platform/docs/per-user-linux-accounts.md

# Per-user Linux accounts

**Status: in progress.** Stage 1 (the account and the privilege-drop mechanism) is being built now.
Agents are explicitly out of scope for the first pass.

## What this is for

Today every `execution` capability — terminal, chat, files, tasks, items, desktop, browser — runs as the
**owner's OS user in the owner's home**. That is why `capabilities/registry.ts` declares them
`kind: 'execution'` and why `authorize.ts` strips them from a grant even if a row somehow contains one.
The registry says so out loud: *"revisit only if per-user home confinement is ever solved — and that is a
project, not a checkbox."*

This is that project. A member gets a real Linux account whose home is the directory the platform already
provisions for them, and the surfaces that execute code run **as that account**. The payoff is three
things at once:

- **Isolation** — a member cannot read another member's files, because the kernel says so rather than
  because a path check happened to be right.
- **Permissions** — "may they see this" becomes a mode bit, checked by the OS on every syscall, instead
  of a predicate the platform has to remember to apply on every route.
- **Separable agents** — `claude` and `opencode` run as the member, with their own `~/.claude`, their own
  transcripts and their own session state, because the CLI groups by HOME and cwd.

## The target for the first test

A member signs in and:

- the **file browser** shows their home as the root and cannot navigate above it;
- the **terminal** lands in their home and has no permission to see anything above it.

Nothing else changes. Agents stay owner-only until this much is solid.

## The layout, and what each mode bit is for

```
data/                       711  service user    traverse only — a member cannot enumerate the members
└── <email>/                711  service user    traverse only — a member cannot see their OWN siblings
    ├── home/               700  the member      their real Linux home
    ├── attachments/        700  service user    platform-written; unreachable even by name
    ├── email_accounts/     700  service user    "
    ├── dashboards/         700  service user    "
    └── …                   700  service user    "
```

The important line is the second one. `data/<email>/` is traverse-only **to its own member**: they need
`x` to reach `home/`, and they must not have `r`, or they could list the platform's private tree beside
it. And because every sibling is `700 service user`, knowing a name does not help — traversal without
read gets you exactly one place, which is where they are going anyway.

This is also what resolves the two-sided ownership problem. The platform runs as the service user and
writes attachments, email databases and dashboards into `data/<email>/`; the member owns only `home/`.
Nobody needs a shared group, a setgid bit or an ACL, and neither side can write where the other lives.

**Members' homes stay under `DATA_PATH`** rather than moving to `/home/<user>`. They are the platform's
data, they belong with the rest of that account's data, and the directory is already provisioned there by
`provisionUserDirs`. A move would also break `getOwnerHomeDir`'s fallback, which is the only shape the
code has ever had for a non-owner home.

## Hard prerequisite: the secrets a shell can currently read

**This must be fixed before any member gets a shell, and it is not optional.**

On this machine, verified 2026-08-11:

| path | mode | consequence |
| --- | --- | --- |
| `/home/pastilhas` | 751 | traversable by anyone (no listing) |
| `…/officer.dev` | 775 | listable by anyone |
| `…/platform/.env` | **664** | **world-readable** |

`platform/.env` holds `POSTGRES_URL`, the JWT signing secret and every service credential. A member with
a real shell could read it and mint themselves an owner token, which makes the whole exercise worse than
not doing it — the capability model would be intact and completely bypassed.

So stage 1 includes: `chmod 600` on every `.env`, `chmod 751` on the project root so the tree is
traversable but not listable, and a **boot-time check that refuses to enable OS users while any `.env`
under the project root is group- or world-readable.** A prerequisite that is merely written down is a
prerequisite that gets skipped.

The same applies to `capabilities/` (775 today) and to the repo checkout itself: a member can read the
platform source. That is acceptable — it is not secret — but anything credential-shaped inside it is not.

## The mechanism, and the trap in it

### `Bun.spawn` silently ignores `uid` and `gid`

Verified on bun 1.3.10, 2026-08-11. From uid 1000:

```js
Bun.spawn(['id', '-u'], { uid: 65534, gid: 65534 })   // exit 0, prints "1000"
```

It does not throw. It does not warn. It accepts the option and runs as the parent. Every agent, task and
script spawn in this codebase goes through `Bun.spawn`.

