gvisor/gofer: Use pivot_root instead of chroot

PiperOrigin-RevId: 231864273
Change-Id: I8545b72b615f5c2945df374b801b80be64ec3e13

runsc/cmd/chroot.go

@@ -36,6 +36,29 @@ func mountInChroot(chroot, src, dst, typ string, flags uint32) error {
 	return nil
 }
 
+func pivotRoot(root string) error {
+	if err := os.Chdir(root); err != nil {
+		return fmt.Errorf("error changing working directory: %v", err)
+	}
+	// pivot_root(new_root, put_old) moves the root filesystem (old_root)
+	// of the calling process to the directory put_old and makes new_root
+	// the new root filesystem of the calling process.
+	//
+	// pivot_root(".", ".") makes a mount of the working directory the new
+	// root filesystem, so it will be moved in "/" and then the old_root
+	// will be moved to "/" too. The parent mount of the old_root will be
+	// new_root, so after umounting the old_root, we will see only
+	// the new_root in "/".
+	if err := syscall.PivotRoot(".", "."); err != nil {
+		return fmt.Errorf("error changing root filesystem: %v", err)
+	}
+
+	if err := syscall.Unmount(".", syscall.MNT_DETACH); err != nil {
+		return fmt.Errorf("error umounting the old root file system: %v", err)
+	}
+	return nil
+}
+
 // setUpChroot creates an empty directory with runsc mounted at /runsc and proc
 // mounted at /proc.
 func setUpChroot(pidns bool) error {
@@ -66,29 +89,9 @@ func setUpChroot(pidns bool) error {
 		}
 	}
 
-	if err := os.Chdir(chroot); err != nil {
-		return fmt.Errorf("error changing working directory: %v", err)
-	}
-
 	if err := syscall.Mount("", chroot, "", syscall.MS_REMOUNT|syscall.MS_RDONLY|syscall.MS_BIND, ""); err != nil {
 		return fmt.Errorf("error remounting chroot in read-only: %v", err)
 	}
-	// pivot_root(new_root, put_old) moves the root filesystem (old_root)
-	// of the calling process to the directory put_old and makes new_root
-	// the new root filesystem of the calling process.
-	//
-	// pivot_root(".", ".") makes a mount of the working directory the new
-	// root filesystem, so it will be moved in "/" and then the old_root
-	// will be moved to "/" too. The parent mount of the old_root will be
-	// new_root, so after umounting the old_root, we will see only
-	// the new_root in "/".
-	if err := syscall.PivotRoot(".", "."); err != nil {
-		return fmt.Errorf("error changing root filesystem: %v", err)
-	}
 
-	if err := syscall.Unmount(".", syscall.MNT_DETACH); err != nil {
+	return pivotRoot(chroot)
-		return fmt.Errorf("error umounting the old root file system: %v", err)
-	}
-
-	return nil
 }

runsc/cmd/gofer.go

@@ -26,6 +26,7 @@ import (
 	"gvisor.googlesource.com/gvisor/pkg/log"
 	"gvisor.googlesource.com/gvisor/pkg/p9"
 	"gvisor.googlesource.com/gvisor/pkg/unet"
+	"gvisor.googlesource.com/gvisor/runsc/boot"
 	"gvisor.googlesource.com/gvisor/runsc/fsgofer"
 	"gvisor.googlesource.com/gvisor/runsc/fsgofer/filter"
 	"gvisor.googlesource.com/gvisor/runsc/specutils"
@@ -54,8 +55,10 @@ type Gofer struct {
 	bundleDir string
 	ioFDs     intFlags
 	applyCaps bool
+	setUpRoot bool
 
 	panicOnWrite bool
+	specFD       int
 }
 
 // Name implements subcommands.Command.
@@ -79,43 +82,83 @@ func (g *Gofer) SetFlags(f *flag.FlagSet) {
 	f.Var(&g.ioFDs, "io-fds", "list of FDs to connect 9P servers. They must follow this order: root first, then mounts as defined in the spec")
 	f.BoolVar(&g.applyCaps, "apply-caps", true, "if true, apply capabilities to restrict what the Gofer process can do")
 	f.BoolVar(&g.panicOnWrite, "panic-on-write", false, "if true, panics on attempts to write to RO mounts. RW mounts are unnaffected")
+	f.BoolVar(&g.setUpRoot, "setup-root", true, "if true, set up an empty root for the process")
+	f.IntVar(&g.specFD, "spec-fd", -1, "required fd with the container spec")
 }
 
