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

3 files changed, 45 insertions, 139 deletions

diff --git a/runsc/sandbox/BUILD b/runsc/sandbox/BUILD
index d6043bcf7..899fd99de 100644
--- a/runsc/sandbox/BUILD
+++ b/runsc/sandbox/BUILD
@@ -5,7 +5,6 @@ package(licenses = ["notice"])  # Apache 2.0
 go_library(
     name = "sandbox",
     srcs = [
-        "chroot.go",
         "network.go",
         "sandbox.go",
     ],
diff --git a/runsc/sandbox/chroot.go b/runsc/sandbox/chroot.go
deleted file mode 100644
index 354049871..000000000
--- a/runsc/sandbox/chroot.go
+++ /dev/null
@@ -1,97 +0,0 @@
-// Copyright 2018 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 sandbox
-
-import (
-	"fmt"
-	"io/ioutil"
-	"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() (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", syscall.MS_NOSUID|syscall.MS_NODEV|syscall.MS_NOEXEC); 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)
-	}
-
-	return chroot, nil
-}
-
-// tearDownChroot unmounts /proc and /runsc from the chroot before deleting the
-// directory.
-func tearDownChroot(chroot string) error {
-	log.Debugf("Removing chroot mounts %q", chroot)
-
-	// 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)
-	}
-
-	// 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 fe55ddab8..411200793 100644
--- a/runsc/sandbox/sandbox.go
+++ b/runsc/sandbox/sandbox.go
@@ -56,10 +56,6 @@ type Sandbox struct {
 	// 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"`
-
 	// Cgroup has the cgroup configuration for the sandbox.
 	Cgroup *cgroup.Cgroup `json:"cgroup"`
 
@@ -491,6 +487,17 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		// rules.
 		cmd.Args = append(cmd.Args, "--apply-caps=true")
 
+		// 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.HasCapabilities(capability.CAP_SYS_ADMIN) {
+			log.Infof("Sandbox will be started in minimal chroot")
+			cmd.Args = append(cmd.Args, "--setup-root")
+		} else {
+			return fmt.Errorf("can't run sandbox process in minimal chroot since we don't have CAP_SYS_ADMIN")
+		}
 	} else {
 		log.Infof("Sandbox will be started in new user namespace")
 		nss = append(nss, specs.LinuxNamespace{Type: specs.UserNamespace})
@@ -499,50 +506,53 @@ func (s *Sandbox) createSandboxProcess(spec *specs.Spec, conf *boot.Config, bund
 		// 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())
+			log.Warningf("Running sandbox in test mode without chroot. This is only safe in tests!")
 		} else if specutils.HasCapabilities(capability.CAP_SETUID, capability.CAP_SETGID) {
 			// Map nobody in the new namespace to nobody in the parent namespace.
+			//
+			// A sandbox process will construct an empty
+			// root for itself, so it has to have the CAP_SYS_ADMIN
+			// capability.
+			//
+			// FIXME: The current implementations of
+			// os/exec doesn't allow to set ambient capabilities if
+			// a process is started in a new user namespace. As a
+			// workaround, we start the sandbox process with the 0
+			// UID and then it constructs a chroot and sets UID to
+			// nobody.  https://github.com/golang/go/issues/2315
 			const nobody = 65534
-			cmd.SysProcAttr.UidMappings = []syscall.SysProcIDMap{{
-				ContainerID: int(nobody),
-				HostID:      int(nobody),
-				Size:        int(1),
-			}}
-			cmd.SysProcAttr.GidMappings = []syscall.SysProcIDMap{{
-				ContainerID: int(nobody),
-				HostID:      int(nobody),
-				Size:        int(1),
-			}}
+			cmd.SysProcAttr.UidMappings = []syscall.SysProcIDMap{
+				{
+					ContainerID: int(0),
+					HostID:      int(nobody - 1),
+					Size:        int(1),
+				},
+				{
+					ContainerID: int(nobody),
+					HostID:      int(nobody),
+					Size:        int(1),
+				},
+			}
+			cmd.SysProcAttr.GidMappings = []syscall.SysProcIDMap{
+				{
+					ContainerID: int(nobody),
+					HostID:      int(nobody),
+					Size:        int(1),
+				},
+			}
 
 			// Set credentials to run as user and group nobody.
 			cmd.SysProcAttr.Credential = &syscall.Credential{
-				Uid: nobody,
+				Uid: 0,
 				Gid: nobody,
 			}
+			cmd.Args = append(cmd.Args, "--setup-root")
 		} 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.HasCapabilities(capability.CAP_SYS_ADMIN, capability.CAP_SYS_CHROOT) {
-		log.Infof("Sandbox will be started in minimal chroot")
-		chroot, err := setUpChroot()
-		if err != nil {
-			return fmt.Errorf("error setting up chroot: %v", err)
-		}
-		s.Chroot = chroot // Remember path so it can cleaned up.
-		cmd.SysProcAttr.Chroot = chroot
-		cmd.Dir = "/"
-		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 and CAP_SYS_CHROOT")
-	}
+	cmd.Args[0] = "runsc-sandbox"
 
 	if s.Cgroup != nil {
 		cpuNum, err := s.Cgroup.NumCPU()
@@ -676,12 +686,6 @@ func (s *Sandbox) destroy() error {
 		}
 	}
 
-	if s.Chroot != "" {
-		if err := tearDownChroot(s.Chroot); err != nil {
-			return err
-		}
-	}
-
 	return nil
 }