syscall: add AmbientCaps to linux SysProcAttr
Fixes #19713
Change-Id: Id1ca61b35bca2a4bea23dd64c7fb001a3a14fd88
Reviewed-on: https://go-review.googlesource.com/43512
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/syscall/exec_linux.go b/src/syscall/exec_linux.go
index 5cbabf7..fd87b86 100644
--- a/src/syscall/exec_linux.go
+++ b/src/syscall/exec_linux.go
@@ -40,6 +40,7 @@
// This parameter is no-op if GidMappings == nil. Otherwise for unprivileged
// users this should be set to false for mappings work.
GidMappingsEnableSetgroups bool
+ AmbientCaps []uintptr // Ambient capabilities (Linux only)
}
var (
@@ -101,6 +102,12 @@
//go:noinline
//go:norace
func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (r1 uintptr, err1 Errno, p [2]int, locked bool) {
+ // Defined in linux/prctl.h starting with Linux 4.3.
+ const (
+ PR_CAP_AMBIENT = 0x2f
+ PR_CAP_AMBIENT_RAISE = 0x2
+ )
+
// vfork requires that the child not touch any of the parent's
// active stack frames. Hence, the child does all post-fork
// processing in this stack frame and never returns, while the
@@ -165,6 +172,14 @@
runtime_AfterForkInChild()
+ // Enable the "keep capabilities" flag to set ambient capabilities later.
+ if len(sys.AmbientCaps) > 0 {
+ _, _, err1 = RawSyscall6(SYS_PRCTL, PR_SET_KEEPCAPS, 1, 0, 0, 0, 0)
+ if err1 != 0 {
+ goto childerror
+ }
+ }
+
// Wait for User ID/Group ID mappings to be written.
if sys.UidMappings != nil || sys.GidMappings != nil {
if _, _, err1 = RawSyscall(SYS_CLOSE, uintptr(p[1]), 0, 0); err1 != 0 {
@@ -279,6 +294,13 @@
}
}
+ for _, c := range sys.AmbientCaps {
+ _, _, err1 = RawSyscall6(SYS_PRCTL, PR_CAP_AMBIENT, uintptr(PR_CAP_AMBIENT_RAISE), c, 0, 0, 0)
+ if err1 != 0 {
+ goto childerror
+ }
+ }
+
// Chdir
if dir != nil {
_, _, err1 = RawSyscall(SYS_CHDIR, uintptr(unsafe.Pointer(dir)), 0, 0)
diff --git a/src/syscall/exec_linux_test.go b/src/syscall/exec_linux_test.go
index 7a88968..854060b 100644
--- a/src/syscall/exec_linux_test.go
+++ b/src/syscall/exec_linux_test.go
@@ -9,13 +9,17 @@
import (
"flag"
"fmt"
+ "io"
"io/ioutil"
"os"
"os/exec"
+ "os/user"
"path/filepath"
+ "strconv"
"strings"
"syscall"
"testing"
+ "unsafe"
)
// Check if we are in a chroot by checking if the inode of / is
@@ -382,3 +386,158 @@
t.Errorf("rmdir failed on %v: %v", d, err)
}
}
+
+type capHeader struct {
+ version uint32
+ pid int
+}
+
+type capData struct {
+ effective uint32
+ permitted uint32
+ inheritable uint32
+}
+
+const CAP_SYS_TIME = 25
+
+type caps struct {
+ hdr capHeader
+ data [2]capData
+}
+
+func getCaps() (caps, error) {
+ var c caps
+
+ // Get capability version
+ if _, _, errno := syscall.Syscall(syscall.SYS_CAPGET, uintptr(unsafe.Pointer(&c.hdr)), uintptr(unsafe.Pointer(nil)), 0); errno != 0 {
+ return c, fmt.Errorf("SYS_CAPGET: %v", errno)
+ }
+
+ // Get current capabilities
+ if _, _, errno := syscall.Syscall(syscall.SYS_CAPGET, uintptr(unsafe.Pointer(&c.hdr)), uintptr(unsafe.Pointer(&c.data[0])), 0); errno != 0 {
+ return c, fmt.Errorf("SYS_CAPGET: %v", errno)
+ }
+
+ return c, nil
+}
+
+func mustSupportAmbientCaps(t *testing.T) {
+ var uname syscall.Utsname
+ if err := syscall.Uname(&uname); err != nil {
+ t.Fatalf("Uname: %v", err)
+ }
+ var buf [65]byte
+ for i, b := range uname.Release {
+ buf[i] = byte(b)
+ }
+ ver := string(buf[:])
+ if i := strings.Index(ver, "\x00"); i != -1 {
+ ver = ver[:i]
+ }
+ if strings.HasPrefix(ver, "2.") ||
+ strings.HasPrefix(ver, "3.") ||
+ strings.HasPrefix(ver, "4.1.") ||
+ strings.HasPrefix(ver, "4.2.") {
+ t.Skipf("kernel version %q predates required 4.3; skipping test", ver)
+ }
+}
+
+// TestAmbientCapsHelper isn't a real test. It's used as a helper process for
+// TestAmbientCaps.
