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

3 files changed, 150 insertions, 4 deletions

diff --git a/runsc/sandbox/BUILD b/runsc/sandbox/BUILD
index 9317b1c14..8ebd14c4e 100644
--- a/runsc/sandbox/BUILD
+++ b/runsc/sandbox/BUILD
@@ -5,6 +5,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library")
 go_library(
     name = "sandbox",
     srcs = [
+        "chroot.go",
         "network.go",
         "sandbox.go",
     ],
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
+}
diff --git a/runsc/sandbox/sandbox.go b/runsc/sandbox/sandbox.go
index dd5a0aa56..f6264d5b2 100644
--- a/runsc/sandbox/sandbox.go
+++ b/runsc/sandbox/sandbox.go
@@ -51,6 +51,10 @@ type Sandbox struct {
 	// Pid is the pid of the running sandbox (immutable). May be 0 is the sandbox
 	// is not running.
 	Pid int `json:"pid"`
+
+	// Chroot is the path to the chroot directory that the sandbox process
+	// is running in.
+	Chroot string `json:"chroot"`
 }
 
 // Create creates the sandbox process.
@@ -392,12 +396,11 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		log.Infof("Sandbox will be started in new user namespace")
 		nss = append(nss, specs.LinuxNamespace{Type: specs.UserNamespace})
 
-		if conf.TestOnlyAllowRunAsCurrentUser {
+		// If we have CAP_SETUID and CAP_SETGID, then we can also run
+		// as user nobody.
+		if conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
 			log.Warningf("Running sandbox in test mode as current user (uid=%d gid=%d). This is only safe in tests!", os.Getuid(), os.Getgid())
 		} else if specutils.CanSetUIDGID() {
-			// If we have CAP_SETUID and CAP_SETGID, then we can also run
-			// as user nobody.
-
 			// Map nobody in the new namespace to nobody in the parent namespace.
 			const nobody = 65534
 			cmd.SysProcAttr.UidMappings = []syscall.SysProcIDMap{{
@@ -419,6 +422,23 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		} else {
 			return fmt.Errorf("can't run sandbox process as user nobody since we don't have CAP_SETUID or CAP_SETGID")
 		}
+
+		// If we have CAP_SYS_ADMIN, we can create an empty chroot and
+		// bind-mount the executable inside it.
+		if conf.TestOnlyAllowRunAsCurrentUserWithoutChroot {
+			log.Warningf("Running sandbox in test mode without chroot. This is only safe in tests!")
+		} else if specutils.HasCapSysAdmin() {
+			log.Infof("Sandbox will be started in minimal chroot")
+			chroot, err := setUpChroot(conf.Platform)
+			if err != nil {
+				return fmt.Errorf("error setting up chroot: %v", err)
+			}
+			cmd.SysProcAttr.Chroot = chroot
+			cmd.Args[0] = "/runsc"
+			cmd.Path = "/runsc"
+		} else {
+			return fmt.Errorf("can't run sandbox process in minimal chroot since we don't have CAP_SYS_ADMIN")
+		}
 	}
 
 	// Log the fds we are donating to the sandbox process.
@@ -525,6 +545,11 @@ func (s *Sandbox) Destroy() error {
 		log.Debugf("Killing sandbox %q", s.ID)
 		signalProcess(s.Pid, unix.SIGKILL)
 	}
+
+	if s.Chroot != "" {
+		return tearDownChroot(s.Chroot)
+	}
+
 	return nil
 }