OpenShell/crates/openshell-driver-vm

openshell-driver-vm

Status: Experimental. The VM compute driver is under active development and the interface still has VM-specific plumbing that will be generalized.

Standalone libkrun-backed ComputeDriver for OpenShell. The gateway spawns this binary as a subprocess, talks to it over a Unix domain socket with the openshell.compute.v1.ComputeDriver gRPC surface, and lets it manage per-sandbox microVMs. The runtime (libkrun + libkrunfw + gvproxy), guest OCI unpacker, and sandbox supervisor are embedded directly in the binary; each sandbox boots from a cached immutable bootstrap ext4 root disk plus a per-sandbox writable overlay disk. When the requested sandbox image differs from the bootstrap image, the driver prepares a read-only image ext4 disk inside a bootstrap VM and mounts that unpacked rootfs as the sandbox lowerdir.

How it fits together

flowchart LR
    subgraph host["Host process"]
        gateway["openshell-server<br/>(compute::vm::spawn)"]
        driver["openshell-driver-vm<br/>├── libkrun (VM)<br/>├── gvproxy (net)<br/>└── openshell-sandbox.zst"]
        gateway <-->|"gRPC over UDS<br/>compute-driver.sock"| driver
    end

    subgraph guest["Per-sandbox microVM"]
        init["/srv/openshell-vm-<br/>sandbox-init.sh"]
        supervisor["/opt/openshell/bin/<br/>openshell-sandbox<br/>(PID 1)"]
        init --> supervisor
    end

    driver -->|"CreateSandbox<br/>boots via libkrun"| guest
    supervisor -.->|"gRPC callback<br/>--grpc-endpoint"| gateway

    client["openshell-cli"] -->|"SSH proxy<br/>127.0.0.1:&lt;port&gt;"| supervisor
    client -->|"CreateSandbox / Watch"| gateway

Sandbox guests execute /opt/openshell/bin/openshell-sandbox as PID 1 inside the VM. gvproxy exposes a single inbound SSH port (host:<allocated> → guest:2222) and provides virtio-net egress.

Quick start (recommended)

mise run gateway:vm

First run takes a few minutes while mise run vm:setup stages libkrun/libkrunfw/gvproxy/umoci and mise run vm:supervisor builds the bundled guest supervisor. Subsequent runs are cached.

By default mise run gateway:vm:

• Listens on plaintext HTTP at 127.0.0.1:18081.
• Registers the CLI gateway vm-dev by writing ~/.config/openshell/gateways/vm-dev/metadata.json. It does not modify the workspace .env.
• Persists the gateway SQLite DB under .cache/gateway-vm/gateway.db.
• Places the VM driver state (per-sandbox overlay.ext4, image cache, and run/compute-driver.sock) under /tmp/openshell-vm-driver-$USER-vm-dev/ so the AF_UNIX socket path stays under macOS SUN_LEN.
• Writes .cache/gateway-vm/gateway.toml with [openshell.drivers.vm].driver_dir = "$PWD/target/debug" so the freshly built openshell-driver-vm is used instead of an older installed copy from ~/.local/libexec/openshell, /usr/libexec/openshell, or /usr/local/libexec.

For GPU passthrough (VFIO), pass -- --gpu and run with root privileges:

sudo -E env "PATH=$PATH" mise run gateway:vm -- --gpu

GPU passthrough uses VFIO and requires host support for IOMMU, root privileges for bind/unbind operations, and a compatible sandbox image. The public GPU overview lives in the repository README.md.

