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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham 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 BE3F8C1975A for ; Thu, 12 Mar 2020 13:46:17 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 492F72067C for ; Thu, 12 Mar 2020 13:46:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 492F72067C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=virtuozzo.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id F27AD6B0007; Thu, 12 Mar 2020 09:46:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id ED7C76B0008; Thu, 12 Mar 2020 09:46:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DEE776B000A; Thu, 12 Mar 2020 09:46:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0203.hostedemail.com [216.40.44.203]) by kanga.kvack.org (Postfix) with ESMTP id C5E656B0007 for ; Thu, 12 Mar 2020 09:46:16 -0400 (EDT) Received: from smtpin22.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 941C8181AEF23 for ; Thu, 12 Mar 2020 13:46:16 +0000 (UTC) X-FDA: 76586834352.22.cars88_70ed145d41f3d X-HE-Tag: cars88_70ed145d41f3d X-Filterd-Recvd-Size: 8284 Received: from relay.sw.ru (relay.sw.ru [185.231.240.75]) by imf11.hostedemail.com (Postfix) with ESMTP for ; Thu, 12 Mar 2020 13:46:15 +0000 (UTC) Received: from dhcp-172-16-24-104.sw.ru ([172.16.24.104]) by relay.sw.ru with esmtp (Exim 4.92.3) (envelope-from ) id 1jCO9a-0007ZC-He; Thu, 12 Mar 2020 16:45:38 +0300 Subject: Re: [PATCH v2 5/5] exec: Add a exec_update_mutex to replace cred_guard_mutex To: "Eric W. Biederman" Cc: Bernd Edlinger , 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> <87v9ne5y4y.fsf_-_@x220.int.ebiederm.org> <87zhcq4jdj.fsf_-_@x220.int.ebiederm.org> <87d09hn4kt.fsf@x220.int.ebiederm.org> From: Kirill Tkhai Message-ID: Date: Thu, 12 Mar 2020 16:45:37 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <87d09hn4kt.fsf@x220.int.ebiederm.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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: On 12.03.2020 15:24, Eric W. Biederman wrote: > Kirill Tkhai writes: > >> On 09.03.2020 00:38, 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. >>> >>> Link: https://lore.kernel.org/lkml/20160921152946.GA24210@dhcp22.suse.cz/ >>> Link: https://lore.kernel.org/lkml/AM6PR03MB5170B06F3A2B75EFB98D071AE4E60@AM6PR03MB5170.eurprd03.prod.outlook.com/ >>> Link: https://lore.kernel.org/linux-fsdevel/20161102181806.GB1112@redhat.com/ >>> Link: https://lore.kernel.org/lkml/20160923095031.GA14923@redhat.com/ >>> Link: https://lore.kernel.org/lkml/20170213141452.GA30203@redhat.com/ >>> Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.") >>> Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2") >>> Signed-off-by: "Eric W. Biederman" >>> --- >>> fs/exec.c | 9 +++++++++ >>> include/linux/sched/signal.h | 9 ++++++++- >>> init/init_task.c | 1 + >>> kernel/fork.c | 1 + >>> 4 files changed, 19 insertions(+), 1 deletion(-) >>> >>> diff --git a/fs/exec.c b/fs/exec.c >>> index d820a7272a76..ffeebb1f167b 100644 >>> --- a/fs/exec.c >>> +++ b/fs/exec.c >>> @@ -1014,6 +1014,7 @@ static int exec_mmap(struct mm_struct *mm) >>> { >>> struct task_struct *tsk; >>> struct mm_struct *old_mm, *active_mm; >>> + int ret; >>> >>> /* Notify parent that we're no longer interested in the old VM */ >>> tsk = current; >>> @@ -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; >> >> You missed old_mm->mmap_sem unlock. See here: > > Duh. Thank you. > > I actually need to switch the lock ordering here, and I haven't yet > because my son was sick yesterday. There is some fundamental problem with your patch, since the below fires in 100% cases on current linux-next: [ 22.838717] kernel BUG at fs/exec.c:1474! diff --git a/fs/exec.c b/fs/exec.c index 47582cd97f86..0f77f8c94905 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1470,8 +1470,10 @@ static void free_bprm(struct linux_binprm *bprm) { free_arg_pages(bprm); if (bprm->cred) { - if (!bprm->mm) + if (!bprm->mm) { + BUG_ON(!mutex_is_locked(¤t->signal->exec_update_mutex)); mutex_unlock(¤t->signal->exec_update_mutex); + } mutex_unlock(¤t->signal->cred_guard_mutex); abort_creds(bprm->cred); } @@ -1521,6 +1523,7 @@ void install_exec_creds(struct linux_binprm *bprm) * credentials; any time after this it may be unlocked. */ security_bprm_committed_creds(bprm); + BUG_ON(!mutex_is_locked(¤t->signal->exec_update_mutex)); mutex_unlock(¤t->signal->exec_update_mutex); mutex_unlock(¤t->signal->cred_guard_mutex); } --------------------------------------------------------------------------------------------- First time the mutex is unlocked in: exec_binprm()->search_binary_handler()->.load_binary->install_exec_creds() Then exec_binprm()->search_binary_handler()->.load_binary->flush_old_exec() clears mm: bprm->mm = NULL; Second time the mutex is unlocked in free_bprm(): if (bprm->cred) { if (!bprm->mm) mutex_unlock(¤t->signal->exec_update_mutex); My opinion is we should not relay on side indicators like bprm->mm. Better you may introduce struct linux_binprm::exec_update_mutex_is_locked. So the next person dealing with this after you won't waste much time on diving into this. Also, if someone decides to change the place, where bprm->mm is set into NULL, this person will bump into hell of dependences between unrelated components like your newly introduced mutex. So, I'm strongly for *struct linux_binprm::exec_update_mutex_is_locked*, since this improves modularity.