microdroid_manager/src/main.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.

//! Microdroid Manager

mod dice;
mod instance;
mod ioutil;
mod payload;
mod swap;
mod vm_payload_service;

use crate::dice::{DiceContext, DiceDriver};
use crate::instance::{ApexData, ApkData, InstanceDisk, MicrodroidData, RootHash};
use crate::vm_payload_service::register_vm_payload_service;
use android_system_virtualizationcommon::aidl::android::system::virtualizationcommon::ErrorCode::ErrorCode;
use android_system_virtualmachineservice::aidl::android::system::virtualmachineservice::IVirtualMachineService::{
        IVirtualMachineService, VM_BINDER_SERVICE_PORT,
};
use android_system_virtualization_payload::aidl::android::system::virtualization::payload::IVmPayloadService::{
    VM_APK_CONTENTS_PATH,
    VM_PAYLOAD_SERVICE_SOCKET_NAME,
};
use anyhow::{anyhow, bail, ensure, Context, Error, Result};
use apkverify::{get_public_key_der, verify, V4Signature};
use binder::Strong;
use diced_utils::cbor::{encode_header, encode_number};
use glob::glob;
use itertools::sorted;
use libc::VMADDR_CID_HOST;
use log::{error, info, warn};
use microdroid_metadata::{write_metadata, Metadata, PayloadMetadata};
use microdroid_payload_config::{OsConfig, Task, TaskType, VmPayloadConfig};
use nix::fcntl::{fcntl, F_SETFD, FdFlag};
use nix::sys::signal::Signal;
use openssl::sha::Sha512;
use payload::{get_apex_data_from_payload, load_metadata, to_metadata};
use rand::Fill;
use rpcbinder::get_vsock_rpc_interface;
use rustutils::sockets::android_get_control_socket;
use rustutils::system_properties;
use rustutils::system_properties::PropertyWatcher;
use std::borrow::Cow::{Borrowed, Owned};
use std::convert::TryInto;
use std::env;
use std::fs::{self, create_dir, OpenOptions};
use std::io::Write;
use std::os::unix::process::ExitStatusExt;
use std::path::Path;
use std::process::{Child, Command, Stdio};
use std::str;
use std::time::{Duration, SystemTime};

const WAIT_TIMEOUT: Duration = Duration::from_secs(10);
const MAIN_APK_PATH: &str = "/dev/block/by-name/microdroid-apk";
const MAIN_APK_IDSIG_PATH: &str = "/dev/block/by-name/microdroid-apk-idsig";
const MAIN_APK_DEVICE_NAME: &str = "microdroid-apk";
const EXTRA_APK_PATH_PATTERN: &str = "/dev/block/by-name/extra-apk-*";
const EXTRA_IDSIG_PATH_PATTERN: &str = "/dev/block/by-name/extra-idsig-*";
const DM_MOUNTED_APK_PATH: &str = "/dev/block/mapper/microdroid-apk";
const APKDMVERITY_BIN: &str = "/system/bin/apkdmverity";
const ZIPFUSE_BIN: &str = "/system/bin/zipfuse";
const AVF_STRICT_BOOT: &str = "/sys/firmware/devicetree/base/chosen/avf,strict-boot";
const AVF_NEW_INSTANCE: &str = "/sys/firmware/devicetree/base/chosen/avf,new-instance";
const DEBUG_MICRODROID_NO_VERIFIED_BOOT: &str =
    "/sys/firmware/devicetree/base/virtualization/guest/debug-microdroid,no-verified-boot";

const APEX_CONFIG_DONE_PROP: &str = "apex_config.done";
const APP_DEBUGGABLE_PROP: &str = "ro.boot.microdroid.app_debuggable";
const APK_MOUNT_DONE_PROP: &str = "microdroid_manager.apk.mounted";

// SYNC WITH virtualizationservice/src/crosvm.rs
const FAILURE_SERIAL_DEVICE: &str = "/dev/ttyS1";