Two honest qualifications, because the danger is narrower than it first looks:

- **Bun's own types do not declare `uid`**, so `bunx tsgo` rejects it. Typed code cannot reach this by
  accident — confirmed while writing the test, which needs a cast to reproduce the behaviour at all.
- What *can* reach it is a spread of untyped config, an `as any`, or a plain-JS sidecar. Two of the four
  sidecars are `.mjs`.

So the exposure is real but bounded, and the mitigation is the same either way: privilege drops go through
an external wrapper, and a test pins Bun's runtime behaviour. If Bun ever implements the option, that test
fails and tells us we may simplify. **A silently absent isolation boundary is the worst possible outcome of
this project**, so it is worth a test that exists only to observe something staying broken.

### `sudo -n setpriv`, and why both words are needed

`runAs` builds:

```
sudo -n setpriv --reuid=<user> --regid=<user> --init-groups --reset-env -- <argv…>
```

- `--reuid`/`--regid` set the real ids, not just effective — there is nothing to switch back to.
- `--init-groups` applies the account's supplementary groups. Without it the process keeps the *owner's*
  groups, which is a quiet way to retain access we just took away.
- `--reset-env` clears the inherited environment and then sets `HOME`, `SHELL`, `USER`, `LOGNAME` and
  `PATH` from the target's passwd entry. Both halves matter: the parent's env contains the owner's `HOME`,
  and on a process started by PM2 in the platform directory it contains everything Bun auto-loaded from
  `.env`.

**`sudo` is not optional, and the reason is not the uid.** Measured 2026-08-11: `--init-groups` fails with
`initgroups failed: Operation not permitted` for an unprivileged caller *even when reuid'ing to its own
account* — `setgroups(2)` is root-only, unconditionally. So there is no unprivileged form of this. `-n`
makes a missing sudoers entry an immediate error rather than a process hanging on a password prompt no
user will ever see.

Verified end to end, dropping to the current account:

```
$ sudo -n setpriv --reuid=pastilhas --regid=pastilhas --init-groups --reset-env -- \
    sh -c 'id -u; id -G; echo HOME=$HOME; echo SECRET=${POSTGRES_URL:-unset}'
1000
1000 4 24 27 30 46 101 988 1001      ← supplementary groups from the account, not inherited
HOME=/home/pastilhas                 ← from passwd, after the reset
SECRET=unset                         ← the platform's .env did NOT cross
```

That last line is the whole security property, demonstrated rather than asserted, and it is pinned by a
test (`os-user.test.ts` → "does not pass the platform environment through").

`sudo -u <user>` alone would also work and be shorter. It is not used because its environment handling is
sudoers *policy* — `env_reset`, `env_keep`, `always_set_home` — and "which variables cross into a member's
shell" must not depend on a config file someone may have edited.

Root is available: `scripts/setup.sh` §4 installs `/etc/sudoers.d/officer-service` granting the service
user `NOPASSWD: ALL` on the full profile. The light profile deliberately skips it, so a light install that
wants OS users needs a **narrow** entry — `useradd`, `chown`, `setpriv` — which is better than the blanket
rule anyway.

### The terminal is the easy one

`sidecar/pty/sessions.mjs` uses **node-pty** under **node**, and node-pty's `spawn` genuinely honours
`uid`/`gid` (it is a native binding, not Bun's spawn). Two options; we take the second:

1. Run the pty sidecar as root and pass `uid`/`gid` per session.
2. Keep the sidecar unprivileged and make the command `setpriv … <shell> -i`.

(2) means no root daemon and one mechanism shared with everything else. A root daemon accepting session
requests over a socket is a bigger promise than this feature needs to make.

## Naming

`officer_<shellname>`, where `<shellname>` is `toShellUsername(username, email)` — the existing
sanitiser, which already lowercases, strips `@…`, replaces illegal characters and truncates to 32. The
combined name is truncated to 32 again.

The prefix earns its ugliness three times: it cannot collide with a system account, it makes every
account this feature created greppable in `/etc/passwd`, and it means a member cannot pick a username
that shadows something real.

