From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751301AbeECV6H (ORCPT ); Thu, 3 May 2018 17:58:07 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:34114 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751216AbeECV6E (ORCPT ); Thu, 3 May 2018 17:58:04 -0400 Subject: Re: [PATCH 0/3] kexec: limit kexec_load syscall From: Mimi Zohar To: "Eric W. Biederman" Cc: Kees Cook , David Howells , Matthew Garrett , linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com Date: Thu, 03 May 2018 17:57:55 -0400 In-Reply-To: <87d0yco1vy.fsf@xmission.com> References: <1523572911-16363-1-git-send-email-zohar@linux.vnet.ibm.com> <87r2mso5up.fsf@xmission.com> <1525383075.3539.67.camel@linux.vnet.ibm.com> <87d0yco1vy.fsf@xmission.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.20.5 (3.20.5-1.fc24) Mime-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 x-cbid: 18050321-0044-0000-0000-0000054EB88E X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18050321-0045-0000-0000-0000288FE16F Message-Id: <1525384675.3539.89.camel@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-03_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805030190 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2018-05-03 at 16:38 -0500, Eric W. Biederman wrote: > Mimi Zohar writes: > > > [Cc'ing Kees and kernel-hardening] > > > > On Thu, 2018-05-03 at 15:13 -0500, Eric W. Biederman wrote: > >> Mimi Zohar writes: > >> > >> > In environments that require the kexec kernel image to be signed, prevent > >> > using the kexec_load syscall. In order for LSMs and IMA to differentiate > >> > between kexec_load and kexec_file_load syscalls, this patch set adds a > >> > call to security_kernel_read_file() in kexec_load_check(). > >> > >> Having thought about it some more this justification for these changes > >> does not work. The functionality of kexec_load is already root-only. > >> So in environments that require the kernel image to be signed just don't > >> use kexec_load. Possibly even compile kexec_load out to save space > >> because you will never need it. You don't need a new security hook to > >> do any of that. Userspace is a very fine mechanism for being the > >> instrument of policy. > > > > True, for those building their own kernel, they can disable the old > > syscalls.  The concern is not for those building their own kernels, > > but for those using stock kernels.   > > > > By adding an LSM hook here in the kexec_load syscall, as opposed to an > > IMA specific hook, other LSMs can piggy back on top of it.  Currently, > > both load_pin and SELinux are gating the kernel module syscalls based > > on security_kernel_read_file. > > > > If there was a similar option for the kernel image, I'm pretty sure > > other LSMs would use it. > > > > From an IMA perspective, there needs to be some method for only > > allowing signed code to be loaded, executed, etc. - kernel modules, > > kernel image/initramfs, firmware, policies. > > What is the IMA perspective. Why can't IMA trust appropriately > authorized userspace? Suppose a system owner wants to define a system wide policy that requires all code be signed - kernel modules, firmware, kexec image & initramfs, executables, mmapped files, etc - without having to rebuild the kernel.  Without a call in kexec_load that isn't possible. > > >> If you don't trust userspace that needs to be spelled out very clearly. > >> You need to talk about what your threat models are. > >> > >> If the only justification is so that that we can't boot windows if > >> someone hacks into userspace it has my nack because that is another kind > >> of complete non-sense. > > > > The usecase is the ability to gate the kexec_load usage in stock > > kernels. > > But kexec_load is already gated. It requires CAP_SYS_BOOT. It isn't a matter of kexec_load already being gated, but of wanting a single place for defining a system wide policy, as described above. Mimi > > >> Given that you are not trusting userspace this changeset also probably > >> needs to have the kernel-hardening list cc'd. Because the only possible > >> justification I can imagine for something like this is kernel-hardening. > > > > Sure, Cc'ing linux-hardening and Kees. > > > > Mimi >