From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751241AbeECVHL (ORCPT ); Thu, 3 May 2018 17:07:11 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:47672 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1750911AbeECVHJ (ORCPT ); Thu, 3 May 2018 17:07:09 -0400 Subject: Re: [PATCH 2/3] kexec: call LSM hook for kexec_load syscall From: Mimi Zohar To: "Eric W. Biederman" , Casey Schaufler Cc: David Howells , Matthew Garrett , linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org Date: Thu, 03 May 2018 17:06:59 -0400 In-Reply-To: <87y3h0pu72.fsf@xmission.com> References: <1523572911-16363-1-git-send-email-zohar@linux.vnet.ibm.com> <1523572911-16363-3-git-send-email-zohar@linux.vnet.ibm.com> <87h8nqglpx.fsf@xmission.com> <1525275904.5669.308.camel@linux.vnet.ibm.com> <87h8nospo5.fsf@xmission.com> <6203b1e4-70c3-6d0e-60e0-56c6e8f72ec9@schaufler-ca.com> <87y3h0pu72.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: 18050321-0008-0000-0000-000004F2B661 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18050321-0009-0000-0000-00001E86DF87 Message-Id: <1525381619.3539.45.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-1805030181 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2018-05-03 at 11:42 -0500, Eric W. Biederman wrote: > Casey Schaufler writes: > > > On 5/3/2018 8:51 AM, Eric W. Biederman wrote: > >> Mimi Zohar writes: > >> > >>> On Wed, 2018-05-02 at 09:45 -0500, Eric W. Biederman wrote: > >>>> Mimi Zohar writes: > >>>> > >>>>> Allow LSMs and IMA to differentiate between the kexec_load and > >>>>> kexec_file_load syscalls by adding an "unnecessary" call to > >>>>> security_kernel_read_file() in kexec_load. This would be similar to the > >>>>> existing init_module syscall calling security_kernel_read_file(). > >>>> Given the reasonable desire to load a policy that ensures everything > >>>> has a signature I don't have fundamental objections. > >>>> > >>>> security_kernel_read_file as a hook seems an odd choice. At the very > >>>> least it has a bad name because there is no file reading going on here. > >>>> > >>>> I am concerned that I don't see CONFIG_KEXEC_VERIFY_SIG being tested > >>>> anywhere. Which means I could have a kernel compiled without that and I > >>>> would be allowed to use kexec_file_load without signature checking. > >>>> While kexec_load would be denied. > >>>> > >>>> Am I missing something here? > >>> The kexec_file_load() calls kernel_read_file_from_fd(), which in turn > >>> calls security_kernel_read_file().  So kexec_file_load and kexec_load > >>> syscall would be using the same method for enforcing signature > >>> verification. > >> Having looked at your patches and the kernel a little more I think > >> this should be a separate security hook that does not take a file > >> parameter. > >> > >> Right now every other security module assumes !file is init_module. > >> So I think this change has the potential to confuse other security > >> modules, with the result of unintended policy being applied. > >> > >> So just for good security module hygeine I think this needs a dedicated > >> kexec_load security hook. > > > > I would rather see the existing modules updated than a new > > hook added. Too many hooks spoil the broth. Two hooks with > > trivial differences just add to the clutter and make it harder > > for non-lsm developers to figure out what to use in their > > code. > > These are not non-trivial differences. There is absolutely nothing > file related about kexec_load. Nor for init_module for that matter. > > If something is called security_kernel_read_file I think it is wholly > appropriate for code that processes such a hook to assume file is > non-NULL. > > When you have to dance a jig (which is what I see the security modules > doing) to figure out who is calling a lsm hook for what purpose I think > it is a maintenance problem waiting to happen and that the hook is badly > designed. > > At this point I don't care what the lsm's do with the hooks but the > hooks need to make sense for people outside of the lsm's and something > about reading a file in a syscall that doesn't read files is complete > and utter nonsense. Sure, we can define a wrapper around the security_kernel_read_file() hook, calling it security_non-fd_syscall() or even security_old_syscall(). Mimi