vault: add auto-unseal

docs/vault/auto-unseal.md

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+# OpenBao TPM2 Auto-Unseal Setup
+
+This document describes the one-time setup process for enabling TPM2-based auto-unsealing on vault01.
+
+## Overview
+
+The auto-unseal feature uses systemd's `LoadCredentialEncrypted` with TPM2 to securely store and retrieve an unseal key. On service start, systemd automatically decrypts the credential using the VM's TPM, and the service unseals OpenBao.
+
+## Prerequisites
+
+- OpenBao must be initialized (`bao operator init` completed)
+- You must have at least one unseal key from the initialization
+- vault01 must have a TPM2 device (virtual TPM for Proxmox VMs)
+
+## Initial Setup
+
+Perform these steps on vault01 after deploying the service configuration:
+
+### 1. Save Unseal Key
+
+```bash
+# Create temporary file with one of your unseal keys
+echo "paste-your-unseal-key-here" > /tmp/unseal-key.txt
+```
+
+### 2. Encrypt with TPM2
+
+```bash
+# Encrypt the key using TPM2 binding
+systemd-creds encrypt \
+  --with-key=tpm2 \
+  --name=unseal-key \
+  /tmp/unseal-key.txt \
+  /var/lib/openbao/unseal-key.cred
+
+# Set proper ownership and permissions
+chown openbao:openbao /var/lib/openbao/unseal-key.cred
+chmod 600 /var/lib/openbao/unseal-key.cred
+```
+
+### 3. Cleanup
+
+```bash
+# Securely delete the plaintext key
+shred -u /tmp/unseal-key.txt
+```
+
+### 4. Test Auto-Unseal
+
+```bash
+# Restart the service - it should auto-unseal
+systemctl restart openbao
+
+# Verify it's unsealed
+bao status
+# Should show: Sealed = false
+```
+
+## TPM PCR Binding
+
+The default `--with-key=tpm2` binds the credential to PCR 7 (Secure Boot state). For stricter binding that includes firmware and boot state:
+
+```bash
+systemd-creds encrypt \
+  --with-key=tpm2 \
+  --tpm2-pcrs=0+7+14 \
+  --name=unseal-key \
+  /tmp/unseal-key.txt \
+  /var/lib/openbao/unseal-key.cred
+```
+
+PCR meanings:
+- **PCR 0**: BIOS/UEFI firmware measurements
+- **PCR 7**: Secure Boot state (UEFI variables)
+- **PCR 14**: MOK (Machine Owner Key) state
+
+**Trade-off**: Stricter PCR binding improves security but may require re-encrypting the credential after firmware updates or kernel changes.
+
+## Re-provisioning
+
+If you need to reprovision vault01 from scratch:
+
+1. **Before destroying**: Back up your root token and all unseal keys (stored securely offline)
+2. **After recreating the VM**:
+   - Initialize OpenBao: `bao operator init`
+   - Follow the setup steps above to encrypt a new unseal key with TPM2
+3. **Restore data** (if migrating): Copy `/var/lib/openbao` from backup
+
+## Handling System Changes
+
+**After firmware updates, kernel updates, or boot configuration changes**, PCR values may change, causing TPM decryption to fail.
+
+### Symptoms
+- Service fails to start
+- Logs show: `Failed to decrypt credentials`
+- OpenBao remains sealed after reboot
+
+### Fix
+1. Unseal manually with one of your offline unseal keys:
+   ```bash
+   bao operator unseal
+   ```
+
+2. Re-encrypt the credential with updated PCR values:
+   ```bash
+   echo "your-unseal-key" > /tmp/unseal-key.txt
+   systemd-creds encrypt \
+     --with-key=tpm2 \
+     --name=unseal-key \
+     /tmp/unseal-key.txt \
+     /var/lib/openbao/unseal-key.cred
+   chown openbao:openbao /var/lib/openbao/unseal-key.cred
+   chmod 600 /var/lib/openbao/unseal-key.cred
+   shred -u /tmp/unseal-key.txt
+   ```
+
+3. Restart the service:
+   ```bash
+   systemctl restart openbao
+   ```
+
+## Security Considerations
+
+### What This Protects Against
+- **Data at rest**: Vault data is encrypted and cannot be accessed without unsealing
+- **VM snapshot theft**: An attacker with a VM snapshot cannot decrypt the unseal key without the TPM state
+- **TPM binding**: The key can only be decrypted by the same VM with matching PCR values
+
+### What This Does NOT Protect Against
+- **Compromised host**: If an attacker gains root access to vault01 while running, they can access unsealed data
+- **Boot-time attacks**: If an attacker can modify the boot process to match PCR values, they may retrieve the key
+- **VM console access**: An attacker with VM console access during boot could potentially access the unsealed vault
+
+### Recommendations
+- **Keep offline backups** of root token and all unseal keys in a secure location (password manager, encrypted USB, etc.)
+- **Use Shamir secret sharing**: The default 5-key threshold means even if the TPM key is compromised, an attacker needs the other keys
+- **Monitor access**: Use OpenBao's audit logging to detect unauthorized access
+- **Consider stricter PCR binding** (PCR 0+7+14) for production, accepting the maintenance overhead
+
+## Troubleshooting
+
+### Check if credential exists
+```bash
+ls -la /var/lib/openbao/unseal-key.cred
+```
+
+### Test credential decryption manually
+```bash
+# Should output your unseal key if TPM decryption works
+systemd-creds decrypt /var/lib/openbao/unseal-key.cred -
+```
+
+### View service logs
+```bash
+journalctl -u openbao -n 50
+```
+
+### Manual unseal
+```bash
+bao operator unseal
+# Enter one of your offline unseal keys when prompted
+```
+
+### Check TPM status
+```bash
+# Check if TPM2 is available
+ls /dev/tpm*
+
+# View TPM PCR values
+tpm2_pcrread
+```
+
+## References
+
+- [systemd.exec - Credentials](https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Credentials)
+- [systemd-creds man page](https://www.freedesktop.org/software/systemd/man/systemd-creds.html)
+- [TPM2 PCR Documentation](https://uapi-group.org/specifications/specs/linux_tpm_pcr_registry/)
+- [OpenBao Documentation](https://openbao.org/docs/)