From c051cd97174a4a9df33368871e0be6c9ea617803 Mon Sep 17 00:00:00 2001 From: atlas Date: Fri, 31 Jul 2026 18:34:34 +0200 Subject: [PATCH] docs(#2862): document the snapshot store, drop the dedicated option MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit mara: the option was the wrong shape for the concern. "this host runs nothing else" is a deployment expectation, not something a module should assert about its own host — and asserting it made co-location look like a config toggle rather than what it is. Replaced with docs/snapshot-store.md, which the module had no docs page at all before: enabling it, why the mesh is the authentication (cryptokey routing already binds source address to pubkey, so certs would authenticate the same fact twice and add an expiry), why the destination is keyed per agent (a per-hive prefix splits an agent's chain the first time it migrates), what the sender may and may not choose, why the firewall rule is interface-scoped, what a snapshot does and does not contain, and what the pull side still needs. The dedicated-host expectation is stated there as an operational assumption with its own failure mode — true on day one, quietly false the day someone notices the box has spare disk — rather than as an assertion someone flips to false to make the build proceed. Linked from CLAUDE.md's reading paths. --- CLAUDE.md | 4 + docs/snapshot-store.md | 186 +++++++++++++++++++++++ nix/host-modules/hive-snapshot-store.nix | 36 +---- 3 files changed, 193 insertions(+), 33 deletions(-) create mode 100644 docs/snapshot-store.md diff --git a/CLAUDE.md b/CLAUDE.md index 1c72fa66..9d811e00 100644 --- a/CLAUDE.md +++ b/CLAUDE.md @@ -165,6 +165,10 @@ read them à la carte. - **"How do I connect two hives into a swarm? How do I declare peer hives and configure TLS trust?"** → [`docs/swarm.md`](docs/swarm.md). +- **"Where do agent snapshots go? How does the swarm's `btrfs receive` + endpoint authenticate a pushing hive, and what does a snapshot + actually contain?"** → + [`docs/snapshot-store.md`](docs/snapshot-store.md). - **"How does the rebuild queue work? What are queue kinds and sources?"** → [`docs/coordinator.md`](docs/coordinator.md). - **"How does the CI runner work? What's the auto-registration flow?"** → diff --git a/docs/snapshot-store.md b/docs/snapshot-store.md new file mode 100644 index 00000000..8bacdd21 --- /dev/null +++ b/docs/snapshot-store.md @@ -0,0 +1,186 @@ +# Snapshot store + +The swarm's `btrfs receive` endpoint. Hives push agent snapshots to it +over the WireGuard mesh; a destination hive later pulls one back to +complete a migration. + +Two things it is not, both worth stating because both are easy to +assume: + +- **It is not the swarm controller**, and does not depend on one. It is + a NixOS host role: a btrfs subvolume tree, a socket-activated + receiver, and the `wg-hive` interface the swarm module already brings + up. That is why it can be deployed before any controller exists. +- **It is not a backup product.** It happens to hold the data a backup + would hold, and it should be operated accordingly (see + [Operating it](#operating-it)) --- but nothing in it does scheduling, + verification, or restore orchestration. + +## Enabling it + +```nix +services.hyperhive.snapshotStore = { + enable = true; + path = "/var/lib/hyperhive-snapshots"; # must be on btrfs + port = 51821; +}; + +# The mesh is a hard requirement, and is asserted: +services.hyperhive.swarm.wireguard = { + enable = true; + address = "10.100.0.9/24"; + privateKeyFile = "/etc/wireguard/hive.key"; +}; +``` + +The store host is a swarm member like any other: peers declare it, and +it declares them, through `services.hyperhive.swarm.peers`. See +[swarm.md](swarm.md) for the mesh itself. + +Note that the mesh is gated on `swarm.wireguard.enable`, **not** on +`c0re.enable` --- a store host runs no hive and would otherwise get no +`wg-hive` interface at all. + +## The mesh is the authentication + +There are no certificates here, and no key material of its own. That is +deliberate rather than an omission. + +WireGuard's cryptokey routing already binds a peer's source address to +its public key: the swarm module configures each peer with +`allowedIPs = [ peer.wireguardAddress ]`, so a packet arriving from +that address provably came from the holder of that private key. A +packet that reaches the receiver has therefore already been +authenticated by the kernel. + +Layering TLS client certs on top would authenticate *the same fact* a +second time, and add a credential with an expiry --- a migration that +fails because a renewal quietly didn't happen, discovered on the day +you need to move an agent. + +## One subvolume per agent, not per hive + +The destination is keyed by **agent**. + +This is not cosmetic. After a migration, an agent's next incremental +send arrives from a *different* hive than the previous one. Keying by +hive would split that agent's snapshot chain across two directories, +and `btrfs send -p` would fail to find its parent --- breaking exactly +the case the store exists to serve. + +## What the sender can and cannot choose + +A `btrfs send` stream carries no notion of *which agent* it belongs to, +and the subvolume name inside it is chosen by the sender. So the +protocol is one `agent ` header line, then the raw stream. + +The rule that matters: + +> **The receiver owns the destination root. The sender-supplied name is +> validated, never used as a path.