From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756739AbcEaVnn (ORCPT ); Tue, 31 May 2016 17:43:43 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36047 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750779AbcEaVnm (ORCPT ); Tue, 31 May 2016 17:43:42 -0400 Date: Tue, 31 May 2016 23:43:38 +0200 From: Oleg Nesterov To: Michal Hocko Cc: linux-mm@kvack.org, Tetsuo Handa , David Rientjes , Vladimir Davydov , Andrew Morton , LKML Subject: Re: [PATCH 4/6] mm, oom: skip vforked tasks from being selected Message-ID: <20160531214338.GB26582@redhat.com> References: <1464613556-16708-1-git-send-email-mhocko@kernel.org> <1464613556-16708-5-git-send-email-mhocko@kernel.org> <20160530192856.GA25696@redhat.com> <20160531074247.GC26128@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160531074247.GC26128@dhcp22.suse.cz> User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.26]); Tue, 31 May 2016 21:43:41 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/31, Michal Hocko wrote: > > On Mon 30-05-16 21:28:57, Oleg Nesterov wrote: > > > > I don't think we can trust vfork_done != NULL. > > > > copy_process() doesn't disallow CLONE_VFORK without CLONE_VM, so with this patch > > it would be trivial to make the exploit which hides a memory hog from oom-killer. > > OK, I wasn't aware of this possibility. Neither was me ;) I noticed this during this review. > > Or I am totally confused? > > I cannot judge I am afraid. You are definitely much more familiar with > all these subtle details than me. OK, I just verified that clone(CLONE_VFORK|SIGCHLD) really works to be sure. > +/* expects to be called with task_lock held */ > +static inline bool in_vfork(struct task_struct *tsk) > +{ > + bool ret; > + > + /* > + * need RCU to access ->real_parent if CLONE_VM was used along with > + * CLONE_PARENT > + */ > + rcu_read_lock(); > + ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm; > + rcu_read_unlock(); > + > + return ret; > +} Yes, but may I ask to add a comment? And note that "expects to be called with task_lock held" looks misleading, we do not need the "stable" tsk->vfork_done since we only need to check if it is NULL or not. It would be nice to explain that 1. we check real_parent->mm == tsk->mm because CLONE_VFORK does not imply CLONE_VM 2. CLONE_VFORK can be used with CLONE_PARENT/CLONE_THREAD and thus ->real_parent is not necessarily the task doing vfork(), so in theory we can't rely on task_lock() if we want to dereference it. And in this case we can't trust the real_parent->mm == tsk->mm check, it can be false negative. But we do not care, if init or another oom-unkillable task does this it should blame itself. Oleg.