+func TestAmbientCapsHelper(*testing.T) {
+ if os.Getenv("GO_WANT_HELPER_PROCESS") != "1" {
+ return
+ }
+ defer os.Exit(0)
+
+ caps, err := getCaps()
+ if err != nil {
+ fmt.Fprintln(os.Stderr, err)
+ os.Exit(2)
+ }
+ if caps.data[0].effective&(1<<uint(CAP_SYS_TIME)) == 0 {
+ fmt.Fprintln(os.Stderr, "CAP_SYS_TIME unexpectedly not in the effective capability mask")
+ os.Exit(2)
+ }
+}
+
+func TestAmbientCaps(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")
+ }
+ mustSupportAmbientCaps(t)
+
+ // 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")
+ }
+
+ caps, err := getCaps()
+ if err != nil {
+ t.Fatal(err)
+ }
+
+ // Add CAP_SYS_TIME to the permitted and inheritable capability mask,
+ // otherwise we will not be able to add it to the ambient capability mask.
+ caps.data[0].permitted |= 1 << uint(CAP_SYS_TIME)
+ caps.data[0].inheritable |= 1 << uint(CAP_SYS_TIME)
+
+ if _, _, errno := syscall.Syscall(syscall.SYS_CAPSET, uintptr(unsafe.Pointer(&caps.hdr)), uintptr(unsafe.Pointer(&caps.data[0])), 0); errno != 0 {
+ t.Fatalf("SYS_CAPSET: %v", errno)
+ }
+
+ u, err := user.Lookup("nobody")
+ if err != nil {
+ t.Fatal(err)
+ }
+ uid, err := strconv.ParseInt(u.Uid, 0, 32)
+ if err != nil {
+ t.Fatal(err)
+ }
+ gid, err := strconv.ParseInt(u.Gid, 0, 32)
+ if err != nil {
+ t.Fatal(err)
+ }
+
+ // Copy the test binary to a temporary location which is readable by nobody.
+ f, err := ioutil.TempFile("", "gotest")
+ if err != nil {
+ t.Fatal(err)
+ }
+ defer os.Remove(f.Name())
+ defer f.Close()
+ e, err := os.Open(os.Args[0])
+ if err != nil {
+ t.Fatal(err)
+ }
+ defer e.Close()
+ if _, err := io.Copy(f, e); err != nil {
+ t.Fatal(err)
+ }
+ if err := f.Chmod(0755); err != nil {
+ t.Fatal(err)
+ }
+ if err := f.Close(); err != nil {
+ t.Fatal(err)
+ }
+
+ cmd := exec.Command(f.Name(), "-test.run=TestAmbientCapsHelper")
+ cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
+ cmd.Stdout = os.Stdout
+ cmd.Stderr = os.Stderr
+ cmd.SysProcAttr = &syscall.SysProcAttr{
+ Credential: &syscall.Credential{
+ Uid: uint32(uid),
+ Gid: uint32(gid),
+ },
+ AmbientCaps: []uintptr{CAP_SYS_TIME},
+ }
+ if err := cmd.Run(); err != nil {
+ t.Fatal(err.Error())
+ }
+}
