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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 700F1C433EF for ; Tue, 7 Sep 2021 21:37:37 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4590961102 for ; Tue, 7 Sep 2021 21:37:37 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1347268AbhIGVim (ORCPT ); Tue, 7 Sep 2021 17:38:42 -0400 Received: from mother.openwall.net ([195.42.179.200]:47524 "HELO mother.openwall.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S229875AbhIGVii (ORCPT ); Tue, 7 Sep 2021 17:38:38 -0400 X-Greylist: delayed 401 seconds by postgrey-1.27 at vger.kernel.org; Tue, 07 Sep 2021 17:38:38 EDT Received: (qmail 8086 invoked from network); 7 Sep 2021 21:30:48 -0000 Received: from localhost (HELO pvt.openwall.com) (127.0.0.1) by localhost with SMTP; 7 Sep 2021 21:30:48 -0000 Received: by pvt.openwall.com (Postfix, from userid 503) id 58A79AB88C; Tue, 7 Sep 2021 23:30:42 +0200 (CEST) Date: Tue, 7 Sep 2021 23:30:42 +0200 From: Solar Designer To: Christian Brauner Cc: CGEL , peterz@infradead.org, tglx@linutronix.de, linux-kernel@vger.kernel.org, Ran Xiaokai , James Morris , Linus Torvalds , Kees Cook , NeilBrown Subject: Re: [PATCH] set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds Message-ID: <20210907213042.GA22626@openwall.com> References: <20210728072629.530435-1-ran.xiaokai@zte.com.cn> <20210728115930.2lzs57h4hvnqipue@wittgenstein> <20210730082329.GA544980@www> <20210803100354.GA607722@www> <20210803140702.f3rdnka3e2x6vj4r@wittgenstein> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210803140702.f3rdnka3e2x6vj4r@wittgenstein> User-Agent: Mutt/1.4.2.3i Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi all, Brad Spengler brought this to my attention on Twitter, and Christian Brauner agreed I should follow up. So here goes, below the quote: On Tue, Aug 03, 2021 at 04:07:02PM +0200, Christian Brauner wrote: > On Tue, Aug 03, 2021 at 03:03:54AM -0700, CGEL wrote: > > On Fri, Jul 30, 2021 at 01:23:31AM -0700, CGEL wrote: > > > On Wed, Jul 28, 2021 at 01:59:30PM +0200, Christian Brauner wrote: > > > > [Ccing a few people that did the PF_NPROC_EXCEEDED changes] > > > > > > > > > > > > Hey Cgel, > > > > Hey Ran, > > > > > > > > The gist seems to me that this code wants to make sure that a program > > > > can't successfully exec if it has gone through a set*id() transition > > > > while exceeding its RLIMIT_NPROC. > > > > > > > > But I agree that the semantics here are a bit strange. > > > > > > > > Iicu, a capable but non-INIT_USER caller getting PF_NPROC_EXCEEDED set > > > > during a set*id() transition wouldn't be able to exec right away if they > > > > still exceed their RLIMIT_NPROC at the time of exec. So their exec would > > > > fail in fs/exec.c: > > > > > > > > if ((current->flags & PF_NPROC_EXCEEDED) && > > > > is_ucounts_overlimit(current_ucounts(), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) { > > > > retval = -EAGAIN; > > > > goto out_ret; > > > > } > > > > > > > > However, if the caller were to fork() right after the set*id() > > > > transition but before the exec while still exceeding their RLIMIT_NPROC > > > > then they would get PF_NPROC_EXCEEDED cleared (while the child would > > > > inherit it): > > > > > > > > retval = -EAGAIN; > > > > if (is_ucounts_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) { > > > > if (p->real_cred->user != INIT_USER && > > > > !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) > > > > goto bad_fork_free; > > > > } > > > > current->flags &= ~PF_NPROC_EXCEEDED; > > > > > > > > which means a subsequent exec by the capable caller would now succeed > > > > even though they could still exceed their RLIMIT_NPROC limit. > > > > > > > > So at first glance, it seems that set_user() should probably get the > > > > same check as it can be circumvented today unless I misunderstand the > > > > original motivation. > > > > > > > > Christian > > > > > > Hi Christian, > > > > > > I think i didn't give enough information in the commit message. > > > When switch to a capable but non-INIT_SUER and the RLIMIT_NPROC limit already exceeded, > > > and calls these funcs: > > > 1. set_xxuid()->exec() > > > ---> fail > > > 2. set_xxuid()->fork()->exec() > > > ---> success > > > Kernel should have the same behavior to uer space. > > > Also i think non init_user CAN exceed the limit when with proper capability, > > > so in the patch, set_user() clear PF_NPROC_EXCEEDED flag if capable() > > > returns true. > > > > Hi, Christian > > > > Do you have any further comments on this patch? > > is there anything i did not give enough infomation ? > > Yeah, this is fine and how I understood it too. I don't see anything > obviously wrong with it and the weird detour workaround via fork() seems > inconsistent. So if I don't here anyone come up with a good reason the > current behavior makes sense I'll pick this up. > > Christian As I understand, the resulting commit: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2863643fb8b92291a7e97ba46e342f1163595fa8 broke RLIMIT_NPROC support for Apache httpd suexec and likely similar. Yes, I can see how having a detour via fork() was inconsistent, but since the privileged process can be assumed non-malicious it was no big deal. suexec just doesn't have fork() in there. Historically, the resetting on fork() appears to have been due to my suggestion here: https://www.openwall.com/lists/kernel-hardening/2011/07/25/4 "Perhaps also reset the flag on fork() because we have an RLIMIT_NPROC check on fork() anyway." Looks like I didn't consider the inconsistency for capable() processes (or maybe that exception wasn't yet in there?) Anyway, now I suggest that 2863643fb8b92291a7e97ba46e342f1163595fa8 be reverted, and if there's any reason to make any change (what reason? mere consistency or any real issue?) then I suggest that the flag resetting on fork() be made conditional. Something like this: if (atomic_read(&p->real_cred->user->processes) >= task_rlimit(p, RLIMIT_NPROC)) { if (p->real_cred->user != INIT_USER && !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) goto bad_fork_free; - } - current->flags &= ~PF_NPROC_EXCEEDED; + } else + current->flags &= ~PF_NPROC_EXCEEDED; Alexander