The resolved name is **stored** on the user row (`users.os_user`) rather than re-derived. `useradd` can
adjust or refuse a name, and re-deriving would mean the platform's idea of who a member is could drift
from what is actually in `/etc/passwd`.

## Out of scope, and honest about it

- **This is not a sandbox.** A member with a shell is on the machine. They cannot read the owner's files
  or another member's, and `sudo` is not theirs — but they can run code, see process names, and reach the
  network. It isolates members from each other and from accidents, not from the host.
- **Agents come later.** `claude-manager.ts` drives turns through `query()` from
  `@anthropic-ai/claude-agent-sdk`, which spawns `claude` itself and takes `env`/`cwd` but has nowhere to
  put a uid. Dropping privileges has to happen *outside* the SDK, which makes a member's turn its own
  process — a change of shape rather than a flag. The credential is not the problem:
  `officer-anthropic-proxy` already holds it, so a member's `claude` needs only `ANTHROPIC_BASE_URL`
  pointed at the proxy and no key of its own.
- **`pty`, `vault` and `opencode` receive no identity at all** (`TODO.md` → Multi-user). pty keys purely
  on a `sessionId` from the query string, and its `/_officer/sessions` endpoints list and kill *every*
  session on the box. Safe today only because terminal is owner-only. **The moment a member has a shell
  that is a cross-user kill switch**, so it is fixed in the same stage as the terminal, not after.
- **Email change orphans a home.** The on-disk layout is keyed on email everywhere. Renaming an account
  would leave its home behind under the old address. Pre-existing, unfixed, worth knowing.

## What the first real run proved, and what it corrected

Stage 1 was exercised end to end against a throwaway `DATA_PATH` with a real `useradd`. Every property
below was **observed**, not reasoned about:

| attempted, as the member | result |
| --- | --- |
| write in own home | OK |
| read `…/<email>/attachments/private.txt` | Permission denied |
| `ls …/<email>/` (their own account dir) | Permission denied |
| `ls $DATA_PATH` (enumerate the members) | Permission denied |
| `ls …/other-member@example.com/home` | Permission denied |
| `cd $HOME/..` | **succeeds** — see below |

Three bugs surfaced only by running it:

1. **`chmod` after `chown` fails forever.** `chmod` requires ownership, so once the home belongs to the
   member the service user cannot set its mode. Both orderings fail unprivileged — the first on the second
   run, the second immediately. Both operations now go through sudo, which is what makes the function
   re-runnable.
2. **A member could read another member's home.** `provisionUserDirs` created directories at the default
   umask (`755`), and the confinement pass only ever ran for the account being created. `DATA_PATH` being
   unlistable is not protection when the child is world-readable and the attacker knows an email address.
   The skeleton is now created closed — `711` on the account directory, `700` inside — so *unconfined* is
   also *unreachable*.
3. **`platform/.env` was readable, and printing `JWT_SECRET` from a member's shell was confirmed.** This is
   the prerequisite above, demonstrated. It is now a boot check (`assertSecretsClosed`) that refuses to
   start with `OFFICER_OS_USERS` on while any `.env` in the project root is group- or world-readable.

**`cd $HOME/..` succeeding is correct and worth being precise about.** `711` grants traversal, so `cd`
works while `ls` does not — they can stand in the directory and see nothing in it. Beyond that, a real
shell can reach `/etc`, `/usr` and anything else the system leaves world-readable, because that is what a
shell is. So:

- the **file browser** genuinely cannot go above the home — that is path containment in `resolveUserPath`,
  enforced by the platform;
- the **terminal** cannot *read* anything above the home, but is not confined to it. Confining it would
  mean a namespace or a chroot, which is a different and much larger feature.

Say "cannot see behind it", not "cannot leave it".

