# crosvm - The Chrome OS Virtual Machine Monitor

This component, known as crosvm, runs untrusted operating systems along with
virtualized devices. This only runs VMs through the Linux's KVM interface. What
makes crosvm unique is a focus on safety within the programming language and a
sandbox around the virtual devices to protect the kernel from attack in case of
an exploit in the devices.

## IRC

The channel #crosvm on [freenode](https://webchat.freenode.net/#crosvm) is used
for technical discussion related to crosvm development and integration.

## Getting started

### Building for CrOS

crosvm on Chromium OS is built with Portage, so it follows the same general
workflow as any `cros_workon` package. The full package name is
`chromeos-base/crosvm`.

See the [Chromium OS developer guide] for more on how to build and deploy with
Portage.

[Chromium OS developer guide]: https://chromium.googlesource.com/chromiumos/docs/+/HEAD/developer_guide.md

### Building with Docker

See the [README](ci/README.md) from the `ci` subdirectory to learn how
to build and test crosvm in enviroments outside of the Chrome OS chroot.

### Building for Linux

>**NOTE:** Building for Linux natively is new and not fully supported.

First, [set up depot_tools] and use `repo` to sync down the crosvm source
tree. This is a subset of the entire Chromium OS manifest with just enough repos
to build crosvm.

```sh
mkdir crosvm
cd crosvm
repo init -g crosvm -u https://chromium.googlesource.com/chromiumos/manifest.git --repo-url=https://chromium.googlesource.com/external/repo.git
repo sync
```

A basic crosvm build links against `libcap`. On a Debian-based system,
you can install `libcap-dev`.

Handy Debian one-liner for all build and runtime deps, particularly if you're
running Crostini:
```sh
sudo apt install build-essential libcap-dev libgbm-dev libvirglrenderer-dev libwayland-bin libwayland-dev pkg-config protobuf-compiler python wayland-protocols
```

Known issues:
*   Seccomp policy files have hardcoded absolute paths. You can either fix up
    the paths locally, or set up an awesome hacky symlink: `sudo mkdir
    /usr/share/policy && sudo ln -s /path/to/crosvm/seccomp/x86_64
    /usr/share/policy/crosvm`. We'll eventually build the precompiled
    policies [into the crosvm binary](http://crbug.com/1052126).
*   Devices can't be jailed if `/var/empty` doesn't exist. `sudo mkdir -p
    /var/empty` to work around this for now.
*   You need read/write permissions for `/dev/kvm` to run tests or other crosvm
    instances. Usually it's owned by the `kvm` group, so `sudo usermod -a -G kvm
    $USER` and then log out and back in again to fix this.
*   Some other features (networking) require `CAP_NET_ADMIN` so those usually
    need to be run as root.

And that's it! You should be able to `cargo build/run/test`.

[set up depot_tools]: https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/depot_tools_tutorial.html#_setting_up

## Usage

To see the usage information for your version of crosvm, run `crosvm` or `crosvm
run --help`.

### Boot a Kernel

To run a very basic VM with just a kernel and default devices:

```bash
$ crosvm run "${KERNEL_PATH}"
```

The uncompressed kernel image, also known as vmlinux, can be found in your kernel
build directory in the case of x86 at `arch/x86/boot/compressed/vmlinux`.

### Rootfs

#### With a disk image

In most cases, you will want to give the VM a virtual block device to use as a
root file system:

```bash
$ crosvm run -r "${ROOT_IMAGE}" "${KERNEL_PATH}"
```

The root image must be a path to a disk image formatted in a way that the kernel
can read. Typically this is a squashfs image made with `mksquashfs` or an ext4
image made with `mkfs.ext4`. By using the `-r` argument, the kernel is
automatically told to use that image as the root, and therefore can only be
given once. More disks can be given with `-d` or `--rwdisk` if a writable disk
is desired.

To run crosvm with a writable rootfs:

>**WARNING:** Writable disks are at risk of corruption by a malicious or
malfunctioning guest OS.
```bash
crosvm run --rwdisk "${ROOT_IMAGE}" -p "root=/dev/vda" vmlinux
```
>**NOTE:** If more disks arguments are added prior to the desired rootfs image,
the `root=/dev/vda` must be adjusted to the appropriate letter.

#### With virtiofs

Linux kernel 5.4+ is required for using virtiofs. This is convenient for testing.
The file system must be named "mtd*" or "ubi*".

```bash
crosvm run --shared-dir "/:mtdfake:type=fs:cache=always" \
    -p "rootfstype=virtiofs root=mtdfake" vmlinux
```

### Control Socket

If the control socket was enabled with `-s`, the main process can be controlled
while crosvm is running. To tell crosvm to stop and exit, for example:

>**NOTE:** If the socket path given is for a directory, a socket name underneath
that path will be generated based on crosvm's PID.
```bash
$ crosvm run -s /run/crosvm.sock ${USUAL_CROSVM_ARGS}
    <in another shell>
$ crosvm stop /run/crosvm.sock
```
>**WARNING:** The guest OS will not be notified or gracefully shutdown.

This will cause the original crosvm process to exit in an orderly fashion,
allowing it to clean up any OS resources that might have stuck around if crosvm
were terminated early.

### Multiprocess Mode

By default crosvm runs in multiprocess mode. Each device that supports running
inside of a sandbox will run in a jailed child process of crosvm. The
appropriate minijail seccomp policy files must be present either in
`/usr/share/policy/crosvm` or in the path specified by the
`--seccomp-policy-dir` argument. The sandbox can be disabled for testing with
the `--disable-sandbox` option.