 // Execute implements subcommands.Command.
 func (g *Gofer) Execute(_ context.Context, f *flag.FlagSet, args ...interface{}) subcommands.ExitStatus {
-	if g.bundleDir == "" || len(g.ioFDs) < 1 {
+	if g.bundleDir == "" || len(g.ioFDs) < 1 || g.specFD < 0 {
 		f.Usage()
 		return subcommands.ExitUsageError
 	}
 
+	specFile := os.NewFile(uintptr(g.specFD), "spec file")
+	defer specFile.Close()
+	spec, err := specutils.ReadSpecFromFile(g.bundleDir, specFile)
+	if err != nil {
+		Fatalf("reading spec: %v", err)
+	}
+
+	// Find what path is going to be served by this gofer.
+	root := spec.Root.Path
+
+	conf := args[0].(*boot.Config)
+
+	if g.setUpRoot && !conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
+		// Convert all shared mounts into slave to be sure that nothing will be
+		// propagated outside of our namespace.
+		if err := syscall.Mount("", "/", "", syscall.MS_SLAVE|syscall.MS_REC, ""); err != nil {
+			Fatalf("error converting mounts: %v", err)
+		}
+
+		// FIXME: runsc can't be re-executed without
+		// /proc, so we create a tmpfs mount, mount ./proc and ./root
+		// there, then move this mount to the root and after
+		// setCapsAndCallSelf, runsc will chroot into /root.
+		//
+		// We need a directory to construct a new root and we know that
+		// runsc can't start without /proc, so we can use it for this.
+		flags := uintptr(syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_NOEXEC)
+		if err := syscall.Mount("runsc-root", "/proc", "tmpfs", flags, ""); err != nil {
+			Fatalf("error mounting tmpfs: %v", err)
+		}
+		os.Mkdir("/proc/proc", 0755)
+		os.Mkdir("/proc/root", 0755)
+		if err := syscall.Mount("runsc-proc", "/proc/proc", "proc", flags|syscall.MS_RDONLY, ""); err != nil {
+			Fatalf("error mounting proc: %v", err)
+		}
+		if err := syscall.Mount(root, "/proc/root", "", syscall.MS_BIND|syscall.MS_REC, ""); err != nil {
+			Fatalf("error mounting root: %v", err)
+		}
+		if err := pivotRoot("/proc"); err != nil {
+			Fatalf("faild to change the root file system: %v", err)
+		}
+		if err := os.Chdir("/"); err != nil {
+			Fatalf("failed to change working directory")
+		}
+	}
+
 	if g.applyCaps {
 		// Disable caps when calling myself again.
 		// Note: minimal argument handling for the default case to keep it simple.
 		args := os.Args
-		args = append(args, "--apply-caps=false")
+		args = append(args, "--apply-caps=false", "--setup-root=false")
 		if err := setCapsAndCallSelf(args, goferCaps); err != nil {
 			Fatalf("Unable to apply caps: %v", err)
 		}
 		panic("unreachable")
 	}
 
-	specFile, err := specutils.OpenCleanSpec(g.bundleDir)
-	if err != nil {
-		Fatalf("opening spec: %v", err)
-	}
-	spec, err := specutils.ReadSpecFromFile(g.bundleDir, specFile)
-	specFile.Close()
-	if err != nil {
-		Fatalf("reading spec: %v", err)
-	}
 	specutils.LogSpec(spec)
 
 	// fsgofer should run with a umask of 0, because we want to preserve file
 	// modes exactly as sent by the sandbox, which will have applied its own umask.
 	syscall.Umask(0)
 
-	// Find what path is going to be served by this gofer.
+	if !conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
-	root := spec.Root.Path
+		root = "/root"
+	}
 	if err := syscall.Chroot(root); err != nil {
 		Fatalf("failed to chroot to %q: %v", root, err)
 	}