#[derive(thiserror::Error, Debug)]
enum MicrodroidError {
    #[error("Cannot connect to virtualization service: {0}")]
    FailedToConnectToVirtualizationService(String),
    #[error("Payload has changed: {0}")]
    PayloadChanged(String),
    #[error("Payload verification has failed: {0}")]
    PayloadVerificationFailed(String),
    #[error("Payload config is invalid: {0}")]
    InvalidConfig(String),
}

fn translate_error(err: &Error) -> (ErrorCode, String) {
    if let Some(e) = err.downcast_ref::<MicrodroidError>() {
        match e {
            MicrodroidError::PayloadChanged(msg) => (ErrorCode::PAYLOAD_CHANGED, msg.to_string()),
            MicrodroidError::PayloadVerificationFailed(msg) => {
                (ErrorCode::PAYLOAD_VERIFICATION_FAILED, msg.to_string())
            }
            MicrodroidError::InvalidConfig(msg) => {
                (ErrorCode::PAYLOAD_CONFIG_INVALID, msg.to_string())
            }

            // Connection failure won't be reported to VS; return the default value
            MicrodroidError::FailedToConnectToVirtualizationService(msg) => {
                (ErrorCode::UNKNOWN, msg.to_string())
            }
        }
    } else {
        (ErrorCode::UNKNOWN, err.to_string())
    }
}

fn write_death_reason_to_serial(err: &Error) -> Result<()> {
    let death_reason = if let Some(e) = err.downcast_ref::<MicrodroidError>() {
        Borrowed(match e {
            MicrodroidError::FailedToConnectToVirtualizationService(_) => {
                "MICRODROID_FAILED_TO_CONNECT_TO_VIRTUALIZATION_SERVICE"
            }
            MicrodroidError::PayloadChanged(_) => "MICRODROID_PAYLOAD_HAS_CHANGED",
            MicrodroidError::PayloadVerificationFailed(_) => {
                "MICRODROID_PAYLOAD_VERIFICATION_FAILED"
            }
            MicrodroidError::InvalidConfig(_) => "MICRODROID_INVALID_PAYLOAD_CONFIG",
        })
    } else {
        // Send context information back after a separator, to ease diagnosis.
        // These errors occur before the payload runs, so this should not leak sensitive
        // information.
        Owned(format!("MICRODROID_UNKNOWN_RUNTIME_ERROR|{:?}", err))
    };

    let death_reason_bytes = death_reason.as_bytes();
    let mut sent_total = 0;
    while sent_total < death_reason_bytes.len() {
        // TODO(b/220071963): Sometimes, sending more than 16 bytes at once makes MM hang.
        let begin = sent_total;
        let end = std::cmp::min(begin.saturating_add(16), death_reason_bytes.len());
        OpenOptions::new()
            .read(false)
            .write(true)
            .open(FAILURE_SERIAL_DEVICE)?
            .write_all(&death_reason_bytes[begin..end])?;
        sent_total = end;
    }

    Ok(())
}

fn get_vms_rpc_binder() -> Result<Strong<dyn IVirtualMachineService>> {
    get_vsock_rpc_interface(VMADDR_CID_HOST, VM_BINDER_SERVICE_PORT as u32)
        .context("Cannot connect to RPC service")
}

fn main() -> Result<()> {
    // If debuggable, print full backtrace to console log with stdio_to_kmsg
    if system_properties::read_bool(APP_DEBUGGABLE_PROP, true)? {
        env::set_var("RUST_BACKTRACE", "full");
    }

    scopeguard::defer! {
        info!("Shutting down...");
        if let Err(e) = system_properties::write("sys.powerctl", "shutdown") {
            error!("failed to shutdown {:?}", e);
        }
    }

    try_main().map_err(|e| {
        error!("Failed with {:?}.", e);
        if let Err(e) = write_death_reason_to_serial(&e) {
            error!("Failed to write death reason {:?}", e);
        }
        e
    })
}

fn set_cloexec_on_vm_payload_service_socket() -> Result<()> {
    let fd = android_get_control_socket(VM_PAYLOAD_SERVICE_SOCKET_NAME)?;

    fcntl(fd, F_SETFD(FdFlag::FD_CLOEXEC))?;

