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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 36F98C433EF for ; Thu, 10 Feb 2022 02:05:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234153AbiBJCDJ convert rfc822-to-8bit (ORCPT ); Wed, 9 Feb 2022 21:03:09 -0500 Received: from gmail-smtp-in.l.google.com ([23.128.96.19]:33174 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234608AbiBJCAR (ORCPT ); Wed, 9 Feb 2022 21:00:17 -0500 Received: from out02.mta.xmission.com (out02.mta.xmission.com [166.70.13.232]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 458DB103F; Wed, 9 Feb 2022 17:57:21 -0800 (PST) Received: from in01.mta.xmission.com ([166.70.13.51]:43770) by out02.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nHyhy-0011Oq-PQ; Wed, 09 Feb 2022 18:57:18 -0700 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:44682 helo=email.froward.int.ebiederm.org.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1nHyhw-00AMiJ-MT; Wed, 09 Feb 2022 18:57:18 -0700 From: "Eric W. Biederman" To: Solar Designer Cc: Michal =?utf-8?Q?Koutn=C3=BD?= , Alexey Gladkov , Kees Cook , Shuah Khan , Christian Brauner , Ran Xiaokai , linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Linux Containers References: <20220207121800.5079-1-mkoutny@suse.com> <20220207121800.5079-2-mkoutny@suse.com> <20220210011405.GA17076@openwall.com> Date: Wed, 09 Feb 2022 19:57:10 -0600 In-Reply-To: <20220210011405.GA17076@openwall.com> (Solar Designer's message of "Thu, 10 Feb 2022 02:14:05 +0100") Message-ID: <87v8xn8ow9.fsf@email.froward.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT X-XM-SPF: eid=1nHyhw-00AMiJ-MT;;;mid=<87v8xn8ow9.fsf@email.froward.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.174.4;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX18XlNqZ8OwOBkY4TyLNlmApbq/5YW22QNU= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [RFC PATCH 1/6] set_user: Perform RLIMIT_NPROC capability check against new user credentials X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Solar Designer writes: > Hi Michal, > > On Mon, Feb 07, 2022 at 01:17:55PM +0100, Michal Koutný wrote: >> The check is currently against the current->cred but since those are >> going to change and we want to check RLIMIT_NPROC condition after the >> switch, supply the capability check with the new cred. >> But since we're checking new_user being INIT_USER any new cred's >> capability-based allowance may be redundant when the check fails and the >> alternative solution would be revert of the commit 2863643fb8b9 >> ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds") >> >> Fixes: 2863643fb8b9 ("set_user: add capability check when rlimit(RLIMIT_NPROC) exceeds") >> >> Cc: Solar Designer >> Cc: Christian Brauner >> Signed-off-by: Michal Koutný >> --- >> kernel/sys.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/kernel/sys.c b/kernel/sys.c >> index 8ea20912103a..48c90dcceff3 100644 >> --- a/kernel/sys.c >> +++ b/kernel/sys.c >> @@ -481,7 +481,8 @@ static int set_user(struct cred *new) >> */ >> if (ucounts_limit_cmp(new->ucounts, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC)) >= 0 && >> new_user != INIT_USER && >> - !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) >> + !security_capable(new, &init_user_ns, CAP_SYS_RESOURCE, CAP_OPT_NONE) && >> + !security_capable(new, &init_user_ns, CAP_SYS_ADMIN, CAP_OPT_NONE)) >> current->flags |= PF_NPROC_EXCEEDED; >> else >> current->flags &= ~PF_NPROC_EXCEEDED; > > Thank you for working on this and CC'ing me on it. This is related to > the discussion Christian and I had in September: > > https://lore.kernel.org/all/20210913100140.bxqlg47pushoqa3r@wittgenstein/ > > Christian was going to revert 2863643fb8b9, but apparently that never > happened. Back then, I also suggested: > > "Alternatively, we could postpone the set_user() calls until we're > running with the new user's capabilities, but that's an invasive change > that's likely to create its own issues." I really think we need to do something like that. Probably just set a flag in commit_creds and test later. I was working on fixes that looked cleaner and I just recently realized that the test in fork is almost as bad. The function has_capability can be used but the same kind of problems exist. I thought I was very quickly going to have patches to post but I need to redo everything now that I have noticed the issue in fork, so it will be a day or so. Eric > The change you propose above is similar to that, but is more limited and > non-invasive. That looks good to me. > > However, I think you need to drop the negations of the return value from > security_capable(). security_capable() returns 0 or -EPERM, while > capable() returns a bool, in kernel/capability.c: ns_capable_common(): > > capable = security_capable(current_cred(), ns, cap, opts); > if (capable == 0) { > current->flags |= PF_SUPERPRIV; > return true; > } > return false; > > Also, your change would result in this no longer setting PF_SUPERPRIV. > This may be fine, but you could want to document it. > > On a related note, this comment in security/commoncap.c needs an update: > > * NOTE WELL: cap_has_capability() cannot be used like the kernel's capable() > * and has_capability() functions. That is, it has the reverse semantics: > * cap_has_capability() returns 0 when a task has a capability, but the > * kernel's capable() and has_capability() returns 1 for this case. > > cap_has_capability() doesn't actually exist, and perhaps the comment > should refer to cap_capable(). > > Alexander