Point the CLI at the gateway with one of:

openshell --gateway vm-dev status
openshell gateway select vm-dev    # then plain `openshell <command>`

Override defaults via environment:

# custom port (fails fast if in use)
OPENSHELL_SERVER_PORT=18091 mise run gateway:vm

# custom CLI gateway name + namespace
OPENSHELL_VM_GATEWAY_NAME=vm-feature-a \
OPENSHELL_SANDBOX_NAMESPACE=vm-feature-a \
mise run gateway:vm

# custom sandbox image
OPENSHELL_SANDBOX_IMAGE=ghcr.io/example/sandbox:latest mise run gateway:vm

# custom bootstrap image for the VM runtime used to prepare/boot target images
OPENSHELL_VM_BOOTSTRAP_IMAGE=ghcr.io/example/bootstrap:latest mise run gateway:vm

Teardown:

rm -rf /tmp/openshell-vm-driver-$USER-vm-dev .cache/gateway-vm
rm -rf "${XDG_CONFIG_HOME:-$HOME/.config}/openshell/gateways/vm-dev"

Manual equivalent

If you want to drive the launch yourself instead of using mise run gateway:vm (i.e. tasks/scripts/gateway-vm.sh):

# 1. Stage runtime artifacts + supervisor bundle into target/vm-runtime-compressed/
mise run vm:setup
mise run vm:supervisor          # if openshell-sandbox.zst is not already present

# 2. Build both binaries with the staged artifacts embedded
OPENSHELL_VM_RUNTIME_COMPRESSED_DIR=$PWD/target/vm-runtime-compressed \
  cargo build -p openshell-server -p openshell-driver-vm

# 3. macOS only: codesign the driver for Hypervisor.framework
codesign \
  --entitlements crates/openshell-driver-vm/entitlements.plist \
  --force -s - target/debug/openshell-driver-vm

# 4. Start the gateway with the VM driver
mkdir -p /tmp/openshell-vm-driver-$USER-vm-dev .cache/gateway-vm
cat > .cache/gateway-vm/gateway.toml <<EOF
[openshell]
version = 1

[openshell.gateway]
compute_drivers = ["vm"]
disable_tls = true

[openshell.drivers.vm]
default_image = "<compatible-image>"
grpc_endpoint = "http://host.containers.internal:18081"
driver_dir = "$PWD/target/debug"
state_dir = "/tmp/openshell-vm-driver-$USER-vm-dev"
EOF

target/debug/openshell-gateway \
  --config .cache/gateway-vm/gateway.toml \
  --drivers vm \
  --disable-tls \
  --db-url "sqlite:.cache/gateway-vm/gateway.db?mode=rwc" \
  --port 18081

The gateway resolves openshell-driver-vm in this order: [openshell.drivers.vm].driver_dir, conventional install locations (~/.local/libexec/openshell, /usr/libexec/openshell, /usr/local/libexec/openshell, /usr/local/libexec), then a sibling of the gateway binary.

Gateway And Driver Configuration

Select the VM driver with --drivers vm, OPENSHELL_DRIVERS=vm, or compute_drivers = ["vm"] in [openshell.gateway]. Configure VM-specific settings in [openshell.drivers.vm].

Configuration key	Default	Purpose
grpc_endpoint	empty	Required. URL the sandbox guest dials to reach the gateway. Use http://host.containers.internal:<port> (or host.docker.internal / host.openshell.internal) so traffic flows through gvproxy's host-loopback NAT (HostIP 192.168.127.254 → host 127.0.0.1). Loopback URLs like http://127.0.0.1:<port> are rewritten automatically by the driver. The bare gateway IP (192.168.127.1) only carries gvproxy's own services and will not reach host-bound ports.
state_dir	target/openshell-vm-driver	Per-sandbox overlay disks, console logs, image cache, and private run/compute-driver.sock UDS.
driver_dir	unset	Override the directory searched for openshell-driver-vm.
default_image	OpenShell base image	Sandbox image used when a create request omits one.
bootstrap_image	unset	VM runtime image used as the immutable bootstrap root disk. Defaults to the sandbox image when unset.
vcpus	2	vCPUs per sandbox.
mem_mib	2048	Memory per sandbox, in MiB.
overlay_disk_mib	4096	Sparse writable overlay disk size per sandbox, in MiB.
krun_log_level	1	libkrun verbosity (0-5).
guest_tls_ca	unset	CA cert for the guest's mTLS client bundle. Required when grpc_endpoint uses https://.
guest_tls_cert	unset	Guest client certificate.
guest_tls_key	unset	Guest client private key.

See openshell-gateway --help for the gateway process flag surface.