    Ok(())
}

fn try_main() -> Result<()> {
    let _ = kernlog::init();
    info!("started.");

    if let Err(e) = set_cloexec_on_vm_payload_service_socket() {
        warn!("Failed to set cloexec on vm payload socket: {:?}", e);
    }

    load_crashkernel_if_supported().context("Failed to load crashkernel")?;

    swap::init_swap().context("Failed to initialise swap")?;
    info!("swap enabled.");

    let service = get_vms_rpc_binder()
        .context("cannot connect to VirtualMachineService")
        .map_err(|e| MicrodroidError::FailedToConnectToVirtualizationService(e.to_string()))?;

    match try_run_payload(&service) {
        Ok(code) => {
            info!("notifying payload finished");
            service.notifyPayloadFinished(code)?;
            if code == 0 {
                info!("task successfully finished");
            } else {
                error!("task exited with exit code: {}", code);
            }
            Ok(())
        }
        Err(err) => {
            let (error_code, message) = translate_error(&err);
            service.notifyError(error_code, &message)?;
            Err(err)
        }
    }
}

fn dice_derivation(
    dice: DiceDriver,
    verified_data: &MicrodroidData,
    payload_metadata: &PayloadMetadata,
) -> Result<DiceContext> {
    // Calculate compound digests of code and authorities
    let mut code_hash_ctx = Sha512::new();
    let mut authority_hash_ctx = Sha512::new();
    code_hash_ctx.update(verified_data.apk_data.root_hash.as_ref());
    authority_hash_ctx.update(verified_data.apk_data.pubkey.as_ref());
    for extra_apk in &verified_data.extra_apks_data {
        code_hash_ctx.update(extra_apk.root_hash.as_ref());
        authority_hash_ctx.update(extra_apk.pubkey.as_ref());
    }
    for apex in &verified_data.apex_data {
        code_hash_ctx.update(apex.root_digest.as_ref());
        authority_hash_ctx.update(apex.public_key.as_ref());
    }
    let code_hash = code_hash_ctx.finish();
    let authority_hash = authority_hash_ctx.finish();

    // {
    //   -70002: "Microdroid payload",
    //   ? -71000: tstr // payload_config_path
    //   ? -71001: PayloadConfig
    // }
    // PayloadConfig = {
    //   1: tstr // payload_binary_path
    // }

    let mut config_desc = vec![
        0xa2, // map(2)
        0x3a, 0x00, 0x01, 0x11, 0x71, // -70002
        0x72, 0x4d, 0x69, 0x63, 0x72, 0x6f, 0x64, 0x72, 0x6f, 0x69, 0x64, 0x20, 0x70, 0x61, 0x79,
        0x6c, 0x6f, 0x61, 0x64, // "Microdroid payload"
    ];

    match payload_metadata {
        PayloadMetadata::config_path(payload_config_path) => {
            encode_negative_number(-71000, &mut config_desc)?;
            encode_tstr(payload_config_path, &mut config_desc)?;
        }
        PayloadMetadata::config(payload_config) => {
            encode_negative_number(-71001, &mut config_desc)?;
            encode_header(5, 1, &mut config_desc)?; // map(1)
            encode_number(1, &mut config_desc)?;
            encode_tstr(&payload_config.payload_binary_path, &mut config_desc)?;
        }
    }

    // Check app debuggability, conervatively assuming it is debuggable
    let app_debuggable = system_properties::read_bool(APP_DEBUGGABLE_PROP, true)?;

