From mboxrd@z Thu Jan 1 00:00:00 1970 From: Oleg Nesterov Subject: Re: [PATCH v3 00/13] Virtually mapped stacks with guard pages (x86, core) Date: Thu, 23 Jun 2016 16:31:26 +0200 Message-ID: <20160623143126.GA16664@redhat.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from mx1.redhat.com ([209.132.183.28]:58637 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751002AbcFWObb (ORCPT ); Thu, 23 Jun 2016 10:31:31 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-arch-owner@vger.kernel.org List-ID: To: Linus Torvalds Cc: Andy Lutomirski , Andy Lutomirski , the arch/x86 maintainers , Linux Kernel Mailing List , "linux-arch@vger.kernel.org" , Borislav Petkov , Nadav Amit , Kees Cook , Brian Gerst , "kernel-hardening@lists.openwall.com" , Josh Poimboeuf , Jann Horn , Heiko Carstens On 06/22, Linus Torvalds wrote: > > Oleg, what do you think? Would it be reasonable to free the stack and > thread_info synchronously at exit time, clear the pointer (to catch > any odd use), and only RCU-delay the task_struct itself? I didn't see the patches yet, quite possibly I misunderstood... But no, I don't this we can do this (if we are not going to move ti->flags to task_struct at least). > (Obviously, we can't release it in do_exit() itself like we do some of > the other state - it would need to be released after we've scheduled > away to another process' stack, but we already have that TASK_DEAD > handling in finish_task_switch for this exact reason). Yes, but the problem is that a zombie thread can do its last schedule before it is reaped. Just for example, syscall_regfunc() does read_lock(&tasklist_lock); for_each_process_thread(p, t) { set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT); } read_unlock(&tasklist_lock); and this can easily hit a TASK_DEAD thread with ->stack == NULL. And we can't free/nullify it when the parent/debuger reaps a zombie, say, mark_oom_victim() expects that get_task_struct() protects thread_info as well. Oleg.