virtualizationservice/src/aidl.rs

// Copyright 2021, The Android Open Source Project
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Implementation of the AIDL interface of the VirtualizationService.

use crate::composite::make_composite_image;
use crate::crosvm::{CrosvmConfig, DiskFile, VmInstance};
use crate::{Cid, FIRST_GUEST_CID};
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::IVirtualizationService::IVirtualizationService;
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::DiskImage::DiskImage;
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::IVirtualMachine::{
    BnVirtualMachine, IVirtualMachine,
};
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::IVirtualMachineCallback::IVirtualMachineCallback;
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::VirtualMachineConfig::VirtualMachineConfig;
use android_system_virtualizationservice::aidl::android::system::virtualizationservice::VirtualMachineDebugInfo::VirtualMachineDebugInfo;
use android_system_virtualizationservice::binder::{
    self, BinderFeatures, ExceptionCode, Interface, ParcelFileDescriptor, Status, Strong, ThreadState,
};
use disk::QcowFile;
use log::{debug, error, warn};
use std::convert::TryInto;
use std::ffi::CString;
use std::fs::{File, create_dir};
use std::os::unix::io::AsRawFd;
use std::path::{Path, PathBuf};
use std::sync::{Arc, Mutex, Weak};

pub const BINDER_SERVICE_IDENTIFIER: &str = "android.system.virtualizationservice";

/// Directory in which to write disk image files used while running VMs.
const TEMPORARY_DIRECTORY: &str = "/data/misc/virtualizationservice";

// TODO(qwandor): Use PermissionController once it is available to Rust.
/// Only processes running with one of these UIDs are allowed to call debug methods.
const DEBUG_ALLOWED_UIDS: [u32; 2] = [0, 2000];

/// Implementation of `IVirtualizationService`, the entry point of the AIDL service.
#[derive(Debug, Default)]
pub struct VirtualizationService {
    state: Mutex<State>,
}

impl Interface for VirtualizationService {}

impl IVirtualizationService for VirtualizationService {
    /// Create and start a new VM with the given configuration, assigning it the next available CID.
    ///
    /// Returns a binder `IVirtualMachine` object referring to it, as a handle for the client.
    fn startVm(
        &self,
        config: &VirtualMachineConfig,
        log_fd: Option<&ParcelFileDescriptor>,
    ) -> binder::Result<Strong<dyn IVirtualMachine>> {
        let state = &mut *self.state.lock().unwrap();
        let log_fd = log_fd.map(clone_file).transpose()?;
        let requester_uid = ThreadState::get_calling_uid();
        let requester_sid = get_calling_sid()?;
        let requester_debug_pid = ThreadState::get_calling_pid();
        let cid = state.allocate_cid()?;

        // Counter to generate unique IDs for temporary image files.
        let mut next_temporary_image_id = 0;
        // Files which are referred to from composite images. These must be mapped to the crosvm
        // child process, and not closed before it is started.
        let mut indirect_files = vec![];

        // Make directory for temporary files.
        let temporary_directory: PathBuf = format!("{}/{}", TEMPORARY_DIRECTORY, cid).into();
        create_dir(&temporary_directory).map_err(|e| {
            error!(
                "Failed to create temporary directory {:?} for VM files: {}",
                temporary_directory, e
            );
            new_binder_exception(
                ExceptionCode::SERVICE_SPECIFIC,
                format!(
                    "Failed to create temporary directory {:?} for VM files: {}",
                    temporary_directory, e
                ),
            )
        })?;

        // Assemble disk images if needed.
        let disks = config
            .disks
            .iter()
            .map(|disk| {
                assemble_disk_image(
                    disk,
                    &temporary_directory,
                    &mut next_temporary_image_id,
                    &mut indirect_files,
                )
            })
            .collect::<Result<Vec<DiskFile>, _>>()?;