    // Send the details to diced
    let hidden = verified_data.salt.clone().try_into().unwrap();
    dice.derive(code_hash, &config_desc, authority_hash, app_debuggable, hidden)
}

fn encode_tstr(tstr: &str, buffer: &mut Vec<u8>) -> Result<()> {
    let bytes = tstr.as_bytes();
    encode_header(3, bytes.len().try_into().unwrap(), buffer)?;
    buffer.extend_from_slice(bytes);
    Ok(())
}

fn encode_negative_number(n: i64, buffer: &mut dyn Write) -> Result<()> {
    ensure!(n < 0);
    let n = -1 - n;
    encode_header(1, n.try_into().unwrap(), buffer)
}

fn is_strict_boot() -> bool {
    Path::new(AVF_STRICT_BOOT).exists()
}

fn is_new_instance() -> bool {
    Path::new(AVF_NEW_INSTANCE).exists()
}

fn is_verified_boot() -> bool {
    !Path::new(DEBUG_MICRODROID_NO_VERIFIED_BOOT).exists()
}

fn try_run_payload(service: &Strong<dyn IVirtualMachineService>) -> Result<i32> {
    let metadata = load_metadata().context("Failed to load payload metadata")?;
    let dice = DiceDriver::new(Path::new("/dev/open-dice0")).context("Failed to load DICE")?;

    let mut instance = InstanceDisk::new().context("Failed to load instance.img")?;
    let saved_data =
        instance.read_microdroid_data(&dice).context("Failed to read identity data")?;

    if is_strict_boot() {
        // Provisioning must happen on the first boot and never again.
        if is_new_instance() {
            ensure!(
                saved_data.is_none(),
                MicrodroidError::InvalidConfig("Found instance data on first boot.".to_string())
            );
        } else {
            ensure!(
                saved_data.is_some(),
                MicrodroidError::InvalidConfig("Instance data not found.".to_string())
            );
        };
    }

    // Verify the payload before using it.
    let verified_data = verify_payload(&metadata, saved_data.as_ref())
        .context("Payload verification failed")
        .map_err(|e| MicrodroidError::PayloadVerificationFailed(e.to_string()))?;

    // In case identity is ignored (by debug policy), we should reuse existing payload data, even
    // when the payload is changed. This is to keep the derived secret same as before.
    let verified_data = if let Some(saved_data) = saved_data {
        if !is_verified_boot() {
            if saved_data != verified_data {
                info!("Detected an update of the payload, but continue (regarding debug policy)")
            }
        } else {
            ensure!(
                saved_data == verified_data,
                MicrodroidError::PayloadChanged(String::from(
                    "Detected an update of the payload which isn't supported yet."
                ))
            );
            info!("Saved data is verified.");
        }
        saved_data
    } else {
        info!("Saving verified data.");
        instance
            .write_microdroid_data(&verified_data, &dice)
            .context("Failed to write identity data")?;
        verified_data
    };

    let payload_metadata = metadata.payload.ok_or_else(|| {
        MicrodroidError::InvalidConfig("No payload config in metadata".to_string())
    })?;

    // To minimize the exposure to untrusted data, derive dice profile as soon as possible.
    info!("DICE derivation for payload");
    let dice = dice_derivation(dice, &verified_data, &payload_metadata)?;

    // Before reading a file from the APK, start zipfuse
    run_zipfuse(
        MountForExec::Allowed,
        "fscontext=u:object_r:zipfusefs:s0,context=u:object_r:system_file:s0",
        Path::new("/dev/block/mapper/microdroid-apk"),
        Path::new(VM_APK_CONTENTS_PATH),
        Some(APK_MOUNT_DONE_PROP),
    )
    .context("Failed to run zipfuse")?;