** + +Validation is a whitelist --- `[A-Za-z0-9_-]+` and nothing else. No +slash and no dot means neither directory traversal nor an absolute path +can survive it. It is deliberately a whitelist and not a list of +forbidden characters: a blocklist only ever excludes the attacks +somebody already thought of. + +## Reachability + +The receiver is socket-activated, and the socket binds **this host's +mesh address**, never a wildcard. Both the mesh being enabled and the +address being set are assertions, not documentation --- bound to +`0.0.0.0` this socket is an unauthenticated remote write into agent +state. + +Binding is not sufficient on its own. NixOS's firewall is default-deny +and filters in netfilter, *before* a packet reaches a bound socket, so +the port is opened explicitly --- and scoped to the mesh interface: + +```nix +networking.firewall.interfaces.wg-hive.allowedTCPPorts = [ cfg.port ]; +``` + +A host-wide `allowedTCPPorts` would open the port on every interface +including a public NIC, leaving only the socket's bind address between +the internet and a root `btrfs receive`. + +## Operating it + +### Confinement is the deployment's job + +`btrfs receive` needs `CAP_SYS_ADMIN`, so the receiver runs as root. +The unit sets `ProtectSystem=strict`, `ProtectHome`, `PrivateTmp` and a +narrow `ReadWritePaths` --- but those are **defence in depth, not a +boundary**: a process holding `CAP_SYS_ADMIN` can call `mount(2)` and +undo the namespace they set up. + +The boundary is the machine. The intended deployments are: + +- **a swarm**: the store is its own small VM. The machine is the + boundary, which is stronger than anything the unit could assert about + itself. +- **all-in-one / local**: the store runs as a container on the c0re + host. + +The second is worth keeping deliberately, and not only for +convenience: it means the confined path is exercised by every local +deployment. The usual failure mode for an isolated variant is that +nobody runs it day to day, so it rots and is discovered broken in +production. + +⚠️ **The assumption to keep true over time:** the store host runs +nothing else. That is true on day one and quietly false the day someone +notices the box has spare disk. Nothing in the config objects when it +stops being true. + +### It holds every agent's state from every hive + +Which makes it the highest-value target in the swarm by a wide margin, +and means it should get the treatment a backup host gets --- restricted +access, and a decision (rather than an omission) on encryption at rest. + +The trap is the label: this box holds backup-grade data while not being +called a backup, so it can end up with backup-grade *exposure* and +non-backup-grade *controls*. Nobody puts a migration staging area on +the access-review list. + +### What a snapshot contains + +The snapshot covers an agent's **state subvolume**, which is the parent +of `state/`, `claude/` and `harness/`. Consequences: + +- The Claude session (`claude/`) travels, so a restored agent keeps its + live `--continue` session rather than needing to log in again. +- `harness/` travels too, including `harness/bash-tasks/`. Task output + is part of an agent's working continuity, so this is wanted --- but it + means anything that has ever leaked into a task's captured output is + in the retained snapshots as well. + +It does **not** cover the agent's applied config (`/applied//`) or +its topology entry, both of which live outside the subvolume. A restore +therefore yields an agent's memory without its definition; closing that +gap is tracked separately. + +### Retention + +Retention lives on the *sending* side (last-N by count, swept +periodically), not here. Count rather than age is deliberate: a count +is bounded by construction, whereas an age policy silently scales disk +usage with how hot a hive runs. + +Per-agent or per-hive `btrfs qgroup` quotas are not configured yet. +Without them one runaway hive can fill the store and take out every +other hive's snapshots. + +## Not built yet + +**The pull side.** Push is safe with minimal authorisation because a +hive can only ever write to a chain it owns. Pull is the direction that +needs a policy: unrestricted, any compromised hive could read every +agent's state from every other hive. It needs a notion of which hive +currently owns which agent, and that ownership record lands with the +swarm controller work. + +With a single hive the question is trivial --- the only peer owns +everything it sends --- which is why the receive half ships first. diff --git a/nix/host-modules/hive-snapshot-store.nix b/nix/host-modules/hive-snapshot-store.nix index 2a31c5f8..5ffa3cbf 100644 --- a/nix/host-modules/hive-snapshot-store.nix +++ b/nix/host-modules/hive-snapshot-store.nix @@ -16,7 +16,8 @@ # Confinement is a property of the DEPLOYMENT, not of this unit: in a # real swarm the store is its own small VM (the machine is the # boundary); in the all-local case it's a container on the c0re host. -# The module therefore hardcodes neither --- see `dedicated` below. +# The module hardcodes neither. docs/snapshot-store.md covers what the +# deployment is expected to provide. { pkgs, lib, @@ -129,23 +130,6 @@ in ''; }; - dedicated = lib.mkOption { - type = lib.types.bool; - default = true; - description = '' - Assert that this host runs no other hyperhive role. The store - aggregates every agent's state from every hive in the swarm, so - the intended deployment is a dedicated machine (or a container - in the all-local case) where the machine itself is the security - boundary. - - That assumption is true on day one and silently false the day - someone notices the box has spare disk. This option makes it a - thing the build checks rather than a thing the deployer - remembers. Set to `false` to co-locate deliberately --- the - point is that it becomes a decision, not an accident. - ''; - }; }; config = lib.mkIf cfg.enable { @@ -170,20 +154,6 @@ in fall back to a wildcard. ''; } - { - assertion = !cfg.dedicated || !config.services.hyperhive.c0re.enable; - message = '' - services.hyperhive.snapshotStore is enabled alongside - services.hyperhive.c0re on the same host. The store holds - every agent's state from every hive, so it is meant to run on - a machine of its own where the machine is the boundary. - - If the co-location is deliberate (the all-local single-host - deployment, where the store runs as a container), set - services.hyperhive.snapshotStore.dedicated = false to record - that decision explicitly. - ''; - } ]; # The store root must exist before the first connection arrives --- @@ -231,7 +201,7 @@ in # nature. The hardening below is defence in depth and NOT a # boundary: a process holding CAP_SYS_ADMIN can call mount(2) and # undo the namespace these directives set up. The real boundary is - # the deployment (dedicated host / container) --- see `dedicated`. + # the deployment --- see docs/snapshot-store.md. systemd.services."hive-snapshot-store@" = { description = "hyperhive swarm snapshot store receiver"; after = [ "hive-snapshot-store.socket" ];