Verifying the gateway

The gateway is auto-registered by mise run gateway:vm. In another terminal:

./scripts/bin/openshell status
./scripts/bin/openshell sandbox create --name demo --from <compatible-image>
./scripts/bin/openshell sandbox connect demo

First sandbox takes 10–30 seconds to boot (image fetch/prepare/cache + libkrun + guest init). If --from is omitted, the VM driver uses the gateway's configured default sandbox image. Without either --from or --sandbox-image, VM sandbox creation fails. Subsequent creates reuse the prepared image cache and create only a sparse per-sandbox overlay.ext4 before boot.

CreateSandbox accepts the sandbox quickly and continues VM provisioning in the background. The driver publishes platform events for image resolution, cache hits/misses, layer pulls, rootfs preparation, overlay creation, and VM launcher startup so the CLI can show progress through the existing sandbox watch stream.

The VM driver keeps two image caches. The bootstrap cache is a controlled rootfs.ext4 used to boot the guest init and OpenShell supervisor. The prepared image cache is used when the requested sandbox image differs from the bootstrap image: the host downloads registry layers into a valid OCI layout, attaches that payload to a temporary bootstrap VM, and guest init runs umoci raw unpack onto Linux-owned ext4 storage. The resulting disk is cached under <state-dir>/images/<cache-id>/rootfs.ext4 and attached read-only to later sandboxes. Local Docker images are still exported as rootfs tar archives and prepared inside the bootstrap VM. Set OPENSHELL_VM_IMAGE_PULL_CONCURRENCY to tune registry layer download parallelism (default 4, maximum 16). Both caches are scoped by source image identity and OpenShell version, so an OpenShell upgrade builds a fresh guest rootfs instead of reusing one with an old embedded supervisor.

Each sandbox gets its own sparse writable <state-dir>/sandboxes/<id>/overlay.ext4. Guest init mounts overlayfs as / with the prepared image rootfs as lowerdir when present, otherwise the bootstrap rootfs is used directly. Writes to /sandbox and other mutable paths land in the overlay while cached image disks remain unchanged. The overlay disk must be large enough to hold the compressed payload, unpacked rootfs, and sandbox writes during the first prepare.

The driver also writes the accepted DriverSandbox launch request to <state-dir>/sandboxes/<id>/sandbox.pb. If the gateway restarts, it starts a new VM driver process; that process scans the sandbox state directories, restarts each persisted VM launcher, and preserves any existing overlay.ext4 instead of cloning a fresh overlay template. If a restart happened before the overlay was created, the driver creates it during the resume attempt.

Logs and debugging

Raise log verbosity for both processes:

RUST_LOG=openshell_server=debug,openshell_driver_vm=debug \
  mise run gateway:vm

The VM guest's serial console is appended to <state-dir>/<sandbox-id>/console.log. Sandbox IDs must match [A-Za-z0-9._-]{1,128} before the driver uses them in host paths. The gateway-owned compute-driver socket lives at <state-dir>/run/compute-driver.sock; OpenShell creates run/ with owner-only permissions, removes same-owner stale sockets, and the gateway removes the socket on clean shutdown via ManagedDriverProcess::drop. UDS clients must match the driver UID and provide the expected gateway process PID by default. Standalone same-UID UDS mode requires the explicit --allow-same-uid-peer development flag. TCP mode is disabled by default because it is unauthenticated; use --allow-unauthenticated-tcp --bind-address 127.0.0.1:50061 only for local development.

Host-side nftables rules

The VM driver creates a per-VM nftables table on the host (openshell_vm_vmtap_<id>) with three chains. These rules serve two purposes: NAT infrastructure (required for VM connectivity) and defense-in-depth host isolation. Primary security enforcement — proxy-only egress and bypass detection — is handled by the sandbox supervisor's own nftables rules inside the VM guest.

postrouting (NAT): Masquerades outbound VM traffic so it can be routed from the VM's private subnet to the external network. This chain handles forwarded traffic (VM → internet), not traffic destined for the host.

forward (defense-in-depth): Accepts all outbound traffic from the VM (security enforcement happens guest-side) and accepts established/related response traffic back to the VM. Drops unsolicited inbound connections to the VM from the broader network. This chain handles forwarded traffic only — packets transiting the host between the TAP interface and other interfaces.

input (defense-in-depth): Accepts traffic from the VM to the gateway port on the host. Drops all other traffic from the VM destined for the host itself. This limits what a compromised guest can reach on the host to the gateway service only.

