From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0360C10F25 for ; Sat, 7 Mar 2020 01:05:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8391E206E2 for ; Sat, 7 Mar 2020 01:05:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727076AbgCGBFa (ORCPT ); Fri, 6 Mar 2020 20:05:30 -0500 Received: from out02.mta.xmission.com ([166.70.13.232]:55008 "EHLO out02.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726231AbgCGBFa (ORCPT ); Fri, 6 Mar 2020 20:05:30 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out02.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1jANuA-0007Im-T1; Fri, 06 Mar 2020 18:05:26 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1jANu8-0006Ld-L3; Fri, 06 Mar 2020 18:05:26 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Bernd Edlinger Cc: Christian Brauner , Kees Cook , Jann Horn , Jonathan Corbet , Alexander Viro , Andrew Morton , Alexey Dobriyan , Thomas Gleixner , Oleg Nesterov , Frederic Weisbecker , Andrei Vagin , Ingo Molnar , "Peter Zijlstra \(Intel\)" , Yuyang Du , David Hildenbrand , Sebastian Andrzej Siewior , Anshuman Khandual , David Howells , James Morris , Greg Kroah-Hartman , Shakeel Butt , Jason Gunthorpe , Christian Kellner , Andrea Arcangeli , Aleksa Sarai , "Dmitry V. Levin" , "linux-doc\@vger.kernel.org" , "linux-kernel\@vger.kernel.org" , "linux-fsdevel\@vger.kernel.org" , "linux-mm\@kvack.org" , "stable\@vger.kernel.org" , "linux-api\@vger.kernel.org" References: <202003021531.C77EF10@keescook> <20200303085802.eqn6jbhwxtmz4j2x@wittgenstein> <87v9nlii0b.fsf@x220.int.ebiederm.org> <87a74xi4kz.fsf@x220.int.ebiederm.org> <87r1y8dqqz.fsf@x220.int.ebiederm.org> <87tv32cxmf.fsf_-_@x220.int.ebiederm.org> <87imjicxjw.fsf_-_@x220.int.ebiederm.org> <87k13yawpp.fsf@x220.int.ebiederm.org> <87sgil87s3.fsf@x220.int.ebiederm.org> <87a74t86cs.fsf@x220.int.ebiederm.org> Date: Fri, 06 Mar 2020 19:03:10 -0600 In-Reply-To: <87a74t86cs.fsf@x220.int.ebiederm.org> (Eric W. Biederman's message of "Fri, 06 Mar 2020 16:29:39 -0600") Message-ID: <87v9nh6koh.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1jANu8-0006Ld-L3;;;mid=<87v9nh6koh.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX19ga0TG/cHLdEFkXj9eUOD1GvS7qbzyimM= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH 2/2] exec: Add a exec_update_mutex to replace cred_guard_mutex X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ebiederm@xmission.com (Eric W. Biederman) writes: > ebiederm@xmission.com (Eric W. Biederman) writes: > >> Bernd Edlinger writes: >> >>> On 3/6/20 6:17 AM, Eric W. Biederman wrote: >>>> Bernd Edlinger writes: >>>> >>>>> On 3/5/20 10:16 PM, Eric W. Biederman wrote: >>>>>> >>>>>> The cred_guard_mutex is problematic. The cred_guard_mutex is held >>>>>> over the userspace accesses as the arguments from userspace are read. >>>>>> The cred_guard_mutex is held of PTRACE_EVENT_EXIT as the the other >>>>>> threads are killed. The cred_guard_mutex is held over >>>>>> "put_user(0, tsk->clear_child_tid)" in exit_mm(). >>>>>> >>> >>> I am all for this patch, and the direction it is heading, Eric. >>> >>> I just wanted to add a note that I think it is >>> possible that exec_mm_release can also invoke put_user(0, tsk->clear_child_tid), >>> under the new exec_update_mutex, since vm_access increments the >>> mm->mm_users, under the cred_update_mutex, but releases the mutex, >>> and the caller can hold the reference for a while and then exec_mmap is not >>> releasing the last reference. >> >> Good catch. I really appreciate your close look at the details. >> >> I am wondering if process_vm_readv and process_vm_writev could be >> safely changed to use mmgrab and mmdrop, instead of mmget and mmput. >> >> That would resolve the potential issue you have pointed out. I just >> haven't figured out if it is safe. The mm code has been seriously >> refactored since I knew how it all worked. > > Nope, mmget can not be replaced by mmgrab. > > It might be possible to do something creative like store a cred in place > of the userns on the mm and use that for mm_access permission checks. > Still we are talking a pretty narrow window, and a case that no one has > figured out how to trigger yet. So I will leave that corner case as > something for future improvements. My brain is restless and keep looking at it. The worst case is processes created with CLONE_VM|CLONE_CHILD_CLEARTID but not CLONE_THREAD. For those that put_user will occur ever time in exec_mmap. The only solution that I can see is to move taking the new mutex after exec_mm_release. Which may be feasible given how close exec_mmap follows de_thread. I am going to sleep on that and perhaps I will be able to see how to move taking the mutex lower. It would be very nice not to have a known issue going into this set of changes. Eric