From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756111Ab3KVUsK (ORCPT ); Fri, 22 Nov 2013 15:48:10 -0500 Received: from mx1.redhat.com ([209.132.183.28]:46416 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755856Ab3KVUsH (ORCPT ); Fri, 22 Nov 2013 15:48:07 -0500 Date: Fri, 22 Nov 2013 21:49:17 +0100 From: Oleg Nesterov To: KOSAKI Motohiro Cc: akpm@linux-foundation.org, viro@ZenIV.linux.org.uk, keescook@chromium.org, mhocko@suse.cz, snanda@chromium.org, dserrg@gmail.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] check_unsafe_exec: kill the dead -EAGAIN and clear_in_exec logic Message-ID: <20131122204917.GA20520@redhat.com> References: <20131122175442.GA31453@redhat.com> <528FBE22.5030208@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <528FBE22.5030208@jp.fujitsu.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/22, KOSAKI Motohiro wrote: > > (11/22/2013 12:54 PM), Oleg Nesterov wrote: > > fs_struct->in_exec == T means that this ->fs is used by a single > > process (thread group), and one of the treads does do_execve(). > > > > To avoid the mt-exec races this code has the following complications: > > > > 1. check_unsafe_exec() returns -EBUSY if ->in_exec was > > already set by another thread. > > > > 2. do_execve_common() records "clear_in_exec" to ensure > > that the error path can only clear ->in_exec if it was > > set by current. > > > > However, after 9b1bf12d5d51 "signals: move cred_guard_mutex from > > task_struct to signal_struct" we do not need these complications: > > > > 1. We can't race with our sub-thread, this is called under > > per-process ->cred_guard_mutex. And we can't race with > > another CLONE_FS task, we already checked that this fs > > is not shared. > > > > We can remove the dead -EAGAIN logic. > > > > 2. "out_unmark:" in do_execve_common() is either called > > under ->cred_guard_mutex, or after de_thread() which > > kills other threads, so we can't race with sub-thread > > which could set ->in_exec. And if ->fs is shared with > > another process ->in_exec should be false anyway. > > > > We can clear in_exec unconditionally. > > > > This also means that check_unsafe_exec() can be void. > > > > Signed-off-by: Oleg Nesterov > > I have found no problem in this patch. However, I have a very basic question. > Why do we need to keep fs->in_exec? To ensure that a sub-thread can't create a new process with the same ->fs while we are doing exec without LSM_UNSAFE_SHARE, I guess. This is only for security/ code. > If it is correct, > can't we move it it to signal->in_exec? Yes, perhaps, I am thinking about more cleanups too. But not that this will add the subtle change. CLONE_THREAD doesn't require CLONE_FS, so copy_fs() can fail even it the caller doesn't share ->fs with the execing thread. And we still need fs->lock to set signal->in_exec, this looks a bit strange. > I am not expert in this area and I may overlook something. Neither me ;) So this patch tries to not change the current logic. I feel that perhaps we can do more cleanups, but I am not really sure and this needs a separate change. Oleg.