        // Actually start the VM.
        let crosvm_config = CrosvmConfig {
            cid,
            bootloader: as_asref(&config.bootloader),
            kernel: as_asref(&config.kernel),
            initrd: as_asref(&config.initrd),
            disks,
            params: config.params.to_owned(),
            protected: config.protected_vm,
        };
        let composite_disk_fds: Vec<_> =
            indirect_files.iter().map(|file| file.as_raw_fd()).collect();
        let instance = VmInstance::start(
            &crosvm_config,
            log_fd,
            &composite_disk_fds,
            temporary_directory,
            requester_uid,
            requester_sid,
            requester_debug_pid,
        )
        .map_err(|e| {
            error!("Failed to start VM with config {:?}: {}", config, e);
            new_binder_exception(
                ExceptionCode::SERVICE_SPECIFIC,
                format!("Failed to start VM: {}", e),
            )
        })?;
        state.add_vm(Arc::downgrade(&instance));
        Ok(VirtualMachine::create(instance))
    }

    /// Initialise an empty partition image of the given size to be used as a writable partition.
    fn initializeWritablePartition(
        &self,
        image_fd: &ParcelFileDescriptor,
        size: i64,
    ) -> binder::Result<()> {
        let size = size.try_into().map_err(|e| {
            new_binder_exception(
                ExceptionCode::ILLEGAL_ARGUMENT,
                format!("Invalid size {}: {}", size, e),
            )
        })?;
        let image = clone_file(image_fd)?;

        QcowFile::new(image, size).map_err(|e| {
            new_binder_exception(
                ExceptionCode::SERVICE_SPECIFIC,
                format!("Failed to create QCOW2 image: {}", e),
            )
        })?;

        Ok(())
    }

    /// Get a list of all currently running VMs. This method is only intended for debug purposes,
    /// and as such is only permitted from the shell user.
    fn debugListVms(&self) -> binder::Result<Vec<VirtualMachineDebugInfo>> {
        check_debug_access()?;

        let state = &mut *self.state.lock().unwrap();
        let vms = state.vms();
        let cids = vms
            .into_iter()
            .map(|vm| VirtualMachineDebugInfo {
                cid: vm.cid as i32,
                temporaryDirectory: vm.temporary_directory.to_string_lossy().to_string(),
                requesterUid: vm.requester_uid as i32,
                requesterSid: vm.requester_sid.clone(),
                requesterPid: vm.requester_debug_pid,
                running: vm.running(),
            })
            .collect();
        Ok(cids)
    }

    /// Hold a strong reference to a VM in VirtualizationService. This method is only intended for
    /// debug purposes, and as such is only permitted from the shell user.
    fn debugHoldVmRef(&self, vmref: &Strong<dyn IVirtualMachine>) -> binder::Result<()> {
        check_debug_access()?;

        let state = &mut *self.state.lock().unwrap();
        state.debug_hold_vm(vmref.clone());
        Ok(())
    }

    /// Drop reference to a VM that is being held by VirtualizationService. Returns the reference if
    /// the VM was found and None otherwise. This method is only intended for debug purposes, and as
    /// such is only permitted from the shell user.
    fn debugDropVmRef(&self, cid: i32) -> binder::Result<Option<Strong<dyn IVirtualMachine>>> {
        check_debug_access()?;

        let state = &mut *self.state.lock().unwrap();
        Ok(state.debug_drop_vm(cid))
    }
}

/// Given the configuration for a disk image, assembles the `DiskFile` to pass to crosvm.
///
/// This may involve assembling a composite disk from a set of partition images.
fn assemble_disk_image(
    disk: &DiskImage,
    temporary_directory: &Path,
    next_temporary_image_id: &mut u64,
    indirect_files: &mut Vec<File>,
) -> Result<DiskFile, Status> {
    let image = if !disk.partitions.is_empty() {
        if disk.image.is_some() {
            warn!("DiskImage {:?} contains both image and partitions.", disk);
            return Err(new_binder_exception(
                ExceptionCode::ILLEGAL_ARGUMENT,
                "DiskImage contains both image and partitions.",
            ));
        }

        let composite_image_filenames =
            make_composite_image_filenames(temporary_directory, next_temporary_image_id);
        let (image, partition_files) = make_composite_image(
            &disk.partitions,
            &composite_image_filenames.composite,
            &composite_image_filenames.header,
            &composite_image_filenames.footer,
        )
        .map_err(|e| {
            error!("Failed to make composite image with config {:?}: {}", disk, e);
            new_binder_exception(
                ExceptionCode::SERVICE_SPECIFIC,
                format!("Failed to make composite image: {}", e),
            )
        })?;

