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REPORT + CONTEXT: F-LEAK closed via the fenced destroy (v0.110.0), all three attempts recorded

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authentik Default Admin
2026-07-28 11:34:48 +02:00
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# REPORT — F-REBOOT + F-LEAK (v0.107.0 → v0.109.0) (2026-07-28)
# REPORT — F-REBOOT + F-LEAK (v0.107.0 → v0.110.0) (2026-07-28)
**Overwritten** per the standing rule. Agent **v0.106.0 → v0.109.0** across three commits, two of them
corrections to the first. Companions: `felhom-controller` v0.180.0 (F-OBS) and `felhom.eu`
**Overwritten** per the standing rule. Agent **v0.106.0 → v0.110.0** across four commits, **three of them
corrections to the first**. F-LEAK took three attempts and two of them were refuted by live test; that
arc is the substance of this report. Companions: `felhom-controller` v0.180.0 (F-OBS) and `felhom.eu`
host-install v1.21.0 (F-LEAK's actual fix).
## Baselines (reconfirmed, not copied)
@@ -68,19 +69,77 @@ that provably cannot work is worse than no path, because it looks like a fix.
`/pool/felhom`, and a guest joins that pool only when its restore **completes**. A *failed*
restore-test therefore leaves a guest that exists, is in no pool, and is outside the token's reach.
**The fix is `felhom-host-install.sh` v1.21.0** — the role granted at each `/vms/990000`…`/vms/990009`,
to both user and token (privsep intersection). `remove_scoped_acl` deletes them before the role delete
(PVE refuses to delete a referenced role, so omitting that would break the uninstall) and `step_verify`
asserts them, because a missing grant is otherwise invisible until a restore-test *fails*.
### Live A/B on demo-hp — same guest, same token, minutes apart
A real PBS restore to `990000` **without `--pool`** reproduced the exact stranded state
(`990000 stopped`, `felhom pool members: [9201]`, `990000 in pool: False`).
### Attempt 2 — the band-scoped ACL. It works exactly ONCE per slot.
`felhom-host-install.sh` v1.21.0 grants the role at each `/vms/990000`…`/vms/990009`. The live A/B on
the same guest, minutes apart, proved it does close the defect:
| | `DELETE /nodes/<node>/lxc/990000` with the agent's own token |
|---|---|
| **grant removed** (the original defect) | `403 Permission check failed (/vms/990000, VM.Allocate)` — guest still present |
| **grant restored** (the fix) | `200 UPID:...:vzdestroy:990000:felhom-agent@pve!agent` — guest gone |
| **grant restored** | `200 UPID:...:vzdestroy:990000:felhom-agent@pve!agent` — guest gone |
**And then the post-run ACL count caught it:** `/vms/990000` had **0 grants** afterwards, while the
other nine slots had 2 each. Confirmed in PVE's own source rather than inferred — the destroy path
calls `AccessControl::remove_vm_access($vmid)` (`API2/LXC.pm:906`), which deletes
`acl_root->children->vms->children->{$vmid}`, i.e. **every ACL at `/vms/<vmid>`**
(`AccessControl.pm:1898`).
**The grant is consumed by the very operation it authorises.** Ten teardowns and the band is ungranted
and the defect is back. A per-VM ACL is structurally the wrong tool; the pool grant is durable only
because it lives on a *pool* path, which `remove_vm_access` leaves alone (it removes the guest's pool
*membership*, not the pool's ACL). I would not have found this without checking the box after the fix.
### Attempt 3 — the fourth root-fenced exception (v0.110.0, SHIPPED)
The route this task offered as the alternative: **the same authority that already performs the create.**
`Privileged.DestroyScratchLXC` joins the fence (previously exactly three: keyctl `pct create`, USB
mount/fstab, SMART/sensors) and is fenced harder than any of them — in three places, none consumed by
use:
| layer | enforcement |
|---|---|
| **sudoers** | `/usr/sbin/pct destroy 99000[0-9] --purge` — sudo matches the vmid **literally**; a compromised agent asking for `9201` is refused by sudo itself |
| **`DestroyScratchLXC`** | re-checks the band before exec; refuses an unconfigured or inverted band rather than defaulting to anything |
| **`teardownScratch`** | acts only on this journal entry's own scratch provenance |
The API destroy is still tried **first** and remains the normal path; the v1.21.0 band ACLs stay
provisioned so the common case needs no privileged call. This is the fallback that makes teardown
deterministic instead of once-per-slot. Ships **with a sudoers change**, deployed alongside the binary.
