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 3302FC433EF for ; Tue, 14 Jun 2022 16:31:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344289AbiFNQbq (ORCPT ); Tue, 14 Jun 2022 12:31:46 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46564 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S243064AbiFNQbg (ORCPT ); Tue, 14 Jun 2022 12:31:36 -0400 Received: from out03.mta.xmission.com (out03.mta.xmission.com [166.70.13.233]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E1D82443C9; Tue, 14 Jun 2022 09:31:35 -0700 (PDT) Received: from in02.mta.xmission.com ([166.70.13.52]:56828) by out03.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1o19S2-00AmWn-9i; Tue, 14 Jun 2022 10:31:34 -0600 Received: from ip68-227-174-4.om.om.cox.net ([68.227.174.4]:40454 helo=email.froward.int.ebiederm.org.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1o19S0-009rSu-TN; Tue, 14 Jun 2022 10:31:33 -0600 From: "Eric W. Biederman" To: Frederick Lawler Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-aio@kvack.org, linux-fsdevel@vger.kernel.org, linux-cachefs@redhat.com, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-mm@kvack.org, linux-nfs@vger.kernel.org, linux-unionfs@vger.kernel.org, linux-security-module@vger.kernel.org, netdev@vger.kernel.org, keyrings@vger.kernel.org, selinux@vger.kernel.org, serge@hallyn.com, amir73il@gmail.com, kernel-team@cloudflare.com, Jeff Moyer , Paul Moore References: <20220608150942.776446-1-fred@cloudflare.com> <87tu8oze94.fsf@email.froward.int.ebiederm.org> <87y1xzyhub.fsf@email.froward.int.ebiederm.org> <859cb593-9e96-5846-2191-6613677b07c5@cloudflare.com> Date: Tue, 14 Jun 2022 11:30:54 -0500 In-Reply-To: <859cb593-9e96-5846-2191-6613677b07c5@cloudflare.com> (Frederick Lawler's message of "Tue, 14 Jun 2022 11:06:24 -0500") Message-ID: <87o7yvxl4x.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 X-XM-SPF: eid=1o19S0-009rSu-TN;;;mid=<87o7yvxl4x.fsf@email.froward.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.174.4;;;frm=ebiederm@xmission.com;;;spf=softfail X-XM-AID: U2FsdGVkX19OwrPXsW8XkXxckntCYt4oo3aNKl7qziA= X-SA-Exim-Connect-IP: 68.227.174.4 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v3] cred: Propagate security_prepare_creds() error code X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Frederick Lawler writes: > On 6/13/22 11:44 PM, Eric W. Biederman wrote: >> Frederick Lawler writes: >> >>> Hi Eric, >>> >>> On 6/13/22 12:04 PM, Eric W. Biederman wrote: >>>> Frederick Lawler writes: >>>> >>>>> While experimenting with the security_prepare_creds() LSM hook, we >>>>> noticed that our EPERM error code was not propagated up the callstack. >>>>> Instead ENOMEM is always returned. As a result, some tools may send a >>>>> confusing error message to the user: >>>>> >>>>> $ unshare -rU >>>>> unshare: unshare failed: Cannot allocate memory >>>>> >>>>> A user would think that the system didn't have enough memory, when >>>>> instead the action was denied. >>>>> >>>>> This problem occurs because prepare_creds() and prepare_kernel_cred() >>>>> return NULL when security_prepare_creds() returns an error code. Later, >>>>> functions calling prepare_creds() and prepare_kernel_cred() return >>>>> ENOMEM because they assume that a NULL meant there was no memory >>>>> allocated. >>>>> >>>>> Fix this by propagating an error code from security_prepare_creds() up >>>>> the callstack. >>>> Why would it make sense for security_prepare_creds to return an error >>>> code other than ENOMEM? >>>> > That seems a bit of a violation of what that function is supposed to do >>>> >>> >>> The API allows LSM authors to decide what error code is returned from the >>> cred_prepare hook. security_task_alloc() is a similar hook, and has its return >>> code propagated. >> It is not an api. It is an implementation detail of the linux kernel. >> It is a set of convenient functions that do a job. >> The general rule is we don't support cases without an in-tree user. I >> don't see an in-tree user. >> >>> I'm proposing we follow security_task_allocs() pattern, and add visibility for >>> failure cases in prepare_creds(). >> I am asking why we would want to. Especially as it is not an API, and I >> don't see any good reason for anything but an -ENOMEM failure to be >> supported. >> > We're writing a LSM BPF policy, and not a new LSM. Our policy aims to solve > unprivileged unshare, similar to Debian's patch [1]. We're in a position such > that we can't use that patch because we can't block _all_ of our applications > from performing an unshare. We prefer a granular approach. LSM BPF seems like a > good choice. I am quite puzzled why doesn't /proc/sys/user/max_user_namespaces work for you? > Because LSM BPF exposes these hooks, we should probably treat them as an > API. From that perspective, userspace expects unshare to return a EPERM > when the call is denied permissions. The BPF code gets to be treated as a out of tree kernel module. >> Without an in-tree user that cares it is probably better to go the >> opposite direction and remove the possibility of return anything but >> memory allocation failure. That will make it clearer to implementors >> that a general error code is not supported and this is not a location >> to implement policy, this is only a hook to allocate state for the LSM. >> > > That's a good point, and it's possible we're using the wrong hook for the > policy. Do you know of other hooks we can look into? Not off the top of my head. >>>> I have probably missed a very interesting discussion where that was >>>> mentioned but I don't see link to the discussion or anything explaining >>>> why we want to do that in this change. >>>> >>> >>> AFAIK, this is the start of the discussion. >> You were on v3 and had an out of tree piece of code so I assumed someone >> had at least thought about why you want to implement policy in a piece >> of code whose only purpose is to allocate memory to store state. >> > > No worries. > >> Eric >> >> > > Links: > 1: > https://sources.debian.org/patches/linux/3.16.56-1+deb8u1/debian/add-sysctl-to-disallow-unprivileged-CLONE_NEWUSER-by-default.patch/ Eric