        // Pass the file descriptors for the various partition files to crosvm when it
        // is run.
        indirect_files.extend(partition_files);

        image
    } else if let Some(image) = &disk.image {
        clone_file(image)?
    } else {
        warn!("DiskImage {:?} didn't contain image or partitions.", disk);
        return Err(new_binder_exception(
            ExceptionCode::ILLEGAL_ARGUMENT,
            "DiskImage didn't contain image or partitions.",
        ));
    };

    Ok(DiskFile { image, writable: disk.writable })
}

/// Generates a unique filename to use for a composite disk image.
fn make_composite_image_filenames(
    temporary_directory: &Path,
    next_temporary_image_id: &mut u64,
) -> CompositeImageFilenames {
    let id = *next_temporary_image_id;
    *next_temporary_image_id += 1;
    CompositeImageFilenames {
        composite: temporary_directory.join(format!("composite-{}.img", id)),
        header: temporary_directory.join(format!("composite-{}-header.img", id)),
        footer: temporary_directory.join(format!("composite-{}-footer.img", id)),
    }
}

/// Filenames for a composite disk image, including header and footer partitions.
#[derive(Clone, Debug, Eq, PartialEq)]
struct CompositeImageFilenames {
    /// The composite disk image itself.
    composite: PathBuf,
    /// The header partition image.
    header: PathBuf,
    /// The footer partition image.
    footer: PathBuf,
}

/// Gets the calling SID of the current Binder thread.
fn get_calling_sid() -> Result<String, Status> {
    ThreadState::with_calling_sid(|sid| {
        if let Some(sid) = sid {
            match sid.to_str() {
                Ok(sid) => Ok(sid.to_owned()),
                Err(e) => {
                    error!("SID was not valid UTF-8: {}", e);
                    Err(new_binder_exception(
                        ExceptionCode::ILLEGAL_ARGUMENT,
                        format!("SID was not valid UTF-8: {}", e),
                    ))
                }
            }
        } else {
            error!("Missing SID on startVm");
            Err(new_binder_exception(ExceptionCode::SECURITY, "Missing SID on startVm"))
        }
    })
}

/// Check whether the caller of the current Binder method is allowed to call debug methods.
fn check_debug_access() -> binder::Result<()> {
    let uid = ThreadState::get_calling_uid();
    log::trace!("Debug method call from UID {}.", uid);
    if DEBUG_ALLOWED_UIDS.contains(&uid) {
        Ok(())
    } else {
        Err(new_binder_exception(ExceptionCode::SECURITY, "Debug access denied"))
    }
}

/// Implementation of the AIDL `IVirtualMachine` interface. Used as a handle to a VM.
#[derive(Debug)]
struct VirtualMachine {
    instance: Arc<VmInstance>,
}

impl VirtualMachine {
    fn create(instance: Arc<VmInstance>) -> Strong<dyn IVirtualMachine> {
        let binder = VirtualMachine { instance };
        BnVirtualMachine::new_binder(binder, BinderFeatures::default())
    }
}

impl Interface for VirtualMachine {}

impl IVirtualMachine for VirtualMachine {
    fn getCid(&self) -> binder::Result<i32> {
        Ok(self.instance.cid as i32)
    }

    fn isRunning(&self) -> binder::Result<bool> {
        Ok(self.instance.running())
    }

    fn registerCallback(
        &self,
        callback: &Strong<dyn IVirtualMachineCallback>,
    ) -> binder::Result<()> {
        // TODO: Should this give an error if the VM is already dead?
        self.instance.callbacks.add(callback.clone());
        Ok(())
    }
}

impl Drop for VirtualMachine {
    fn drop(&mut self) {
        debug!("Dropping {:?}", self);
        self.instance.kill();
    }
}

