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

4 files changed, 150 insertions, 3 deletions

diff --git a/runsc/cmd/BUILD b/runsc/cmd/BUILD
index 394bb0e1f..a908172af 100644
--- a/runsc/cmd/BUILD
+++ b/runsc/cmd/BUILD
@@ -8,6 +8,7 @@ go_library(
         "boot.go",
         "capability.go",
         "checkpoint.go",
+        "chroot.go",
         "cmd.go",
         "create.go",
         "debug.go",
diff --git a/runsc/cmd/boot.go b/runsc/cmd/boot.go
index bb3435284..7ca2744bd 100644
--- a/runsc/cmd/boot.go
+++ b/runsc/cmd/boot.go
@@ -60,6 +60,9 @@ type Boot struct {
 	// to the process.
 	applyCaps bool
 
+	// setUpChroot is set to true if the sandbox is started in an empty root.
+	setUpRoot bool
+
 	// cpuNum number of CPUs to create inside the sandbox.
 	cpuNum int
 
@@ -99,6 +102,7 @@ func (b *Boot) SetFlags(f *flag.FlagSet) {
 	f.Var(&b.stdioFDs, "stdio-fds", "list of FDs containing sandbox stdin, stdout, and stderr in that order")
 	f.BoolVar(&b.console, "console", false, "set to true if the sandbox should allow terminal ioctl(2) syscalls")
 	f.BoolVar(&b.applyCaps, "apply-caps", false, "if true, apply capabilities defined in the spec to the process")
+	f.BoolVar(&b.setUpRoot, "setup-root", false, "if true, set up an empty root for the process")
 	f.IntVar(&b.cpuNum, "cpu-num", 0, "number of CPUs to create inside the sandbox")
 	f.Uint64Var(&b.totalMem, "total-memory", 0, "sets the initial amount of total memory to report back to the container")
 	f.IntVar(&b.userLogFD, "user-log-fd", 0, "file descriptor to write user logs to. 0 means no logging.")
@@ -116,6 +120,31 @@ func (b *Boot) Execute(_ context.Context, f *flag.FlagSet, args ...interface{})
 	// Ensure that if there is a panic, all goroutine stacks are printed.
 	debug.SetTraceback("all")
 
+	if b.setUpRoot {
+		if err := setUpChroot(); err != nil {
+			Fatalf("error setting up chroot: %v", err)
+		}
+
+		specutils.ExePath = "/runsc"
+		if !b.applyCaps {
+			// Remove --setup-root arg to call myself.
+			var args []string
+			for _, arg := range os.Args {
+				if !strings.Contains(arg, "setup-root") {
+					args = append(args, arg)
+				}
+			}
+			// Note that we've already read the spec from the spec FD, and
+			// we will read it again after the exec call. This works
+			// because the ReadSpecFromFile function seeks to the beginning
+			// of the file before reading.
+			if err := callSelfAsNobody(args); err != nil {
+				Fatalf("%v", err)
+			}
+			panic("callSelfAsNobody must never return success")
+		}
+	}
+
 	// Get the spec from the specFD.
 	specFile := os.NewFile(uintptr(b.specFD), "spec file")
 	defer specFile.Close()
@@ -144,7 +173,7 @@ func (b *Boot) Execute(_ context.Context, f *flag.FlagSet, args ...interface{})
 		// Remove --apply-caps arg to call myself.
 		var args []string
 		for _, arg := range os.Args {
-			if !strings.Contains(arg, "apply-caps") {
+			if !strings.Contains(arg, "setup-root") && !strings.Contains(arg, "apply-caps") {
 				args = append(args, arg)
 			}
 		}
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
+}
diff --git a/runsc/cmd/cmd.go b/runsc/cmd/cmd.go
index a1c3491a3..fbfc18fc9 100644
--- a/runsc/cmd/cmd.go
+++ b/runsc/cmd/cmd.go
@@ -86,6 +86,28 @@ func setCapsAndCallSelf(args []string, caps *specs.LinuxCapabilities) error {
 	}
 
 	log.Infof("Execve %q again, bye!", binPath)
-	syscall.Exec(binPath, args, []string{})
-	panic("unreachable")
+	err = syscall.Exec(binPath, args, []string{})
+	return fmt.Errorf("error executing %s: %v", binPath, err)
+}
+
+// callSelfAsNobody sets UID and GID to nobody and then execve's itself again.
+func callSelfAsNobody(args []string) error {
+	// Keep thread locked while user/group are changed.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	const nobody = 65534
+
+	if _, _, err := syscall.RawSyscall(syscall.SYS_SETGID, uintptr(nobody), 0, 0); err != 0 {
+		return fmt.Errorf("error setting uid: %v", err)
+	}
+	if _, _, err := syscall.RawSyscall(syscall.SYS_SETUID, uintptr(nobody), 0, 0); err != 0 {
+		return fmt.Errorf("error setting gid: %v", err)
+	}
+
+	binPath := "/runsc"
+
+	log.Infof("Execve %q again, bye!", binPath)
+	err := syscall.Exec(binPath, args, []string{})
+	return fmt.Errorf("error executing %s: %v", binPath, err)
 }