    // Restricted APIs are only allowed to be used by platform or test components. Infer this from
    // the use of a VM config file since those can only be used by platform and test components.
    let allow_restricted_apis = match payload_metadata {
        PayloadMetadata::config_path(_) => true,
        PayloadMetadata::config(_) => false,
    };

    let config = load_config(payload_metadata).context("Failed to load payload metadata")?;

    let task = config
        .task
        .as_ref()
        .ok_or_else(|| MicrodroidError::InvalidConfig("No task in VM config".to_string()))?;

    if config.extra_apks.len() != verified_data.extra_apks_data.len() {
        return Err(anyhow!(
            "config expects {} extra apks, but found {}",
            config.extra_apks.len(),
            verified_data.extra_apks_data.len()
        ));
    }
    mount_extra_apks(&config)?;

    // Wait until apex config is done. (e.g. linker configuration for apexes)
    // TODO(jooyung): wait until sys.boot_completed?
    wait_for_apex_config_done()?;

    // Start tombstone_transmit if enabled
    if config.export_tombstones {
        control_service("start", "tombstone_transmit")?;
    } else {
        control_service("stop", "tombstoned")?;
    }

    // Start authfs if enabled
    if config.enable_authfs {
        control_service("start", "authfs_service")?;
    }

    // Wait until zipfuse has mounted the APK so we can access the payload
    wait_for_property_true(APK_MOUNT_DONE_PROP).context("Failed waiting for APK mount done")?;

    register_vm_payload_service(allow_restricted_apis, service.clone(), dice)?;

    system_properties::write("dev.bootcomplete", "1").context("set dev.bootcomplete")?;
    exec_task(task, service).context("Failed to run payload")
}

fn control_service(action: &str, service: &str) -> Result<()> {
    system_properties::write(&format!("ctl.{}", action), service)
        .with_context(|| format!("Failed to {} {}", action, service))
}

struct ApkDmverityArgument<'a> {
    apk: &'a str,
    idsig: &'a str,
    name: &'a str,
    saved_root_hash: Option<&'a RootHash>,
}

fn run_apkdmverity(args: &[ApkDmverityArgument]) -> Result<Child> {
    let mut cmd = Command::new(APKDMVERITY_BIN);

    cmd.stdin(Stdio::null()).stdout(Stdio::null()).stderr(Stdio::null());

    for argument in args {
        cmd.arg("--apk").arg(argument.apk).arg(argument.idsig).arg(argument.name);
        if let Some(root_hash) = argument.saved_root_hash {
            cmd.arg(&to_hex_string(root_hash));
        } else {
            cmd.arg("none");
        }
    }

    cmd.spawn().context("Spawn apkdmverity")
}

enum MountForExec {
    Allowed,
    Disallowed,
}

fn run_zipfuse(
    noexec: MountForExec,
    option: &str,
    zip_path: &Path,
    mount_dir: &Path,
    ready_prop: Option<&str>,
) -> Result<Child> {
    let mut cmd = Command::new(ZIPFUSE_BIN);
    if let MountForExec::Disallowed = noexec {
        cmd.arg("--noexec");
    }
    if let Some(property_name) = ready_prop {
        cmd.args(["-p", property_name]);
    }
    cmd.arg("-o")
        .arg(option)
        .arg(zip_path)
        .arg(mount_dir)
        .stdin(Stdio::null())
        .stdout(Stdio::null())
        .stderr(Stdio::null())
        .spawn()
        .context("Spawn zipfuse")
}

fn write_apex_payload_data(
    saved_data: Option<&MicrodroidData>,
    apex_data_from_payload: &[ApexData],
) -> Result<()> {
    if let Some(saved_apex_data) = saved_data.map(|d| &d.apex_data) {
        // We don't support APEX updates. (assuming that update will change root digest)
        ensure!(
            saved_apex_data == apex_data_from_payload,
            MicrodroidError::PayloadChanged(String::from("APEXes have changed."))
        );
        let apex_metadata = to_metadata(apex_data_from_payload);
        // Pass metadata(with public keys and root digests) to apexd so that it uses the passed
        // metadata instead of the default one (/dev/block/by-name/payload-metadata)
        OpenOptions::new()
            .create_new(true)
            .write(true)
            .open("/apex/vm-payload-metadata")
            .context("Failed to open /apex/vm-payload-metadata")
            .and_then(|f| write_metadata(&apex_metadata, f))?;
    }
    Ok(())
}

