runsc: Run sandbox process inside minimal chroot.

We construct a dir with the executable bind-mounted at /exe, and proc mounted
at /proc.  Runsc now executes the sandbox process inside this chroot, thus
limiting access to the host filesystem.  The mounts and chroot dir are removed
when the sandbox is destroyed.

Because this requires bind-mounts, we can only do the chroot if we have
CAP_SYS_ADMIN.

PiperOrigin-RevId: 211994001
Change-Id: Ia71c515e26085e0b69b833e71691830148bc70d1

1 files changed, 120 insertions, 0 deletions

diff --git a/runsc/sandbox/chroot.go b/runsc/sandbox/chroot.go
new file mode 100644
index 000000000..a77a186c2
--- /dev/null
+++ b/runsc/sandbox/chroot.go
@@ -0,0 +1,120 @@
+// Copyright 2018 Google Inc.
+//
+// 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 sandbox
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/runsc/boot"
+	"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)
+
+	return specutils.Mount(src, chrootDst, typ, flags)
+}
+
+// setUpChroot creates an empty directory with runsc mounted at /runsc, proc
+// mounted at /proc, and any dev files needed for the platform.
+func setUpChroot(platform boot.PlatformType) (string, error) {
+	// Create the chroot directory and make it accessible to all users.
+	chroot, err := ioutil.TempDir("", "runsc-sandbox-chroot-")
+	if err != nil {
+		return "", fmt.Errorf("TempDir() failed: %v", err)
+	}
+	if err := os.Chmod(chroot, 0777); err != nil {
+		return "", fmt.Errorf("Chmod(%q) failed: %v", chroot, err)
+	}
+	log.Infof("Setting up sandbox chroot in %q", chroot)
+
+	// Mount /proc.
+	if err := mountInChroot(chroot, "proc", "/proc", "proc", 0); err != nil {
+		return "", fmt.Errorf("error mounting proc in chroot: %v", err)
+	}
+
+	// Mount runsc at /runsc in the chroot.
+	binPath, err := specutils.BinPath()
+	if err != nil {
+		return "", err
+	}
+	if err := mountInChroot(chroot, binPath, chrootBinPath, "bind", syscall.MS_BIND|syscall.MS_RDONLY); err != nil {
+		return "", fmt.Errorf("error mounting runsc in chroot: %v", err)
+	}
+
+	// Mount dev files needed for platform.
+	var devMount string
+	switch platform {
+	case boot.PlatformKVM:
+		devMount = "/dev/kvm"
+	}
+	if devMount != "" {
+		if err := mountInChroot(chroot, devMount, devMount, "bind", syscall.MS_BIND); err != nil {
+			return "", fmt.Errorf("error mounting platform device in chroot: %v", err)
+		}
+	}
+
+	return chroot, nil
+}
+
+// tearDownChroot unmounts /proc and /runsc from the chroot before deleting the
+// directory.
+func tearDownChroot(chroot string) error {
+	// Unmount /proc.
+	proc := filepath.Join(chroot, "proc")
+	if err := syscall.Unmount(proc, 0); err != nil {
+		return fmt.Errorf("error unmounting %q: %v", proc, err)
+	}
+
+	// Unmount /runsc.
+	exe := filepath.Join(chroot, chrootBinPath)
+	if err := syscall.Unmount(exe, 0); err != nil {
+		return fmt.Errorf("error unmounting %q: %v", exe, err)
+	}
+
+	// Unmount platform dev files.
+	devFiles := []string{"dev/kvm"}
+	for _, f := range devFiles {
+		devPath := filepath.Join(chroot, f)
+		if _, err := os.Stat(devPath); err != nil {
+			if os.IsNotExist(err) {
+				continue
+			}
+			return fmt.Errorf("Stat(%q) failed: %v", devPath, err)
+		}
+		if err := syscall.Unmount(devPath, 0); err != nil {
+			return fmt.Errorf("error unmounting %q: %v", devPath, err)
+		}
+	}
+
+	// Remove chroot directory.
+	if err := os.RemoveAll(chroot); err != nil {
+		return fmt.Errorf("error removing %q: %v", chroot, err)
+	}
+
+	return nil
+}