From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from youngberry.canonical.com ([91.189.89.112]:58962 "EHLO youngberry.canonical.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727377AbeHCSNm (ORCPT ); Fri, 3 Aug 2018 14:13:42 -0400 Received: from mail-io0-f197.google.com ([209.85.223.197]) by youngberry.canonical.com with esmtps (TLS1.0:RSA_AES_128_CBC_SHA1:16) (Exim 4.76) (envelope-from ) id 1flcks-0006Mp-5A for linux-integrity@vger.kernel.org; Fri, 03 Aug 2018 16:16:42 +0000 Received: by mail-io0-f197.google.com with SMTP id y4-v6so4504367iol.2 for ; Fri, 03 Aug 2018 09:16:42 -0700 (PDT) Date: Fri, 3 Aug 2018 11:16:36 -0500 From: Seth Forshee To: Mimi Zohar Cc: Eric Richter , linux-integrity , linux-security-module , linux-efi , linux-kernel , David Howells , Justin Forbes Subject: Re: [PATCH 3/4] ima: add support for KEXEC_ORIG_KERNEL_CHECK Message-ID: <20180803161636.GX3001@ubuntu-xps13> References: <20180725233200.761-1-erichte@linux.vnet.ibm.com> <20180725233200.761-4-erichte@linux.vnet.ibm.com> <20180803131129.GS3001@ubuntu-xps13> <1533308099.4337.424.camel@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 In-Reply-To: <1533308099.4337.424.camel@linux.ibm.com> Sender: linux-integrity-owner@vger.kernel.org List-ID: On Fri, Aug 03, 2018 at 10:54:59AM -0400, Mimi Zohar wrote: > On Fri, 2018-08-03 at 08:11 -0500, Seth Forshee wrote: > > On Wed, Jul 25, 2018 at 06:31:59PM -0500, Eric Richter wrote: > > > IMA can verify the signature of kernel images loaded with kexec_file_load, > > > but can not verify images loaded with the regular kexec_load syscall. > > > Therefore, the appraisal will automatically fail during kexec_load when an > > > appraise policy rule is set for func=KEXEC_KERNEL_CHECK. This can be used > > > to effectively disable the kexec_load syscall, while still allowing the > > > kexec_file_load to operate so long as the target kernel image is signed. > > > > > > However, this conflicts with CONFIG_KEXEC_VERIFY_SIG. If that option is > > > enabled and there is an appraise rule set, then the target kernel would > > > have to be verifiable by both IMA and the architecture specific kernel > > > verification procedure. > > > > > > This patch adds a new func= for IMA appraisal specifically for the original > > > kexec_load syscall. Therefore, the kexec_load syscall can be effectively > > > disabled via IMA policy, leaving the kexec_file_load syscall able to do its > > > own signature verification, and not require it to be signed via IMA. To > > > retain compatibility, the existing func=KEXEC_KERNEL_CHECK flag is > > > unchanged, and thus enables appraisal for both kexec syscalls. > > > > This seems like a roundabout way to disallow the kexec_load syscall. > > Wouldn't it make more sense to simply disallow kexec_load any time > > CONFIG_KEXEC_VERIFY_SIG is enabled, since it effectively renders that > > option impotent? Or has that idea already been rejected? > > Agreed! We can modify the "case LOADING_KEXEC_IMAGE" in > ima_load_data() to prevent the kexec_load based on > CONFIG_KEXEC_VERIFY_SIG. > > The architecture specific policy would only include the IMA appraise > rule if CONFIG_KEXEC_VERIFY_SIG was not defined. After looking at this some more I'm having second thoughts about my suggestion. As a distro we produce a kernel that needs to be flexible enough for a variety of scenarios, and if we completely close off the ability to load an unsigned kernel for kexec that's almost certainly going to end up breaking some use cases. So I think it is necessary to make this a run-time decision rather than a compile-time decision. The patch as provided does this based on whether or not the kernel was booted under secure boot. That might be reasonable, though I still find this mechanism kind of awkward. It seems like ideally there would instead be some logic that would accept the image if the KEXEC_VERIFY_SIG verification had passed, and otherwise require IMA signature verification. Thanks, Seth