPM has brute-force protection, so the PIN isn't actually that unsafe to use).
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## Check if you actually have the TPM module
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First, you will want to verify that your machine even has the TPM module. To do so, you can use the following command:
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```shell
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bootctl status
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```
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You should see `TPM2 Support: yes` in the output.
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## Enable TPM2 support
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First, we will want to enable the TPM support in our configuration:
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```nix
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# Points TPM2TOOLS_TCTI and TPM2_PKCS11_TCTI to /dev/tpmrm0
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# and allows you to read and write to /dev/tpmrm0
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security.tpm2.enable = true;
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# Tools we will need to interact with the TPM module
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environment.systemPackages = with pkgs; [ tpm2-tss tpm2-tools ];
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# Use systemd in initrd. Only systemd based initrd systems can request data from the TPM module,
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# busybox based ones do not have support for this.
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boot.initrd.systemd.enable = true;
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```
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## Choosing PCRs
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PCR stands for Platform Configuration Register, and all TPM v2 modules have a bunch of these registers, which hold
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hashes about the system's state. These registers are read-only, and their value is set by the TPM module itself.
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The data held by the TPM module (our LUKS encryption key) can then only be accessed when all of the selected PCR
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registers contain the expected values. You can find a list of the PCR registers on [Arch
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Wiki](https://wiki.archlinux.org/title/Trusted_Platform_Module#Accessing_PCR_registers).
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You can look at the current values of these registers with this command:
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```bash
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systemd-analyze pcrs
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```
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For our purposes, we will choose these:
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- **PCR0:** Hash of the UEFI firmware executable code (may change if you update UEFI)
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- **PCR7:** Secure boot state - contains the certificates used to validate each boot application
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- **PCR12:** Overridden kernel command line, credentials
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> [!IMPORTANT]
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> If you're using a boot loader (rather than booting directly from the Unified Kernel Images - EFI files), it is crucial
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> that we choose all 3, including PCR12, as many tutorials only recommend 0 and 7, which would however lead to a
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> security hole, where an attacker would be able to remove the drive with the (unencrypted) EFI partition, and modify
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> the boot loader config. (With systemd-boot, this would be `loaders/loader.conf`).
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>
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> From there, the attacker could simply add a kernel argument like `init=/bin/bash`, or just enable editor support,
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> allowing them to edit the parameters from the boot menu on the fly (This is actually the default for systemd-boot on
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> NixOS, editor support is enabled). This would then bypass systemd as the init system and instead make the kernel run
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> bash executable as the PID=1 (init) program. This would mean you would get directly into bash console that is running
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> as root, without any need to enter a password.
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>
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> This wouldn't violate secure boot, as the `.efi` image files were unchanged, and are still signed, so the attacker
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> would be able to boot into the system without issues.
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>
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> However, with PCR12, this is prevented, as it detects that the kernel cmdline arguments which were used, and if they
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> don't match the recorded parameters during enrollment, TPM will not release the key.
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>
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> The nice thing about also selecting PCR12 is that it will actually allow us to securely keep systemd-boot editor
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> support, which can be very useful for debugging, as all that will happen if we do edit the kernel command line will be
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> that the TPM module will not release the credentials, and the initrd will just ask us to enter the password manually.
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Optionally, you may also consider these:
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- **PCR1:** Hash of the UEFI firmware data (changes when you change your BIOS settings)
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- **PCR4:** Boot manager (changes when you change the boot manager)
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## Enroll a new key into TPM
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The following command will enroll a new randomly generated key into the TPM module and add it as a new keyslot of the
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specified LUKS2 encrypted device (make sure to use the path to your device, it might not be the same as the one I'm
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using):
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```shell
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sudo systemd-cryptenroll --tpm2-device=auto --tpm2-pcrs=0+7+12 /dev/disk/by-label/NIXCRYPTFS
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```
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> [!NOTE]
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> If you already had something in the tpm2 module, you'll want to add `--wipe-slot=tpm2` too.
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> [!NOTE]
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> If you're extra paranoid, you can also provide `--tpm2-with-pin=yes`, to prompt for a PIN code on each boot.
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>
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> I have mentioned why you may want to do this in the beginning.
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You will now be prompted for an existing LUKS password (needed to add a new LUKS keyslot).
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## Reboot
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All that remains now is rebooting. The system should now get unlocked automatically, without prompting for the password.
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If you're using a bootloader, I'd recommend also trying to modify the kernel parameters, to make sure that TPM does not
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release the key anymore, and you will be prompted to enter it manually.
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## Remove existing key
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Once you have confirmed that TPM unlocking is working, you can now optionally get rid of your original LUKS key.
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Since the passphrase generated by systemd-cryptenroll is guaranteed to have a high encryption, getting rid of your, likely much less secure, chosen key can be a good idea, as it further improves your chances if someone attempts a brute-force decryption of your drive.
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That said, if you wish to proceed with this, I **heavily** recommend first creating an equally secure recovery key, so
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that you have a way to decrypt your data from the drive in case your computer breaks. To generate a recovery key, you
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can actually also just use `systemd-cryptenroll` (though you can also do it manually with `cryptsetup`):
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```shell
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systemd-cryptenroll /dev/disk/by-label/NIXCRYPTFS --recovery-key
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```
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This will give you a randomized key, using characters that are easy to type. You will even be given a QR code that can
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be scanned directly to save the password on your phone.
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Before proceeding with removing your own key, let's first make absolutely certain that the recovery key you saved does
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in fact work. Without doing this, you may get locked out!
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```shell
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cryptsetup luksOpen /dev/disk/by-label/NIXCRYPTFS crypttemp # enter the recovery key
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cryptsetup luksClose crypttemp
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```
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If this worked, proceed to:
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```shell
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cryptsetup luksRemoveKey /dev/disk/by-label/NIXCRYPTFS # Enter your key to be deleted
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```
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## Removing the key from TPM
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In case you'd ever want to remove the LUKS key from TPM, you can do so simply with:
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```bash
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csystemd-cryptenroll --wipe-slot=tpm2
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```
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This will actually also remove the LUKS key from the `/dev/disk/by-label/NIXCRYPTFS` device.
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## Sources / Attribution
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- <https://nixos.wiki/wiki/TPM>
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- <https://discourse.nixos.org/t/full-disk-encryption-tpm2/29454/6>
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- <https://wiki.archlinux.org/title/systemd-cryptenroll>
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- <https://wiki.archlinux.org/title/Trusted_Platform_Module#Accessing_PCR_registers>
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- <https://pawitp.medium.com/full-disk-encryption-on-arch-linux-backed-by-tpm-2-0-c0892cab9704>
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