syscall: fix ordering of Unshare and chroot on Linux

When unshare specifies a new namespace, the syscall
package changes / to make namespace changes private.

If a chroot is specified, the unshare must be done first.
If the chroot is done first then the unshare will
not specify the correct /.

A new test is included which test combining chroot
and CLONE_NEWNS; it fails without the patch and works with
it.

Fixes #20103

Change-Id: I86022803c784bd418a30383321f3d64103d95c62
Reviewed-on: https://go-review.googlesource.com/41626Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/exec_linux.go

@@ -195,14 +195,6 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		}
 	}
 
-	// Chroot
-	if chroot != nil {
-		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
-		if err1 != 0 {
-			goto childerror
-		}
-	}
-
 	// Unshare
 	if sys.Unshareflags != 0 {
 		_, _, err1 = RawSyscall(SYS_UNSHARE, sys.Unshareflags, 0, 0)
@@ -224,6 +216,14 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		}
 	}
 
+	// Chroot
+	if chroot != nil {
+		_, _, err1 = RawSyscall(SYS_CHROOT, uintptr(unsafe.Pointer(chroot)), 0, 0)
+		if err1 != 0 {
+			goto childerror
+		}
+	}
+
 	// User and groups
 	if cred := sys.Credential; cred != nil {
 		ngroups := uintptr(len(cred.Groups))

src/syscall/exec_linux_test.go

@@ -12,6 +12,7 @@ import (
 	"io/ioutil"
 	"os"
 	"os/exec"
+	"path/filepath"
 	"strings"
 	"syscall"
 	"testing"
@@ -265,7 +266,6 @@ func TestUnshareMountNameSpaceHelper(*testing.T) {
 		return
 	}
 	defer os.Exit(0)
-
 	if err := syscall.Mount("none", flag.Args()[0], "proc", 0, ""); err != nil {
 		fmt.Fprintf(os.Stderr, "unshare: mount %v failed: %v", os.Args, err)
 		os.Exit(2)
@@ -320,3 +320,65 @@ func TestUnshareMountNameSpace(t *testing.T) {
 		}
 	}
 }
+
+// Test for Issue 20103: unshare fails when chroot is used
+func TestUnshareMountNameSpaceChroot(t *testing.T) {
+	// Make sure we are running as root so we have permissions to use unshare
+	// and create a network namespace.
+	if os.Getuid() != 0 {
+		t.Skip("kernel prohibits unshare in unprivileged process, unless using user namespace")
+	}
+
+	// When running under the Go continuous build, skip tests for
+	// now when under Kubernetes. (where things are root but not quite)
+	// Both of these are our own environment variables.
+	// See Issue 12815.
+	if os.Getenv("GO_BUILDER_NAME") != "" && os.Getenv("IN_KUBERNETES") == "1" {
+		t.Skip("skipping test on Kubernetes-based builders; see Issue 12815")
+	}
+
+	d, err := ioutil.TempDir("", "unshare")
+	if err != nil {
+		t.Fatalf("tempdir: %v", err)
+	}
+
+	// Since we are doing a chroot, we need the binary there,
+	// and it must be statically linked.
+	x := filepath.Join(d, "syscall.test")
+	cmd := exec.Command("go", "test", "-c", "-o", x, "syscall")
+	cmd.Env = append(os.Environ(), "CGO_ENABLED=0")
+	if o, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("Build of syscall in chroot failed, output %v, err %v", o, err)
+	}
+
+	cmd = exec.Command("/syscall.test", "-test.run=TestUnshareMountNameSpaceHelper", "/")
+	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+	cmd.SysProcAttr = &syscall.SysProcAttr{Chroot: d, Unshareflags: syscall.CLONE_NEWNS}
+
+	o, err := cmd.CombinedOutput()
+	if err != nil {
+		if strings.Contains(err.Error(), ": permission denied") {
+			t.Skipf("Skipping test (golang.org/issue/19698); unshare failed due to permissions: %s, %v", o, err)
+		}
+		t.Fatalf("unshare failed: %s, %v", o, err)
+	}
+
+	// How do we tell if the namespace was really unshared? It turns out
+	// to be simple: just try to remove the executable. If it's still mounted
+	// on, the rm will fail. Then we have some cleanup to do:
+	// we must force unmount it, then try to remove it again.
+
+	if err := os.Remove(x); err != nil {
+		t.Errorf("rm failed on %v: %v", x, err)
+		if err := syscall.Unmount(d, syscall.MNT_FORCE); err != nil {
+			t.Fatalf("Can't unmount %v: %v", d, err)
+		}
+		if err := os.Remove(x); err != nil {
+			t.Fatalf("rm failed on %v: %v", x, err)
+		}
+	}
+
+	if err := os.Remove(d); err != nil {
+		t.Errorf("rmdir failed on %v: %v", d, err)
+	}
+}