Add an option to prevent putting the shim in a new mount namespace

This is needed for users on kernel older than 3.18 so they can avoid EBUSY
errors when trying to unlink, rename or remove a mountpoint that is present in
a shim namespace.

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

container_test.go

@@ -1216,9 +1216,9 @@ func TestContainerExitedAtSet(t *testing.T) {
 	}
 
 	status := <-statusC
-	code, _, _ := status.Result()
+	code, _, err := status.Result()
 	if code != 0 {
-		t.Errorf("expected status 0 but received %d", code)
+		t.Errorf("expected status 0 but received %d (err: %v)", code, err)
 	}
 
 	if s, err := task.Status(ctx); err != nil {

docs/ops.md

@@ -212,4 +212,7 @@ The linux runtime allows a few options to be set to configure the shim and the r
 	no_shim = false
 	# display shim logs in the containerd daemon's log output
 	shim_debug = true
+	# do not put the shim in its own mount namespace
+	# (this only need to be set on kernel < 3.18)
+	shim_no_newns = true
 ```

linux/bundle.go

@@ -73,10 +73,10 @@ type bundle struct {
 type shimOpt func(*bundle, string, *runcopts.RuncOptions) (client.Config, client.ClientOpt)
 
 // ShimRemote is a shimOpt for connecting and starting a remote shim
-func ShimRemote(shim, daemonAddress, cgroup string, debug bool, exitHandler func()) shimOpt {
+func ShimRemote(shim, daemonAddress, cgroup string, nonewns, debug bool, exitHandler func()) shimOpt {
 	return func(b *bundle, ns string, ropts *runcopts.RuncOptions) (client.Config, client.ClientOpt) {
 		return b.shimConfig(ns, ropts),
-			client.WithStart(shim, b.shimAddress(ns), daemonAddress, cgroup, debug, exitHandler)
+			client.WithStart(shim, b.shimAddress(ns), daemonAddress, cgroup, nonewns, debug, exitHandler)
 	}
 }
 

linux/runtime.go

@@ -77,6 +77,17 @@ type Config struct {
 	NoShim bool `toml:"no_shim"`
 	// Debug enable debug on the shim
 	ShimDebug bool `toml:"shim_debug"`
+	// ShimNoMountNS prevents the runtime from putting shims into their own mount namespace.
+	//
+	// Putting the shim in its own mount namespace ensure that any mounts made
+	// by it in order to get the task rootfs ready will be undone regardless
+	// on how the shim dies.
+	//
+	// NOTE: This should only be used in kernel older than 3.18 to avoid shims
+	// from causing a DoS in their parent namespace due to having a copy of
+	// mounts previously there which would prevent unlink, rename and remove
+	// operations on those mountpoints.
+	ShimNoMountNS bool `toml:"shim_no_newns"`
 }
 
 // New returns a configured runtime
@@ -180,7 +191,8 @@ func (r *Runtime) Create(ctx context.Context, id string, opts runtime.CreateOpts
 			}
 			cgroup = v.(*runcopts.CreateOptions).ShimCgroup
 		}
-		shimopt = ShimRemote(r.config.Shim, r.address, cgroup, r.config.ShimDebug, func() {
+		exitHandler := func() {
+			log.G(ctx).WithField("id", id).Info("shim reaped")
 			t, err := r.tasks.Get(ctx, id)
 			if err != nil {
 				// Task was never started or was already sucessfully deleted
@@ -209,7 +221,9 @@ func (r *Runtime) Create(ctx context.Context, id string, opts runtime.CreateOpts
 					"namespace": namespace,
 				}).Warn("failed to clen up after killed shim")
 			}
-		})
+		}
+		shimopt = ShimRemote(r.config.Shim, r.address, cgroup,
+			r.config.ShimNoMountNS, r.config.ShimDebug, exitHandler)
 	}
 
 	s, err := bundle.NewShimClient(ctx, namespace, shimopt, ropts)

linux/shim/client.go

@@ -31,7 +31,7 @@ import (
 type ClientOpt func(context.Context, Config) (shim.ShimClient, io.Closer, error)
 
 // WithStart executes a new shim process
-func WithStart(binary, address, daemonAddress, cgroup string, debug bool, exitHandler func()) ClientOpt {
+func WithStart(binary, address, daemonAddress, cgroup string, nonewns, debug bool, exitHandler func()) ClientOpt {
 	return func(ctx context.Context, config Config) (_ shim.ShimClient, _ io.Closer, err error) {
 		socket, err := newSocket(address)
 		if err != nil {
@@ -44,7 +44,7 @@ func WithStart(binary, address, daemonAddress, cgroup string, debug bool, exitHa
 		}
 		defer f.Close()
 
-		cmd := newCommand(binary, daemonAddress, debug, config, f)
+		cmd := newCommand(binary, daemonAddress, nonewns, debug, config, f)
 		ec, err := reaper.Default.Start(cmd)
 		if err != nil {
 			return nil, nil, errors.Wrapf(err, "failed to start shim")
@@ -84,7 +84,7 @@ func WithStart(binary, address, daemonAddress, cgroup string, debug bool, exitHa
 	}
 }
 
-func newCommand(binary, daemonAddress string, debug bool, config Config, socket *os.File) *exec.Cmd {
+func newCommand(binary, daemonAddress string, nonewns, debug bool, config Config, socket *os.File) *exec.Cmd {
 	args := []string{
 		"--namespace", config.Namespace,
 		"--workdir", config.WorkDir,
@@ -109,7 +109,7 @@ func newCommand(binary, daemonAddress string, debug bool, config Config, socket
 	// make sure the shim can be re-parented to system init
 	// and is cloned in a new mount namespace because the overlay/filesystems
 	// will be mounted by the shim
-	cmd.SysProcAttr = &atter
+	cmd.SysProcAttr = getSysProcAttr(nonewns)
 	cmd.ExtraFiles = append(cmd.ExtraFiles, socket)
 	if debug {
 		cmd.Stdout = os.Stdout

linux/shim/client_linux.go

@@ -10,9 +10,14 @@ import (
 	"github.com/pkg/errors"
 )
 
-var atter = syscall.SysProcAttr{
-	Cloneflags: syscall.CLONE_NEWNS,
-	Setpgid:    true,
+func getSysProcAttr(nonewns bool) *syscall.SysProcAttr {
+	attr := syscall.SysProcAttr{
+		Setpgid: true,
+	}
+	if !nonewns {
+		attr.Cloneflags = syscall.CLONE_NEWNS
+	}
+	return &attr
 }
 
 func setCgroup(cgroupPath string, cmd *exec.Cmd) error {

linux/shim/client_unix.go

@@ -7,8 +7,10 @@ import (
 	"syscall"
 )
 
-var atter = syscall.SysProcAttr{
-	Setpgid: true,
+func getSysProcAttr(nonewns bool) *syscall.SysProcAttr {
+	return &syscall.SysProcAttr{
+		Setpgid: true,
+	}
 }
 
 func setCgroup(cgroupPath string, cmd *exec.Cmd) error {