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// Copyright 2024 syzkaller project authors. All rights reserved.
// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
package qemu
import (
"encoding/binary"
"fmt"
"net"
"path/filepath"
"sync/atomic"
"syscall"
"time"
"unsafe"
"github.com/google/syzkaller/pkg/flatrpc"
"golang.org/x/sys/unix"
)
type snapshot struct {
ivsListener *net.UnixListener
ivsConn *net.UnixConn
doorbellFD int
eventFD int
shmemFD int
shmem []byte
input []byte
header *flatrpc.SnapshotHeaderT
}
func (inst *instance) snapshotClose() {
if inst.ivsListener != nil {
inst.ivsListener.Close()
}
if inst.ivsConn != nil {
inst.ivsConn.Close()
}
if inst.doorbellFD != 0 {
syscall.Close(inst.doorbellFD)
}
if inst.eventFD != 0 {
syscall.Close(inst.eventFD)
}
if inst.shmemFD != 0 {
syscall.Close(inst.shmemFD)
}
if inst.shmem != nil {
syscall.Munmap(inst.shmem)
}
}
func (inst *instance) snapshotEnable() ([]string, error) {
// We use ivshmem device (Inter-VM Shared Memory) for communication with the VM,
// it allows to have a shared memory region directly accessible by both host and target:
// https://www.qemu.org/docs/master/system/devices/ivshmem.html
//
// The shared memory region is not restored as part of snapshot restore since we set:
// migrate_set_capability x-ignore-shared on
// This allows to write a new input into ivshmem before each restore.
//
// We also use doorbell (interrupt) capability of ivshmem to notify host about
// program execution completion. Doorbell also allows to send interrupts in the other direction
// (from host to target), but we don't need/use this since we arrange things such that
// snapshot restore serves as a signal to execute new input.
//
// Ideally we use a single ivshmem device for both purposes (shmem+doorbell).
// But unfortunately it seems that the doorbell device is always restored on snapshot restore
// (at least I did not find a way to make it not restored, maybe can be solved with qemu change).
// So we use 2 separate devices for these purposes.
shmemFD, err := unix.MemfdCreate("syz-qemu-shmem", 0)
if err != nil {
return nil, fmt.Errorf("qemu: memfd_create failed: %w", err)
}
inst.shmemFD = shmemFD
if err := syscall.Ftruncate(shmemFD, int64(flatrpc.ConstSnapshotShmemSize)); err != nil {
return nil, fmt.Errorf("qemu: ftruncate failed: %w", err)
}
shmem, err := syscall.Mmap(shmemFD, 0, int(flatrpc.ConstSnapshotShmemSize),
syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
if err != nil {
return nil, fmt.Errorf("qemu: shmem mmap failed: %w", err)
}
inst.shmem = shmem
inst.input = shmem[:flatrpc.ConstMaxInputSize:flatrpc.ConstMaxInputSize]
inst.header = (*flatrpc.SnapshotHeaderT)(unsafe.Pointer(&shmem[flatrpc.ConstMaxInputSize]))
shmemFile := fmt.Sprintf("/proc/%v/fd/%v", syscall.Getpid(), shmemFD)
doorbellFD, err := unix.MemfdCreate("syz-qemu-doorbell", 0)
if err != nil {
return nil, fmt.Errorf("qemu: memfd_create failed: %w", err)
}
if err := syscall.Ftruncate(doorbellFD, int64(flatrpc.ConstSnapshotDoorbellSize)); err != nil {
return nil, fmt.Errorf("qemu: ftruncate failed: %w", err)
}
inst.doorbellFD = doorbellFD
eventFD, err := unix.Eventfd(0, unix.EFD_SEMAPHORE)
if err != nil {
return nil, fmt.Errorf("qemu: eventfd failed: %w", err)
}
inst.eventFD = eventFD
sockPath := filepath.Join(inst.workdir, "ivs.sock")
ln, err := net.ListenUnix("unix", &net.UnixAddr{Name: sockPath, Net: "unix"})
if err != nil {
return nil, fmt.Errorf("qemu: unix listen on %v failed: %w", sockPath, err)
}
inst.ivsListener = ln
return []string{
// migratable=on is required to take snapshots.
// tsc=off disables RDTSC timestamp counter, it's not virtualized/restored as part of snapshots,
// so the target kernel sees a large jump in time and always declares TSC as unstable after restore.
"-cpu", "host,migratable=on,tsc=off",
"-chardev", fmt.Sprintf("socket,path=%v,id=snapshot-doorbell", sockPath),
"-device", "ivshmem-doorbell,master=on,vectors=1,chardev=snapshot-doorbell",
"-device", "ivshmem-plain,master=on,memdev=snapshot-shmem",
"-object", fmt.Sprintf("memory-backend-file,size=%v,share=on,discard-data=on,id=snapshot-shmem,mem-path=%v",
uint64(flatrpc.ConstSnapshotShmemSize), shmemFile),
}, nil
}
func (inst *instance) snapshotHandshake() error {
// ivshmem-doorbell expects an external server that communicates via a unix socket.