### Virtio Wayland

Virtio Wayland support requires special support on the part of the guest and as
such is unlikely to work out of the box unless you are using a Chrome OS kernel
along with a `termina` rootfs.

To use it, ensure that the `XDG_RUNTIME_DIR` enviroment variable is set and that
the path `$XDG_RUNTIME_DIR/wayland-0` points to the socket of the Wayland
compositor you would like the guest to use.

### GDB Support

crosvm supports [GDB Remote Serial Protocol] to allow developers to debug guest
kernel via GDB.

You can enable the feature by `--gdb` flag:

```sh
# Use uncompressed vmlinux
$ crosvm run --gdb <port> ${USUAL_CROSVM_ARGS} vmlinux
```

Then, you can start GDB in another shell.

```sh
$ gdb vmlinux
(gdb) target remote :<port>
(gdb) hbreak start_kernel
(gdb) c
<start booting in the other shell>
```

For general techniques for debugging the Linux kernel via GDB, see this
[kernel documentation].

[GDB Remote Serial Protocol]: https://sourceware.org/gdb/onlinedocs/gdb/Remote-Protocol.html
[kernel documentation]: https://www.kernel.org/doc/html/latest/dev-tools/gdb-kernel-debugging.html

## Defaults

The following are crosvm's default arguments and how to override them.

* 256MB of memory (set with `-m`)
* 1 virtual CPU (set with `-c`)
* no block devices (set with `-r`, `-d`, or `--rwdisk`)
* no network (set with `--host_ip`, `--netmask`, and `--mac`)
* virtio wayland support if `XDG_RUNTIME_DIR` enviroment variable is set (disable with `--no-wl`)
* only the kernel arguments necessary to run with the supported devices (add more with `-p`)
* run in multiprocess mode (run in single process mode with `--disable-sandbox`)
* no control socket (set with `-s`)

## System Requirements

A Linux kernel with KVM support (check for `/dev/kvm`) is required to run
crosvm. In order to run certain devices, there are additional system
requirements:

* `virtio-wayland` - The `memfd_create` syscall, introduced in Linux 3.17, and a Wayland compositor.
* `vsock` - Host Linux kernel with vhost-vsock support, introduced in Linux 4.8.
* `multiprocess` - Host Linux kernel with seccomp-bpf and Linux namespacing support.
* `virtio-net` - Host Linux kernel with TUN/TAP support (check for `/dev/net/tun`) and running with `CAP_NET_ADMIN` privileges.

## Emulated Devices

| Device           | Description                                                                        |
|------------------|------------------------------------------------------------------------------------|
| `CMOS/RTC`       | Used to get the current calendar time.                                             |
| `i8042`          | Used by the guest kernel to exit crosvm.                                           |
| `serial`         | x86 I/O port driven serial devices that print to stdout and take input from stdin. |
| `virtio-block`   | Basic read/write block device.                                                     |
| `virtio-net`     | Device to interface the host and guest networks.                                   |
| `virtio-rng`     | Entropy source used to seed guest OS's entropy pool.                               |
| `virtio-vsock`   | Enabled VSOCKs for the guests.                                                     |
| `virtio-wayland` | Allowed guest to use host Wayland socket.                                          |

## Contributing

### Code Health

#### `test_all`

Crosvm provides docker containers to build and run tests for both x86_64 and
aarch64, which can be run with the `./test_all` script.
See `ci/README.md` for more details on how to use the containers for local
development.

#### `rustfmt`

All code should be formatted with `rustfmt`. We have a script that applies
rustfmt to all Rust code in the crosvm repo: please run `bin/fmt` before
checking in a change. This is different from `cargo fmt --all` which formats
multiple crates but a single workspace only; crosvm consists of multiple
workspaces.

#### `clippy`

The `clippy` linter is used to check for common Rust problems.  The crosvm
project uses a specific set of `clippy` checks; please run `bin/clippy` before
checking in a change.

#### Dependencies

With a few exceptions, external dependencies inside of the `Cargo.toml` files
are not allowed. The reason being that community made crates tend to explode the
binary size by including dozens of transitive dependencies. All these
dependencies also must be reviewed to ensure their suitability to the crosvm
project. Currently allowed crates are:

* `cc` - Build time dependency needed to build C source code used in crosvm.
* `libc` - Required to use the standard library, this crate is a simple wrapper around `libc`'s symbols.

### Code Overview

The crosvm source code is written in Rust and C. To build, crosvm generally
requires the most recent stable version of rustc.

Source code is organized into crates, each with their own unit tests. These
crates are:

* `crosvm` - The top-level binary front-end for using crosvm.
* `devices` - Virtual devices exposed to the guest OS.
* `kernel_loader` - Loads elf64 kernel files to a slice of memory.
* `kvm_sys` - Low-level (mostly) auto-generated structures and constants for using KVM.
* `kvm` - Unsafe, low-level wrapper code for using `kvm_sys`.
* `net_sys` - Low-level (mostly) auto-generated structures and constants for creating TUN/TAP devices.
* `net_util` - Wrapper for creating TUN/TAP devices.
* `sys_util` - Mostly safe wrappers for small system facilities such as `eventfd` or `syslog`.
* `syscall_defines` - Lists of syscall numbers in each architecture used to make syscalls not supported in `libc`.
* `vhost` - Wrappers for creating vhost based devices.
* `virtio_sys` - Low-level (mostly) auto-generated structures and constants for interfacing with kernel vhost support.
* `vm_control` - IPC for the VM.
* `x86_64` - Support code specific to 64 bit intel machines.

The `seccomp` folder contains minijail seccomp policy files for each sandboxed
device. Because some syscalls vary by architecture, the seccomp policies are
split by architecture.