diff --git a/kernel/fork.c b/kernel/fork.c
index 1bfefc6..d544ae9 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1866,13 +1866,21 @@
 				CLONE_NEWUSER|CLONE_NEWPID))
 		return -EINVAL;
 	/*
-	 * Not implemented, but pretend it works if there is nothing to
-	 * unshare. Note that unsharing CLONE_THREAD or CLONE_SIGHAND
-	 * needs to unshare vm.
+	 * Not implemented, but pretend it works if there is nothing
+	 * to unshare.  Note that unsharing the address space or the
+	 * signal handlers also need to unshare the signal queues (aka
+	 * CLONE_THREAD).
 	 */
 	if (unshare_flags & (CLONE_THREAD | CLONE_SIGHAND | CLONE_VM)) {
-		/* FIXME: get_task_mm() increments ->mm_users */
-		if (atomic_read(&current->mm->mm_users) > 1)
+		if (!thread_group_empty(current))
+			return -EINVAL;
+	}
+	if (unshare_flags & (CLONE_SIGHAND | CLONE_VM)) {
+		if (atomic_read(&current->sighand->count) > 1)
+			return -EINVAL;
+	}
+	if (unshare_flags & CLONE_VM) {
+		if (!current_is_single_threaded())
 			return -EINVAL;
 	}
 
@@ -1941,16 +1949,16 @@
 	if (unshare_flags & CLONE_NEWUSER)
 		unshare_flags |= CLONE_THREAD | CLONE_FS;
 	/*
-	 * If unsharing a thread from a thread group, must also unshare vm.
-	 */
-	if (unshare_flags & CLONE_THREAD)
-		unshare_flags |= CLONE_VM;
-	/*
 	 * If unsharing vm, must also unshare signal handlers.
 	 */
 	if (unshare_flags & CLONE_VM)
 		unshare_flags |= CLONE_SIGHAND;
 	/*
+	 * If unsharing a signal handlers, must also unshare the signal queues.
+	 */
+	if (unshare_flags & CLONE_SIGHAND)
+		unshare_flags |= CLONE_THREAD;
+	/*
 	 * If unsharing namespace, must also unshare filesystem information.
 	 */
 	if (unshare_flags & CLONE_NEWNS)