// The protocol is not documented, for details see:
// https://github.com/qemu/qemu/blob/master/hw/misc/ivshmem.c
// https://github.com/qemu/qemu/blob/master/contrib/ivshmem-server/ivshmem-server.c
conn, err := inst.ivsListener.AcceptUnix()
if err != nil {
return fmt.Errorf("qemu: unix accept failed: %w", err)
}
inst.ivsListener.Close()
inst.ivsListener = nil
inst.ivsConn = conn
msg := make([]byte, 8)
// Send protocol version 0.
binary.LittleEndian.PutUint64(msg, 0)
if _, err := conn.Write(msg); err != nil {
return fmt.Errorf("qemu: ivs conn write failed: %w", err)
}
// Send VM id 0.
binary.LittleEndian.PutUint64(msg, 0)
if _, err := conn.Write(msg); err != nil {
return fmt.Errorf("qemu: ivs conn write failed: %w", err)
}
// Send shared memory file FD.
binary.LittleEndian.PutUint64(msg, ^uint64(0))
rights := syscall.UnixRights(inst.doorbellFD)
if _, _, err := conn.WriteMsgUnix(msg, rights, nil); err != nil {
return fmt.Errorf("qemu: ivs conn sendmsg failed: %w", err)
}
// Send event FD for VM 1 interrupt vector 0.
binary.LittleEndian.PutUint64(msg, 1)
rights = syscall.UnixRights(inst.eventFD)
if _, _, err := conn.WriteMsgUnix(msg, rights, nil); err != nil {
return fmt.Errorf("qemu: ivs conn sendmsg failed: %w", err)
}
return nil
}
func (inst *instance) SetupSnapshot(input []byte) error {
copy(inst.input, input)
// Tell executor that we are ready to snapshot and wait for an ack.
inst.header.UpdateState(flatrpc.SnapshotStateHandshake)
if !inst.waitSnapshotStateChange(flatrpc.SnapshotStateHandshake, 10*time.Minute) {
return fmt.Errorf("executor does not start snapshot handshake\n%s", inst.readOutput())
}
if _, err := inst.hmp("migrate_set_capability x-ignore-shared on", 0); err != nil {
return err
}
if _, err := inst.hmp("savevm syz", 0); err != nil {
return err
}
if inst.debug {
inst.hmp("info snapshots", 0) // this prints size of the snapshot
}
inst.header.UpdateState(flatrpc.SnapshotStateSnapshotted)
if !inst.waitSnapshotStateChange(flatrpc.SnapshotStateSnapshotted, time.Minute) {
return fmt.Errorf("executor has not confirmed snapshot handshake\n%s", inst.readOutput())
}
return nil
}
func (inst *instance) RunSnapshot(timeout time.Duration, input []byte) (result, output []byte, err error) {
copy(inst.input, input)
inst.header.OutputOffset = 0
inst.header.OutputSize = 0
inst.header.UpdateState(flatrpc.SnapshotStateExecute)
if _, err := inst.hmp("loadvm syz", 0); err != nil {
return nil, nil, fmt.Errorf("%w\n%s", err, inst.readOutput())
}
inst.waitSnapshotStateChange(flatrpc.SnapshotStateExecute, timeout)
resStart := int(flatrpc.ConstMaxInputSize) + int(atomic.LoadUint32(&inst.header.OutputOffset))
resEnd := resStart + int(atomic.LoadUint32(&inst.header.OutputSize))
var res []byte
if resEnd <= len(inst.shmem) {
res = inst.shmem[resStart:resEnd:resEnd]
}
output = inst.readOutput()
return res, output, nil
}
func (inst *instance) waitSnapshotStateChange(state flatrpc.SnapshotState, timeout time.Duration) bool {
deadline := time.Now().Add(timeout)
timeoutMs := int(timeout / time.Millisecond)
fds := []unix.PollFd{{
Fd: int32(inst.eventFD),
Events: unix.POLLIN,
}}
for {
if n, _ := unix.Poll(fds, timeoutMs); n == 1 {
var buf [8]byte
syscall.Read(inst.eventFD, buf[:])
}
if inst.header.LoadState() != state {
return true
}
remain := time.Until(deadline)
if remain < time.Millisecond {
return false
}
timeoutMs = int(remain / time.Millisecond)
}
}
func (inst *instance) readOutput() []byte {
var output []byte
// If output channel has overflown, then wait for more output from the merger goroutine.
wait := cap(inst.merger.Output)
for {
select {
case out := <-inst.merger.Output:
output = append(output, out...)
wait--
default:
if wait > 0 {
return output
}
// After the first overflow we wait after every read because the goroutine
// may be running and sending more output to the channel concurrently.
wait = 1
time.Sleep(10 * time.Millisecond)
}
}
}
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