// Verify payload before executing it. For APK payload, Full verification (which is slow) is done
// when the root_hash values from the idsig file and the instance disk are different. This function
// returns the verified root hash (for APK payload) and pubkeys (for APEX payloads) that can be
// saved to the instance disk.
fn verify_payload(
    metadata: &Metadata,
    saved_data: Option<&MicrodroidData>,
) -> Result<MicrodroidData> {
    let start_time = SystemTime::now();

    // Verify main APK
    let root_hash = saved_data.map(|d| &d.apk_data.root_hash);
    let root_hash_from_idsig = get_apk_root_hash_from_idsig(MAIN_APK_IDSIG_PATH)?;
    let root_hash_trustful = root_hash == Some(&root_hash_from_idsig);

    // If root_hash can be trusted, pass it to apkdmverity so that it uses the passed root_hash
    // instead of the value read from the idsig file.
    let main_apk_argument = {
        ApkDmverityArgument {
            apk: MAIN_APK_PATH,
            idsig: MAIN_APK_IDSIG_PATH,
            name: MAIN_APK_DEVICE_NAME,
            saved_root_hash: if root_hash_trustful {
                Some(root_hash_from_idsig.as_ref())
            } else {
                None
            },
        }
    };
    let mut apkdmverity_arguments = vec![main_apk_argument];

    // Verify extra APKs
    // For now, we can't read the payload config, so glob APKs and idsigs.
    // Later, we'll see if it matches with the payload config.

    // sort globbed paths to match apks (extra-apk-{idx}) and idsigs (extra-idsig-{idx})
    // e.g. "extra-apk-0" corresponds to "extra-idsig-0"
    let extra_apks =
        sorted(glob(EXTRA_APK_PATH_PATTERN)?.collect::<Result<Vec<_>, _>>()?).collect::<Vec<_>>();
    let extra_idsigs =
        sorted(glob(EXTRA_IDSIG_PATH_PATTERN)?.collect::<Result<Vec<_>, _>>()?).collect::<Vec<_>>();
    if extra_apks.len() != extra_idsigs.len() {
        return Err(anyhow!(
            "Extra apks/idsigs mismatch: {} apks but {} idsigs",
            extra_apks.len(),
            extra_idsigs.len()
        ));
    }
    let extra_apks_count = extra_apks.len();

    let (extra_apk_names, extra_root_hashes_from_idsig): (Vec<_>, Vec<_>) = extra_idsigs
        .iter()
        .enumerate()
        .map(|(i, extra_idsig)| {
            (
                format!("extra-apk-{}", i),
                get_apk_root_hash_from_idsig(extra_idsig)
                    .expect("Can't find root hash from extra idsig"),
            )
        })
        .unzip();

    let saved_extra_root_hashes: Vec<_> = saved_data
        .map(|d| d.extra_apks_data.iter().map(|apk_data| &apk_data.root_hash).collect())
        .unwrap_or_else(Vec::new);
    let extra_root_hashes_trustful: Vec<_> = extra_root_hashes_from_idsig
        .iter()
        .enumerate()
        .map(|(i, root_hash_from_idsig)| {
            saved_extra_root_hashes.get(i).copied() == Some(root_hash_from_idsig)
        })
        .collect();

    for i in 0..extra_apks_count {
        apkdmverity_arguments.push({
            ApkDmverityArgument {
                apk: extra_apks[i].to_str().unwrap(),
                idsig: extra_idsigs[i].to_str().unwrap(),
                name: &extra_apk_names[i],
                saved_root_hash: if extra_root_hashes_trustful[i] {
                    Some(&extra_root_hashes_from_idsig[i])
                } else {
                    None
                },
            }
        });
    }