The input and postrouting chains handle different traffic paths: input covers packets addressed to the host (VM → host), while postrouting covers packets the host is forwarding on behalf of the VM (VM → internet). A packet from the VM goes through one path or the other, never both.

All chains use policy accept, so non-TAP traffic is unaffected. Because nftables evaluates multiple base chains on the same hook independently, host firewalls interact with these rules as follows:

• Open host (no other firewall): Our chains are the only filter. The defense-in-depth drop rules block unsolicited inbound and non-gateway host access. Non-TAP traffic passes through.
• Restrictive host firewall (e.g. firewalld): The host firewall's chains may additionally drop TAP traffic that our chains accept. A drop verdict from any chain is final — our accept cannot override it. If VM connectivity fails, verify that the host firewall allows forwarding and input for vmtap-* interfaces.

Each table is created atomically via nft -f on VM start and torn down atomically via nft delete table when the VM is destroyed.

Prerequisites

• macOS on Apple Silicon, or Linux on aarch64/x86_64 with KVM
• Rust toolchain
• e2fsprogs (mke2fs or mkfs.ext4, plus debugfs) for root and overlay disk image creation and QEMU environment injection
• Guest-supervisor cross-compile toolchain (needed on macOS, and on Linux when host arch ≠ guest arch):
  • Matching rustup target: rustup target add aarch64-unknown-linux-gnu (or x86_64-unknown-linux-gnu for an amd64 guest)
  • cargo install --locked cargo-zigbuild and brew install zig (or distro equivalent). vm:supervisor uses cargo zigbuild to cross-compile the in-VM openshell-sandbox supervisor binary.
• mise task runner
• Docker or Podman socket on the local CLI/gateway host when using openshell sandbox create --from ./Dockerfile or --from ./dir; the CLI builds the image and the VM driver exports it via the local container engine. Docker is tried first; if unavailable, the driver falls back to the Podman socket. On Linux, enable the Podman API socket with systemctl --user start podman.socket
• gh CLI (used by mise run vm:setup to download pre-built runtime artifacts)

Releases

openshell-driver-vm is published as a normal OpenShell release artifact:

• development builds: the rolling dev release
• tagged builds: the corresponding v* release
• runtime tarballs: the rolling vm-runtime release, rebuilt on demand by release-vm-kernel.yml

On Debian-family Linux amd64 and arm64 systems, install.sh installs the Debian package from the selected OPENSHELL_VERSION release tag. That package includes openshell-gateway and openshell-driver-vm, but leaves OPENSHELL_DRIVERS unset so the gateway uses its normal runtime auto-detection. Set OPENSHELL_DRIVERS=vm to force the VM driver.

On RPM-family Linux x86_64 and aarch64 systems, install.sh installs the openshell and openshell-gateway RPM packages from the selected release tag. The RPM gateway package is configured for the Podman driver.

On Apple Silicon macOS, install.sh stages the generated openshell.rb formula from the selected release in the nvidia/openshell Homebrew tap. Homebrew installs openshell, openshell-gateway, and openshell-driver-vm, ad-hoc signs the driver with the Hypervisor entitlement in post_install, and owns the brew services gateway lifecycle. The service also leaves OPENSHELL_DRIVERS unset so driver choice remains automatic unless the user explicitly overrides it.

TODOs

• The gateway still configures the driver via CLI args; this will move to a gRPC bootstrap call so the driver interface is uniform across backends. See the TODO(driver-abstraction) notes in crates/openshell-server/src/lib.rs and crates/openshell-server/src/compute/vm.rs.
• macOS local builds are codesigned by tasks/scripts/gateway-vm.sh; the generated Homebrew formula signs the release tarball driver for local installs.