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,URIBL_BLOCKED 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 7CA94C18E5A for ; Tue, 10 Mar 2020 21:33:15 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 42FB424658 for ; Tue, 10 Mar 2020 21:33:15 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 42FB424658 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=xmission.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id B8AD46B0003; Tue, 10 Mar 2020 17:33:14 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B14436B0006; Tue, 10 Mar 2020 17:33:14 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 98E8E6B0007; Tue, 10 Mar 2020 17:33:14 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0046.hostedemail.com [216.40.44.46]) by kanga.kvack.org (Postfix) with ESMTP id 7DEFA6B0003 for ; Tue, 10 Mar 2020 17:33:14 -0400 (EDT) Received: from smtpin15.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 5636752BA for ; Tue, 10 Mar 2020 21:33:14 +0000 (UTC) X-FDA: 76580753508.15.crow55_90a1877d5044e X-HE-Tag: crow55_90a1877d5044e X-Filterd-Recvd-Size: 7914 Received: from out03.mta.xmission.com (out03.mta.xmission.com [166.70.13.233]) by imf24.hostedemail.com (Postfix) with ESMTP for ; Tue, 10 Mar 2020 21:33:13 +0000 (UTC) Received: from in01.mta.xmission.com ([166.70.13.51]) by out03.mta.xmission.com with esmtps (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.90_1) (envelope-from ) id 1jBmUw-0002iF-Vy; Tue, 10 Mar 2020 15:33:11 -0600 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 1jBmUw-0004iW-59; Tue, 10 Mar 2020 15:33:10 -0600 From: ebiederm@xmission.com (Eric W. Biederman) To: Jann Horn Cc: Bernd Edlinger , Christian Brauner , Kees Cook , 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: <87k142lpfz.fsf@x220.int.ebiederm.org> <875zfmloir.fsf@x220.int.ebiederm.org> <87v9nmjulm.fsf@x220.int.ebiederm.org> <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> <87v9ne5y4y.fsf_-_@x220.int.ebiederm.org> <87zhcq4jdj.fsf_-_@x220.int.ebiederm.org> Date: Tue, 10 Mar 2020 16:30:51 -0500 In-Reply-To: (Jann Horn's message of "Tue, 10 Mar 2020 22:21:28 +0100") Message-ID: <87wo7roq2c.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=1jBmUw-0004iW-59;;;mid=<87wo7roq2c.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/8FoRYPhgRSLS/IXD3oa0BUqZPsIotcuI= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v2 5/5] 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) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Jann Horn writes: > On Sun, Mar 8, 2020 at 10:41 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(). >> >> Any of those can result in deadlock, as the cred_guard_mutex is held >> over a possible indefinite userspace waits for userspace. >> >> Add exec_update_mutex that is only held over exec updating process >> with the new contents of exec, so that code that needs not to be >> confused by exec changing the mm and the cred in ways that can not >> happen during ordinary execution of a process. >> >> The plan is to switch the users of cred_guard_mutex to >> exec_udpate_mutex one by one. This lets us move forward while still >> being careful and not introducing any regressions. > [...] >> @@ -1034,6 +1035,11 @@ static int exec_mmap(struct mm_struct *mm) >> return -EINTR; >> } >> } >> + >> + ret = mutex_lock_killable(&tsk->signal->exec_update_mutex); >> + if (ret) >> + return ret; > > We're already holding the old mmap_sem, and now nest the > exec_update_mutex inside it; but then while still holding the > exec_update_mutex, we do mmput(), which can e.g. end up in ksm_exit(), > which can do down_write(&mm->mmap_sem) from __ksm_exit(). So I think > at least lockdep will be unhappy, and I'm not sure whether it's an > actual problem or not. Good point. I should double check the lock ordering here with mmap_sem. It doesn't look like mmput takes mmap_sem, but still there might be a lock inversion of some kind here. At least as far as lockdep is concerned and we don't want anything like that. Eric