syscall: add GidMappingsEnableSetgroups to Linux SysProcAttr

Linux 3.19 made a change in the handling of setgroups and the 'gid_map' file to
address a security issue.
The upshot of the 3.19 changes is that in order to update the 'gid_maps' file,
use of the setgroups() system call in this user namespace must first be disabled
by writing "deny" to one of the /proc/PID/setgroups files for this namespace.

Also added tests for remapping uid_map and gid_map inside new user
namespace.

Fixes #10626

Change-Id: I4d2539acbab741a37092d277e10f31fc39a8feb7
Reviewed-on: https://go-review.googlesource.com/10670
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

diff --git a/src/syscall/exec_linux.go b/src/syscall/exec_linux.go
index 3aa30c736404adb970ae6116ce69da623c571d20..9bac04212488763d3ac3be0d339c0c1cb90e91ac 100644 (file)
--- a/src/syscall/exec_linux.go
+++ b/src/syscall/exec_linux.go
@@ -33,6 +33,11 @@ type SysProcAttr struct {
        Cloneflags  uintptr        // Flags for clone calls (Linux only)
        UidMappings []SysProcIDMap // User ID mappings for user namespaces.
        GidMappings []SysProcIDMap // Group ID mappings for user namespaces.
+       // GidMappingsEnableSetgroups enabling setgroups syscall.
+       // If false, then setgroups syscall will be disabled for the child process.
+       // This parameter is no-op if GidMappings == nil. Otherwise for unprivileged
+       // users this should be set to false for mappings work.
+       GidMappingsEnableSetgroups bool
 }
 
 // Implemented in runtime package.
@@ -366,6 +371,32 @@ func writeIDMappings(path string, idMap []SysProcIDMap) error {
        return nil
 }
 
+// writeSetgroups writes to /proc/PID/setgroups "deny" if enable is false
+// and "allow" if enable is true.
+// This is needed since kernel 3.19, because you can't write gid_map without
+// disabling setgroups() system call.
+func writeSetgroups(pid int, enable bool) error {
+       sgf := "/proc/" + itoa(pid) + "/setgroups"
+       fd, err := Open(sgf, O_RDWR, 0)
+       if err != nil {
+               return err
+       }
+
+       var data []byte
+       if enable {
+               data = []byte("allow")
+       } else {
+               data = []byte("deny")
+       }
+
+       if _, err := Write(fd, data); err != nil {
+               Close(fd)
+               return err
+       }
+
+       return Close(fd)
+}
+
 // writeUidGidMappings writes User ID and Group ID mappings for user namespaces
 // for a process and it is called from the parent process.
 func writeUidGidMappings(pid int, sys *SysProcAttr) error {
@@ -377,6 +408,10 @@ func writeUidGidMappings(pid int, sys *SysProcAttr) error {
        }
 
        if sys.GidMappings != nil {
+               // If the kernel is too old to support /proc/PID/setgroups, writeSetGroups will return ENOENT; this is OK.
+               if err := writeSetgroups(pid, sys.GidMappingsEnableSetgroups); err != nil && err != ENOENT {
+                       return err
+               }
                gidf := "/proc/" + itoa(pid) + "/gid_map"
                if err := writeIDMappings(gidf, sys.GidMappings); err != nil {
                        return err

diff --git a/src/syscall/exec_linux_test.go b/src/syscall/exec_linux_test.go
new file mode 100644 (file)
index 0000000..a39fd58
--- /dev/null
+++ b/src/syscall/exec_linux_test.go
@@ -0,0 +1,84 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build linux
+
+package syscall_test
+
+import (
+       "os"
+       "os/exec"
+       "strings"
+       "syscall"
+       "testing"
+)
+
+func whoamiCmd(t *testing.T, uid int, setgroups bool) *exec.Cmd {
+       if _, err := os.Stat("/proc/self/ns/user"); err != nil {
+               if os.IsNotExist(err) {
+                       t.Skip("kernel doesn't support user namespaces")
+               }
+               t.Fatalf("Failed to stat /proc/self/ns/user: %v", err)
+       }
+       cmd := exec.Command("whoami")
+       cmd.SysProcAttr = &syscall.SysProcAttr{
+               Cloneflags: syscall.CLONE_NEWUSER,
+               UidMappings: []syscall.SysProcIDMap{
+                       {ContainerID: 0, HostID: uid, Size: 1},
+               },
+               GidMappings: []syscall.SysProcIDMap{
+                       {ContainerID: 0, HostID: uid, Size: 1},
+               },
+               GidMappingsEnableSetgroups: setgroups,
+       }
+       return cmd
+}
+
+func testNEWUSERRemap(t *testing.T, uid int, setgroups bool) {
+       cmd := whoamiCmd(t, uid, setgroups)
+       out, err := cmd.CombinedOutput()
+       if err != nil {
+               t.Fatalf("Cmd failed with err %v, output: %s", err, out)
+       }
+       sout := strings.TrimSpace(string(out))
+       want := "root"
+       if sout != want {
+               t.Fatalf("whoami = %q; want %q", out, want)
+       }
+}
+
+func TestCloneNEWUSERAndRemapRootDisableSetgroups(t *testing.T) {
+       if os.Getuid() != 0 {
+               t.Skip("skipping root only test")
+       }
+       testNEWUSERRemap(t, 0, false)
+}
+
+func TestCloneNEWUSERAndRemapRootEnableSetgroups(t *testing.T) {
+       if os.Getuid() != 0 {
+               t.Skip("skipping root only test")
+       }
+       testNEWUSERRemap(t, 0, false)
+}
+
+func TestCloneNEWUSERAndRemapNoRootDisableSetgroups(t *testing.T) {
+       if os.Getuid() == 0 {
+               t.Skip("skipping unprivileged user only test")
+       }
+       testNEWUSERRemap(t, os.Getuid(), false)
+}
+
+func TestCloneNEWUSERAndRemapNoRootSetgroupsEnableSetgroups(t *testing.T) {
+       if os.Getuid() == 0 {
+               t.Skip("skipping unprivileged user only test")
+       }
+       cmd := whoamiCmd(t, os.Getuid(), true)
+       err := cmd.Run()
+       if err == nil {
+               t.Skip("probably old kernel without security fix")
+       }
+       if !strings.Contains(err.Error(), "operation not permitted") {
+               t.Fatalf("Unprivileged gid_map rewriting with GidMappingsEnableSetgroups must fail")
+       }
+}