Fix thread safety of SelinuxEnabled and getSelinuxMountPoint

Both suffered from different race conditions.

SelinuxEnabled assigned selinuxEnabledChecked before selinuxEnabled.
Thus racing callers could see the wrong selinuxEnabled.

getSelinuxMountPoint assigned selinuxfs to "" before it know the right
value. Thus racing could see "" improperly.

The gate selinuxfs, enabled, and mclist all on the same lock

libcontainer/selinux/selinux.go

@@ -32,33 +32,73 @@ const (
 	stRdOnly         = 0x01
 )
 
+type selinuxState struct {
+	enabledSet   bool
+	enabled      bool
+	selinuxfsSet bool
+	selinuxfs    string
+	mcsList      map[string]bool
+	sync.Mutex
+}
+
 var (
-	assignRegex           = regexp.MustCompile(`^([^=]+)=(.*)$`)
-	mcsList               = make(map[string]bool)
-	mcsLock               sync.Mutex
-	selinuxfs             = "unknown"
-	selinuxEnabled        = false // Stores whether selinux is currently enabled
-	selinuxEnabledChecked = false // Stores whether selinux enablement has been checked or established yet
+	assignRegex = regexp.MustCompile(`^([^=]+)=(.*)$`)
+	state       = selinuxState{
+		mcsList: make(map[string]bool),
+	}
 )
 
 type SELinuxContext map[string]string
 
-// SetDisabled disables selinux support for the package
-func SetDisabled() {
-	selinuxEnabled, selinuxEnabledChecked = false, true
+func (s *selinuxState) setEnable(enabled bool) bool {
+	s.Lock()
+	defer s.Unlock()
+	s.enabledSet = true
+	s.enabled = enabled
+	return s.enabled
 }
 
-// getSelinuxMountPoint returns the path to the mountpoint of an selinuxfs
-// filesystem or an empty string if no mountpoint is found.  Selinuxfs is
-// a proc-like pseudo-filesystem that exposes the selinux policy API to
-// processes.  The existence of an selinuxfs mount is used to determine
-// whether selinux is currently enabled or not.
-func getSelinuxMountPoint() string {
-	if selinuxfs != "unknown" {
+func (s *selinuxState) getEnabled() bool {
+	s.Lock()
+	enabled := s.enabled
+	enabledSet := s.enabledSet
+	s.Unlock()
+	if enabledSet {
+		return enabled
+	}
+
+	enabled = false
+	if fs := getSelinuxMountPoint(); fs != "" {
+		if con, _ := Getcon(); con != "kernel" {
+			enabled = true
+		}
+	}
+	return s.setEnable(enabled)
+}
+
+// SetDisabled disables selinux support for the package
+func SetDisabled() {
+	state.setEnable(false)
+}
+
+func (s *selinuxState) setSELinuxfs(selinuxfs string) string {
+	s.Lock()
+	defer s.Unlock()
+	s.selinuxfsSet = true
+	s.selinuxfs = selinuxfs
+	return s.selinuxfs
+}
+
+func (s *selinuxState) getSELinuxfs() string {
+	s.Lock()
+	selinuxfs := s.selinuxfs
+	selinuxfsSet := s.selinuxfsSet
+	s.Unlock()
+	if selinuxfsSet {
 		return selinuxfs
 	}
-	selinuxfs = ""
 
+	selinuxfs = ""
 	f, err := os.Open("/proc/self/mountinfo")
 	if err != nil {
 		return selinuxfs
@@ -91,21 +131,21 @@ func getSelinuxMountPoint() string {
 			selinuxfs = ""
 		}
 	}
-	return selinuxfs
+	return s.setSELinuxfs(selinuxfs)
+}
+
+// getSelinuxMountPoint returns the path to the mountpoint of an selinuxfs
+// filesystem or an empty string if no mountpoint is found.  Selinuxfs is
+// a proc-like pseudo-filesystem that exposes the selinux policy API to
+// processes.  The existence of an selinuxfs mount is used to determine
+// whether selinux is currently enabled or not.
+func getSelinuxMountPoint() string {
+	return state.getSELinuxfs()
 }
 
 // SelinuxEnabled returns whether selinux is currently enabled.
 func SelinuxEnabled() bool {
-	if selinuxEnabledChecked {
-		return selinuxEnabled
-	}
-	selinuxEnabledChecked = true
-	if fs := getSelinuxMountPoint(); fs != "" {
-		if con, _ := Getcon(); con != "kernel" {
-			selinuxEnabled = true
-		}
-	}
-	return selinuxEnabled
+	return state.getEnabled()
 }
 
 func readConfig(target string) (value string) {
@@ -283,19 +323,19 @@ func SelinuxGetEnforceMode() int {
 }
 
 func mcsAdd(mcs string) error {
-	mcsLock.Lock()
-	defer mcsLock.Unlock()
-	if mcsList[mcs] {
+	state.Lock()
+	defer state.Unlock()
+	if state.mcsList[mcs] {
 		return fmt.Errorf("MCS Label already exists")
 	}
-	mcsList[mcs] = true
+	state.mcsList[mcs] = true
 	return nil
 }
 
 func mcsDelete(mcs string) {
-	mcsLock.Lock()
-	mcsList[mcs] = false
-	mcsLock.Unlock()
+	state.Lock()
+	defer state.Unlock()
+	state.mcsList[mcs] = false
 }
 
 func IntToMcs(id int, catRange uint32) string {