runsc: set up a minimal chroot from the sandbox process

In this case, new mounts are not created in the host mount namspaces, so
tearDownChroot isn't needed, because chroot will be destroyed with a
sandbox mount namespace.

In additional, pivot_root can't be called instead of chroot.

PiperOrigin-RevId: 229250871
Change-Id: I765bdb587d0b8287a6a8efda8747639d37c7e7b6

1 files changed, 95 insertions, 0 deletions

diff --git a/runsc/cmd/chroot.go b/runsc/cmd/chroot.go
new file mode 100644
index 000000000..b53085934
--- /dev/null
+++ b/runsc/cmd/chroot.go
@@ -0,0 +1,95 @@
+// Copyright 2019 Google LLC
+//
+// 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.
+
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/runsc/specutils"
+)
+
+// chrootBinPath is the location inside the chroot where the runsc binary will
+// be mounted.
+const chrootBinPath = "/runsc"
+
+// mountInChroot creates the destination mount point in the given chroot and
+// mounts the source.
+func mountInChroot(chroot, src, dst, typ string, flags uint32) error {
+	chrootDst := filepath.Join(chroot, dst)
+	log.Infof("Mounting %q at %q", src, chrootDst)
+
+	if err := specutils.Mount(src, chrootDst, typ, flags); err != nil {
+		return fmt.Errorf("error mounting %q at %q: %v", src, chrootDst, err)
+	}
+	return nil
+}
+
+// setUpChroot creates an empty directory with runsc mounted at /runsc and proc
+// mounted at /proc.
+func setUpChroot() error {
+	// We are a new mount namespace, so we can use /tmp as a directory to
+	// construct a new root.
+	chroot := os.TempDir()
+
+	log.Infof("Setting up sandbox chroot in %q", chroot)
+
+	// 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 {
+		return fmt.Errorf("error converting mounts: %v", err)
+	}
+
+	if err := syscall.Mount("runsc-root", chroot, "tmpfs", syscall.MS_NOSUID|syscall.MS_NODEV|syscall.MS_NOEXEC, ""); err != nil {
+		return fmt.Errorf("error mounting tmpfs in choot: %v", err)
+	}
+
+	if err := mountInChroot(chroot, "/proc", "/proc", "bind", syscall.MS_BIND|syscall.MS_RDONLY|syscall.MS_REC); err != nil {
+		return fmt.Errorf("error mounting proc in chroot: %v", err)
+	}
+
+	if err := mountInChroot(chroot, specutils.ExePath, chrootBinPath, "bind", syscall.MS_BIND|syscall.MS_RDONLY); err != nil {
+		return fmt.Errorf("error mounting runsc in chroot: %v", err)
+	}
+
+	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 fmt.Errorf("error umounting the old root file system: %v", err)
+	}
+
+	return nil
+}