v1 runtime: reduce permissions for bundle dir

Bundle directory permissions should be 0700 by default.  On Linux with
user namespaces enabled, the remapped root also needs access to the
bundle directory.  In this case, the bundle directory is modified to
0710 and group ownership is changed to the remapped root group.

Port of the same change for the v2 runtime

Signed-off-by: Samuel Karp <skarp@amazon.com>

runtime/v1/linux/bundle.go

@@ -22,6 +22,7 @@ package linux
 import (
 	"context"
 	"crypto/sha256"
+	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"os"
@@ -31,6 +32,7 @@ import (
 	"github.com/containerd/containerd/runtime/linux/runctypes"
 	"github.com/containerd/containerd/runtime/v1/shim"
 	"github.com/containerd/containerd/runtime/v1/shim/client"
+	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/pkg/errors"
 )
 
@@ -49,7 +51,7 @@ func newBundle(id, path, workDir string, spec []byte) (b *bundle, err error) {
 		return nil, err
 	}
 	path = filepath.Join(path, id)
-	if err := os.Mkdir(path, 0711); err != nil {
+	if err := os.Mkdir(path, 0700); err != nil {
 		return nil, err
 	}
 	defer func() {
@@ -57,6 +59,9 @@ func newBundle(id, path, workDir string, spec []byte) (b *bundle, err error) {
 			os.RemoveAll(path)
 		}
 	}()
+	if err := prepareBundleDirectoryPermissions(path, spec); err != nil {
+		return nil, err
+	}
 	workDir = filepath.Join(workDir, id)
 	if err := os.MkdirAll(workDir, 0711); err != nil {
 		return nil, err
@@ -78,6 +83,55 @@ func newBundle(id, path, workDir string, spec []byte) (b *bundle, err error) {
 	}, err
 }
 
+// prepareBundleDirectoryPermissions prepares the permissions of the bundle
+// directory. When user namespaces are enabled, the permissions are modified
+// to allow the remapped root GID to access the bundle.
+func prepareBundleDirectoryPermissions(path string, spec []byte) error {
+	gid, err := remappedGID(spec)
+	if err != nil {
+		return err
+	}
+	if gid == 0 {
+		return nil
+	}
+	if err := os.Chown(path, -1, int(gid)); err != nil {
+		return err
+	}
+	return os.Chmod(path, 0710)
+}
+
+// ociSpecUserNS is a subset of specs.Spec used to reduce garbage during
+// unmarshal.
+type ociSpecUserNS struct {
+	Linux *linuxSpecUserNS
+}
+
+// linuxSpecUserNS is a subset of specs.Linux used to reduce garbage during
+// unmarshal.
+type linuxSpecUserNS struct {
+	GIDMappings []specs.LinuxIDMapping
+}
+
+// remappedGID reads the remapped GID 0 from the OCI spec, if it exists. If
+// there is no remapping, remappedGID returns 0. If the spec cannot be parsed,
+// remappedGID returns an error.
+func remappedGID(spec []byte) (uint32, error) {
+	var ociSpec ociSpecUserNS
+	err := json.Unmarshal(spec, &ociSpec)
+	if err != nil {
+		return 0, err
+	}
+	if ociSpec.Linux == nil || len(ociSpec.Linux.GIDMappings) == 0 {
+		return 0, nil
+	}
+	for _, mapping := range ociSpec.Linux.GIDMappings {
+		if mapping.ContainerID == 0 {
+			return mapping.HostID, nil
+		}
+	}
+	return 0, nil
+}
+
 type bundle struct {
 	id      string
 	path    string