ntp: drop privileges to nobody/nogroup, retain CAP_SYS_TIME

cmd/ntp/ntp.go

@@ -23,6 +23,8 @@ func set() error {
 func main() {
 	log.SetFlags(log.LstdFlags | log.Lshortfile)
 
+	mustDropPrivileges()
+
 	for {
 		if err := set(); err != nil {
 			log.Fatalf("setting time failed: %v", err)

cmd/ntp/privdrop.go

@@ -0,0 +1,84 @@
+// +build go1.9
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"syscall"
+	"unsafe"
+)
+
+type capHeader struct {
+	version uint32
+	pid     int
+}
+
+type capData struct {
+	effective   uint32
+	permitted   uint32
+	inheritable uint32
+}
+
+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
+}
+
+// mustDropPrivileges executes the program in a child process, dropping root
+// privileges, but retaining the CAP_SYS_TIME capability to change the system
+// clock.
+func mustDropPrivileges() {
+	if os.Getenv("NTP_PRIVILEGES_DROPPED") == "1" {
+		return
+	}
+
+	// From include/uapi/linux/capability.h:
+	// Allow setting the real-time clock
+	const CAP_SYS_TIME = 25
+
+	caps, err := getCaps()
+	if err != nil {
+		log.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 {
+		log.Fatalf("SYS_CAPSET: %v", errno)
+	}
+
+	cmd := exec.Command(os.Args[0])
+	cmd.Env = append(os.Environ(), "NTP_PRIVILEGES_DROPPED=1")
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Credential: &syscall.Credential{
+			Uid: 65534,
+			Gid: 65534,
+		},
+		AmbientCaps: []uintptr{CAP_SYS_TIME},
+	}
+	log.Fatal(cmd.Run())
+}

cmd/ntp/privdrop_dummy.go

@@ -0,0 +1,8 @@
+// +build !go1.9
+
+package main
+
+// mustDropPrivileges (on Go < 1.9) does nothing, as the AmbientCaps field in
+// syscall.SysProcAttr was only introduced in Go 1.9.
+func mustDropPrivileges() {
+}