/// A set of Binders to be called back in response to various events on the VM, such as when it
/// dies.
#[derive(Debug, Default)]
pub struct VirtualMachineCallbacks(Mutex<Vec<Strong<dyn IVirtualMachineCallback>>>);

impl VirtualMachineCallbacks {
    /// Call all registered callbacks to say that the VM has died.
    pub fn callback_on_died(&self, cid: Cid) {
        let callbacks = &*self.0.lock().unwrap();
        for callback in callbacks {
            if let Err(e) = callback.onDied(cid as i32) {
                error!("Error calling callback: {}", e);
            }
        }
    }

    /// Add a new callback to the set.
    fn add(&self, callback: Strong<dyn IVirtualMachineCallback>) {
        self.0.lock().unwrap().push(callback);
    }
}

/// The mutable state of the VirtualizationService. There should only be one instance of this
/// struct.
#[derive(Debug)]
struct State {
    /// The next available unused CID.
    next_cid: Cid,

    /// The VMs which have been started. When VMs are started a weak reference is added to this list
    /// while a strong reference is returned to the caller over Binder. Once all copies of the
    /// Binder client are dropped the weak reference here will become invalid, and will be removed
    /// from the list opportunistically the next time `add_vm` is called.
    vms: Vec<Weak<VmInstance>>,

    /// Vector of strong VM references held on behalf of users that cannot hold them themselves.
    /// This is only used for debugging purposes.
    debug_held_vms: Vec<Strong<dyn IVirtualMachine>>,
}

impl State {
    /// Get a list of VMs which still have Binder references to them.
    fn vms(&self) -> Vec<Arc<VmInstance>> {
        // Attempt to upgrade the weak pointers to strong pointers.
        self.vms.iter().filter_map(Weak::upgrade).collect()
    }

    /// Add a new VM to the list.
    fn add_vm(&mut self, vm: Weak<VmInstance>) {
        // Garbage collect any entries from the stored list which no longer exist.
        self.vms.retain(|vm| vm.strong_count() > 0);

        // Actually add the new VM.
        self.vms.push(vm);
    }

    /// Store a strong VM reference.
    fn debug_hold_vm(&mut self, vm: Strong<dyn IVirtualMachine>) {
        self.debug_held_vms.push(vm);
    }

    /// Retrieve and remove a strong VM reference.
    fn debug_drop_vm(&mut self, cid: i32) -> Option<Strong<dyn IVirtualMachine>> {
        let pos = self.debug_held_vms.iter().position(|vm| vm.getCid() == Ok(cid))?;
        Some(self.debug_held_vms.swap_remove(pos))
    }

    /// Get the next available CID, or an error if we have run out.
    fn allocate_cid(&mut self) -> binder::Result<Cid> {
        // TODO(qwandor): keep track of which CIDs are currently in use so that we can reuse them.
        let cid = self.next_cid;
        self.next_cid = self.next_cid.checked_add(1).ok_or(ExceptionCode::ILLEGAL_STATE)?;
        Ok(cid)
    }
}

impl Default for State {
    fn default() -> Self {
        State { next_cid: FIRST_GUEST_CID, vms: vec![], debug_held_vms: vec![] }
    }
}

/// Converts an `&Option<T>` to an `Option<U>` where `T` implements `AsRef<U>`.
fn as_asref<T: AsRef<U>, U>(option: &Option<T>) -> Option<&U> {
    option.as_ref().map(|t| t.as_ref())
}

/// Converts a `&ParcelFileDescriptor` to a `File` by cloning the file.
fn clone_file(file: &ParcelFileDescriptor) -> Result<File, Status> {
    file.as_ref().try_clone().map_err(|e| {
        new_binder_exception(
            ExceptionCode::BAD_PARCELABLE,
            format!("Failed to clone File from ParcelFileDescriptor: {}", e),
        )
    })
}

/// Constructs a new Binder error `Status` with the given `ExceptionCode` and message.
fn new_binder_exception<T: AsRef<str>>(exception: ExceptionCode, message: T) -> Status {
    Status::new_exception(exception, CString::new(message.as_ref()).ok().as_deref())
}