pidns: Outlaw thread creation after unshare(CLONE_NEWPID)

The sequence:
unshare(CLONE_NEWPID)
clone(CLONE_THREAD|CLONE_SIGHAND|CLONE_VM)

Creates a new process in the new pid namespace without setting
pid_ns->child_reaper.  After forking this results in a NULL
pointer dereference.

Avoid this and other nonsense scenarios that can show up after
creating a new pid namespace with unshare by adding a new
check in copy_prodcess.
Pointed-out-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

kernel/fork.c

@@ -1166,6 +1166,14 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 				current->signal->flags & SIGNAL_UNKILLABLE)
 		return ERR_PTR(-EINVAL);
 
+	/*
+	 * If the new process will be in a different pid namespace
+	 * don't allow the creation of threads.
+	 */
+	if ((clone_flags & (CLONE_VM|CLONE_NEWPID)) &&
+	    (task_active_pid_ns(current) != current->nsproxy->pid_ns))
+		return ERR_PTR(-EINVAL);
+
 	retval = security_task_create(clone_flags);
 	if (retval)
 		goto fork_out;