runsc/container/container.go

@@ -787,11 +787,50 @@ func (c *Container) waitForStopped() error {
 func (c *Container) createGoferProcess(spec *specs.Spec, conf *boot.Config, bundleDir string) ([]*os.File, error) {
 	// Start with the general config flags.
 	args := conf.ToFlags()
+
+	var goferEnds []*os.File
+
+	// nextFD is the next available file descriptor for the gofer process.
+	// It starts at 3 because 0-2 are used by stdin/stdout/stderr.
+	nextFD := 3
+
+	if conf.LogFilename != "" {
+		logFile, err := os.OpenFile(conf.LogFilename, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+		if err != nil {
+			return nil, fmt.Errorf("opening log file %q: %v", conf.LogFilename, err)
+		}
+		defer logFile.Close()
+		goferEnds = append(goferEnds, logFile)
+		args = append(args, "--log-fd="+strconv.Itoa(nextFD))
+		nextFD++
+	}
+
+	if conf.DebugLog != "" {
+		debugLogFile, err := specutils.DebugLogFile(conf.DebugLog, "gofer")
+		if err != nil {
+			return nil, fmt.Errorf("opening debug log file in %q: %v", conf.DebugLog, err)
+		}
+		defer debugLogFile.Close()
+		goferEnds = append(goferEnds, debugLogFile)
+		args = append(args, "--debug-log-fd="+strconv.Itoa(nextFD))
+		nextFD++
+	}
+
 	args = append(args, "gofer", "--bundle", bundleDir)
 	if conf.Overlay {
 		args = append(args, "--panic-on-write=true")
 	}
 
+	// Open the spec file to donate to the sandbox.
+	specFile, err := specutils.OpenCleanSpec(bundleDir)
+	if err != nil {
+		return nil, fmt.Errorf("opening spec file: %v", err)
+	}
+	defer specFile.Close()
+	goferEnds = append(goferEnds, specFile)
+	args = append(args, "--spec-fd="+strconv.Itoa(nextFD))
+	nextFD++
+
 	// Add root mount and then add any other additional mounts.
 	mountCount := 1
 
@@ -802,12 +841,8 @@ func (c *Container) createGoferProcess(spec *specs.Spec, conf *boot.Config, bund
 		}
 	}
 	sandEnds := make([]*os.File, 0, mountCount)
-	goferEnds := make([]*os.File, 0, mountCount)
 
-	// nextFD is the next available file descriptor for the gofer process.
+	for i := 0; i < mountCount; i++ {
-	// It starts at 3 because 0-2 are used by stdin/stdout/stderr.
-	nextFD := 3
-	for ; nextFD-3 < mountCount; nextFD++ {
 		fds, err := syscall.Socketpair(syscall.AF_UNIX, syscall.SOCK_STREAM|syscall.SOCK_CLOEXEC, 0)
 		if err != nil {
 			return nil, err
@@ -819,6 +854,7 @@ func (c *Container) createGoferProcess(spec *specs.Spec, conf *boot.Config, bund
 		goferEnds = append(goferEnds, goferEnd)
 
 		args = append(args, fmt.Sprintf("--io-fds=%d", nextFD))
+		nextFD++
 	}
 
 	binPath := specutils.ExePath

runsc/main.go

@@ -179,8 +179,8 @@ func main() {
 
 		// Quick sanity check to make sure no other commands get passed
 		// a log fd (they should use log dir instead).
-		if subcommand != "boot" {
+		if subcommand != "boot" && subcommand != "gofer" {
-			cmd.Fatalf("flag --debug-log-fd should only be passed to 'boot' command, but was passed to %q", subcommand)
+			cmd.Fatalf("flag --debug-log-fd should only be passed to 'boot' and 'gofer' command, but was passed to %q", subcommand)
 		}
 
 		// If we are the boot process, then we own our stdio FDs and

runsc/test/root/chroot_test.go

@@ -118,10 +118,7 @@ func TestChrootGofer(t *testing.T) {
 
 	// This where the root directory is mapped on the host and that's where the
 	// gofer must have chroot'd to.
-	root, err := d.RootDirInHost()
+	root := "/root"
-	if err != nil {
-		t.Fatalf("Docker.RootDirInHost(): %v", err)
-	}
 
 	for _, child := range children {
 		childPID, err := strconv.Atoi(child)

runsc/test/testutil/docker.go

@@ -297,15 +297,6 @@ func (d *Docker) SandboxPid() (int, error) {
 	return pid, nil
 }
 
-// RootDirInHost returns where the root directory is mapped on the host.
-func (d *Docker) RootDirInHost() (string, error) {
-	out, err := do("inspect", "-f={{.GraphDriver.Data.MergedDir}}", d.Name)
-	if err != nil {
-		return "", fmt.Errorf("error retrieving pid: %v", err)
-	}
-	return strings.TrimSuffix(string(out), "\n"), nil
-}
-
 // ID returns the container ID.
 func (d *Docker) ID() (string, error) {
 	out, err := do("inspect", "-f={{.Id}}", d.Name)