### The fence, proven live by ASKING sudo rather than executing
Executing the negative case against a live guest is exactly what must not happen, so `sudo -l` was used
to ask whether each command is permitted:
| command | verdict |
|---|---|
| `pct destroy 990000 / 990005 / 990009 --purge` | **PERMITTED** |
| `pct destroy 9201 --purge` (the live customer guest) | **REFUSED** |
| `pct destroy 9100 / 9999 / 1 --purge` | **REFUSED** |
| `pct destroy 990010 --purge` (one past the band) | **REFUSED** |
| `pct start 990000`, `pct set 990000 -memory 512` | **REFUSED** — the grant is this one op, not this vmid |
### The end-to-end proof, in the ACL's natural post-consumption state
The best possible setup arrived by itself: after the attempt-2 destroy, `/vms/990000` had **0 grants**.
A fresh PBS restore to `990000` **without `--pool`** then reproduced the stranded guest with the ACL
genuinely absent — not artificially removed:
```
/vms/990000 grants: 0
990000 stopped (restore rc=0, in no pool)
DELETE via token -> HTTP 403 Permission check failed (/vms/990000, VM.Allocate) ← the defect
runuser -u felhom-agent -- sudo -n /usr/sbin/pct destroy 990000 --purge
Logical volume "vm-990000-disk-0" successfully removed.
Logical volume "vm-990000-disk-1" successfully removed.
Logical volume "vm-990000-disk-2" successfully removed.
purging CT 990000 from related configurations.. rc=0
after: pct list -> 9201 only · 990000 LVs remaining: 0 · /etc/pve/lxc/990000.conf: absent
```
That command vector is byte-identical to what `Privileged.run` issues, executed as the same
unprivileged service user the agent runs as. The token was refused and the fenced path reclaimed the
guest **and its three disks** — which is the actual harm F-LEAK described.
The band grants were re-applied on both boxes afterwards, so the common case still needs no privileged
call.
### Scenario E — it still cannot destroy a non-scratch guest
| target | result |
@@ -115,7 +174,7 @@ it would have the heartbeat claim liveness for a watchdog doing nothing.
---
## Red-proofs — 8 total, all observed failing
## Red-proofs — 10 total, all observed failing
| # | red-proof | observed failure |
|---|---|---|
@@ -127,6 +186,8 @@ it would have the heartbeat claim liveness for a watchdog doing nothing.
| 6 | unfiltered guest list on error | `acted with unproven ownership` |
| 7 | remove the liveness observable | `no liveness observable after 10 sweeps — silence is indistinguishable from a dead watchdog` |
| 8 | count aborted sweeps as healthy | `an aborted sweep was counted as healthy (sweeps=20)` |
| 9 | remove the band check from `DestroyScratchLXC` | `REFUSAL FAILED: executed [pct destroy 9201 --purge] for out-of-band vmid 9201` |
| 10 | remove the unconfigured-band check | `an unconfigured band admitted vmid 0 and EXECUTED [pct destroy 0 --purge]` |
`go build`, `go vet ./...`, `go test ./...` — 29 packages, `rc=0`, run separately from every commit.
@@ -140,7 +201,8 @@ committed, then **rebuilt and redeployed from a genuinely clean tree** (`88b3cf0
---
## Deployed & fleet state
Agent **0.109.0** on **demo-hp** and **demo-felhom**, both `active`. Scratch-band ACLs applied on
Agent **0.110.0** on **demo-hp** and **demo-felhom**, both `active`, with the updated sudoers installed
and `visudo -cf` clean on both. Band ACLs at 20 rows (10 vmids x user+token) on **both** boxes. Scratch-band ACLs applied on
**both** boxes. No leftover scratch guests — `pct list` shows only `9201(running)` on demo-hp.
The only WARN on demo-hp after restart is **F-CRIT-2's fix working**: it rejected the 1-byte phantom