    // Start apkdmverity and wait for the dm-verify block
    let mut apkdmverity_child = run_apkdmverity(&apkdmverity_arguments)?;

    // While waiting for apkdmverity to mount APK, gathers public keys and root digests from
    // APEX payload.
    let apex_data_from_payload = get_apex_data_from_payload(metadata)?;

    // Writing /apex/vm-payload-metadata is to verify that the payload isn't changed.
    // Skip writing it if the debug policy ignoring identity is on
    if is_verified_boot() {
        write_apex_payload_data(saved_data, &apex_data_from_payload)?;
    }

    // Start apexd to activate APEXes
    system_properties::write("ctl.start", "apexd-vm")?;

    // TODO(inseob): add timeout
    apkdmverity_child.wait()?;

    // Do the full verification if the root_hash is un-trustful. This requires the full scanning of
    // the APK file and therefore can be very slow if the APK is large. Note that this step is
    // taken only when the root_hash is un-trustful which can be either when this is the first boot
    // of the VM or APK was updated in the host.
    // TODO(jooyung): consider multithreading to make this faster
    let main_apk_pubkey = get_public_key_from_apk(DM_MOUNTED_APK_PATH, root_hash_trustful)?;
    let extra_apks_data = extra_root_hashes_from_idsig
        .into_iter()
        .enumerate()
        .map(|(i, extra_root_hash)| {
            let mount_path = format!("/dev/block/mapper/{}", &extra_apk_names[i]);
            let apk_pubkey = get_public_key_from_apk(&mount_path, extra_root_hashes_trustful[i])?;
            Ok(ApkData { root_hash: extra_root_hash, pubkey: apk_pubkey })
        })
        .collect::<Result<Vec<_>>>()?;

    info!("payload verification successful. took {:#?}", start_time.elapsed().unwrap());

    // Use the salt from a verified instance, or generate a salt for a new instance.
    let salt = if let Some(saved_data) = saved_data {
        saved_data.salt.clone()
    } else {
        let mut salt = vec![0u8; 64];
        salt.as_mut_slice().try_fill(&mut rand::thread_rng())?;
        salt
    };

    // At this point, we can ensure that the root_hash from the idsig file is trusted, either by
    // fully verifying the APK or by comparing it with the saved root_hash.
    Ok(MicrodroidData {
        salt,
        apk_data: ApkData { root_hash: root_hash_from_idsig, pubkey: main_apk_pubkey },
        extra_apks_data,
        apex_data: apex_data_from_payload,
    })
}

fn mount_extra_apks(config: &VmPayloadConfig) -> Result<()> {
    // For now, only the number of apks is important, as the mount point and dm-verity name is fixed
    for i in 0..config.extra_apks.len() {
        let mount_dir = format!("/mnt/extra-apk/{}", i);
        create_dir(Path::new(&mount_dir)).context("Failed to create mount dir for extra apks")?;

        // don't wait, just detach
        run_zipfuse(
            MountForExec::Disallowed,
            "fscontext=u:object_r:zipfusefs:s0,context=u:object_r:extra_apk_file:s0",
            Path::new(&format!("/dev/block/mapper/extra-apk-{}", i)),
            Path::new(&mount_dir),
            None,
        )
        .context("Failed to zipfuse extra apks")?;
    }