## SSH: two keys, two directions

A member is meant to behave like a real user on the machine — reachable over SSH, able to push to Gitea as
themselves, able to have an agent do the same on their behalf. That needs two keys, and they are **not**
alternatives:

| | where | who holds the private half | what it is for |
| --- | --- | --- | --- |
| **inbound** | `~/.ssh/authorized_keys` | the member, on their laptop | *they* SSH into this machine |
| **outbound** | `~/.ssh/id_ed25519` | this machine, generated here | *the machine* authenticates to Gitea as them |

The tempting simplification is "if they pasted a key, skip generating one." It breaks the actual goal.
Agent forwarding covers a human in an interactive session; a **platform-spawned agent has no agent socket
to borrow**, so an edge checkout it is asked to commit and push needs a key that lives on the box. So the
inbound key is optional — an account without one is simply platform-only — and the outbound keypair is
generated regardless.

**No Linux password, ever.** `useradd` is called with none, which leaves `!` in shadow. That blocks
*password* login and does **not** block key auth, so "real user, reachable over SSH, no password anywhere"
is the resting state. The privilege drop is `sudo -n setpriv` performed by the platform, so there is nothing
to authenticate. Keeping the platform password and the machine out of each other's business is the point: a
Linux password would be a second door that changing the platform password does not close and deleting the
platform account does not lock.

**Validation is about line count, not key shape.** Every line of `authorized_keys` is a credential, so a
pasted value containing a newline would silently install a *second* authorized key. `validatePublicKey`
refuses anything multi-line, refuses a private key with a message saying so, and refuses an options prefix
(`command="…" ssh-ed25519 …`) — legitimate OpenSSH, but not something anyone pastes by accident, and it can
force a command.

**Everything is written with `sudo install`.** The home is 700 and owned by the member, so the service user
cannot create `.ssh` at all. `install` sets content, owner and mode in one step, which also closes the
window where a key file briefly exists at the process umask. File content goes via a temp path rather than
shell text, so nothing has to reason about quoting a value that came from a form.

**`StrictHostKeyChecking accept-new`, not a seeded `known_hosts`.** The Gitea SSH endpoint is not knowable
at account-creation time — the platform stores an HTTP base URL, and SSH may be a different host or port.
The failure this avoids is specific: the default setting makes a first connection *prompt*, and a prompt in
a non-interactive agent turn is a hang, not an error. `accept-new` trusts on first use and still refuses a
*changed* host key, which is the attack that matters.

**The generated public key is stored on the user row** (`users.os_ssh_public_key`) and shown after creation
and on the user's row afterwards. It is public by definition, and it has an errand attached that nothing
else will remind anyone about: it has to be added to that person's Gitea account or their pushes fail with
a permission error that says nothing about a missing key.

Verified end to end with a real `useradd`: `.ssh` 700 and `id_ed25519` 600 both owned by the member and
readable by them, `authorized_keys` byte-identical to what was pasted, the key **not** rotated on a second
run (it has been added to Gitea by then), and a multi-line paste refused with `authorized_keys` left
untouched.

## Follow-ups this creates

- **Deleting a member no longer removes their home.** It belongs to their uid, so the platform cannot
  remove it — `rm -rf` fails with EPERM, which is how it was noticed. `deleteUserHandler` does not touch
  disk today so nothing is broken, but account deletion will need `userdel` and a sudo `rm` to stop
  leaving an orphaned, unremovable directory behind.
- **`confineUserTree` sets `DATA_PATH` itself to 711.** If a container ever bind-mounts a path under
  `DATA_PATH` and runs as another uid, it will traverse but not list. Nothing does today.

## Stages

1. **The account and the mechanism.** `users.os_user`; `ensureOsUser` (useradd + chown + the mode bits
   above); `runAs`; the `.env` permission gate; tests including the Bun-ignores-uid pin. **No behaviour
   change** — accounts are created and nothing uses them yet.
2. **`getOwnerHomeDir` honours its email argument.** It takes an email and throws it away whenever
   `HOME_DIR` is set, which is always on a real install. Seven call sites; this one change repoints the
   file browser, chat, tasks, agents and the VNC password file per account.
3. **The file browser**, rooted at the member's home. Containment already exists — `resolveUserPath` +
   `isInside`, which has the `..`-escape fix in it — so this is a root-resolution change, not new
   security code.
4. **The terminal**, via `setpriv`, plus pty identity. One `execution` capability reopened.
5. **Agents.** Separately, later, with the SDK problem solved first.