kernel: GetMountNamespace has to check that mntns isn't nil

Reported-by: syzbot+e21bed832e505430bb27@syzkaller.appspotmail.com
PiperOrigin-RevId: 553568085

pkg/sentry/kernel/task.go

@@ -795,8 +795,11 @@ func (t *Task) MountNamespace() *vfs.MountNamespace {
 func (t *Task) GetMountNamespace() *vfs.MountNamespace {
 	t.mu.Lock()
 	defer t.mu.Unlock()
-	t.mountNamespace.IncRef()
-	return t.mountNamespace
+	mntns := t.mountNamespace
+	if mntns != nil {
+		mntns.IncRef()
+	}
+	return mntns
 }
 
 // AbstractSockets returns t's AbstractSocketNamespace.

pkg/sentry/kernel/task_exit.go

@@ -288,12 +288,11 @@ func (*runExitMain) execute(t *Task) taskRunState {
 	mntns := t.mountNamespace
 	t.mountNamespace = nil
 	ipcns := t.ipcns
+	t.ipcns = nil
 	netns := t.netns
 	t.netns = nil
 	t.mu.Unlock()
-	if mntns != nil {
-		mntns.DecRef(t)
-	}
+	mntns.DecRef(t)
 	ipcns.DecRef(t)
 	netns.DecRef(t)