    Ok(())
}

// Waits until linker config is generated
fn wait_for_apex_config_done() -> Result<()> {
    wait_for_property_true(APEX_CONFIG_DONE_PROP).context("Failed waiting for apex config done")
}

fn wait_for_property_true(property_name: &str) -> Result<()> {
    let mut prop = PropertyWatcher::new(property_name)?;
    loop {
        prop.wait()?;
        if system_properties::read_bool(property_name, false)? {
            break;
        }
    }
    Ok(())
}

fn get_apk_root_hash_from_idsig<P: AsRef<Path>>(idsig_path: P) -> Result<Box<RootHash>> {
    Ok(V4Signature::from_idsig_path(idsig_path)?.hashing_info.raw_root_hash)
}

fn get_public_key_from_apk(apk: &str, root_hash_trustful: bool) -> Result<Box<[u8]>> {
    if !root_hash_trustful {
        verify(apk).context(MicrodroidError::PayloadVerificationFailed(format!(
            "failed to verify {}",
            apk
        )))
    } else {
        get_public_key_der(apk)
    }
}

fn load_config(payload_metadata: PayloadMetadata) -> Result<VmPayloadConfig> {
    match payload_metadata {
        PayloadMetadata::config_path(path) => {
            let path = Path::new(&path);
            info!("loading config from {:?}...", path);
            let file = ioutil::wait_for_file(path, WAIT_TIMEOUT)
                .with_context(|| format!("Failed to read {:?}", path))?;
            Ok(serde_json::from_reader(file)?)
        }
        PayloadMetadata::config(payload_config) => {
            let task = Task {
                type_: TaskType::MicrodroidLauncher,
                command: payload_config.payload_binary_path,
            };
            Ok(VmPayloadConfig {
                os: OsConfig { name: "microdroid".to_owned() },
                task: Some(task),
                apexes: vec![],
                extra_apks: vec![],
                prefer_staged: false,
                export_tombstones: false,
                enable_authfs: false,
            })
        }
    }
}

/// Loads the crashkernel into memory using kexec if the VM is loaded with `crashkernel=' parameter
/// in the cmdline.
fn load_crashkernel_if_supported() -> Result<()> {
    let supported = std::fs::read_to_string("/proc/cmdline")?.contains(" crashkernel=");
    info!("ramdump supported: {}", supported);
    if supported {
        let status = Command::new("/system/bin/kexec_load").status()?;
        if !status.success() {
            return Err(anyhow!("Failed to load crashkernel: {:?}", status));
        }
    }
    Ok(())
}

/// Executes the given task.
fn exec_task(task: &Task, service: &Strong<dyn IVirtualMachineService>) -> Result<i32> {
    info!("executing main task {:?}...", task);
    let mut command = match task.type_ {
        TaskType::Executable => Command::new(&task.command),
        TaskType::MicrodroidLauncher => {
            let mut command = Command::new("/system/bin/microdroid_launcher");
            command.arg(find_library_path(&task.command)?);
            command
        }
    };
    command.stdin(Stdio::null()).stdout(Stdio::null()).stderr(Stdio::null());

    info!("notifying payload started");
    service.notifyPayloadStarted()?;

    let exit_status = command.spawn()?.wait()?;
    match exit_status.code() {
        Some(exit_code) => Ok(exit_code),
        None => Err(match exit_status.signal() {
            Some(signal) => anyhow!(
                "Payload exited due to signal: {} ({})",
                signal,
                Signal::try_from(signal).map_or("unknown", |s| s.as_str())
            ),
            None => anyhow!("Payload has neither exit code nor signal"),
        }),
    }
}

fn find_library_path(name: &str) -> Result<String> {
    let mut watcher = PropertyWatcher::new("ro.product.cpu.abilist")?;
    let value = watcher.read(|_name, value| Ok(value.trim().to_string()))?;
    let abi = value.split(',').next().ok_or_else(|| anyhow!("no abilist"))?;
    let path = format!("{}/lib/{}/{}", VM_APK_CONTENTS_PATH, abi, name);

    let metadata = fs::metadata(&path).with_context(|| format!("Unable to access {}", path))?;
    if !metadata.is_file() {
        bail!("{} is not a file", &path);
    }

    Ok(path)
}

fn to_hex_string(buf: &[u8]) -> String {
    buf.iter().map(|b| format!("{:02X}", b)).collect()
}