Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752259AbdLGISW (ORCPT ); Thu, 7 Dec 2017 03:18:22 -0500 Received: from mail-wr0-f194.google.com ([209.85.128.194]:38313 "EHLO mail-wr0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750862AbdLGISU (ORCPT ); Thu, 7 Dec 2017 03:18:20 -0500 X-Google-Smtp-Source: AGs4zMZIQlRSEu3JW4YSG0kPbImaVqag93mZgpsdwg1w9CwFWCafCGfsm/04rzDY9cn3h3fPgwiXLw== Date: Thu, 7 Dec 2017 09:18:16 +0100 From: Ingo Molnar To: Gary Lin Cc: Josh Boyer , x86 , "Linux-Kernel@Vger. Kernel. Org" , "linux-efi@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "H. Peter Anvin" , Thomas Gleixner , Ard Biesheuvel , Ingo Molnar , Matt Fleming , Catalin Marinas , Will Deacon , Joey Lee Subject: Re: [RFC v3 PATCH 0/2] Introduce Security Version to EFI Stub Message-ID: <20171207081816.jy2rw5y5iyxeqw6n@gmail.com> References: <20171205100148.5757-1-glin@suse.com> <20171206032437.pifxxcewgozlghfs@GaryWorkstation> <20171206183734.xmgecyrd5suiurm6@gmail.com> <20171207015903.jaos5siysggzz4nc@GaryWorkstation> <20171207060927.i4b4h6ahas3iiyrc@gmail.com> <20171207075203.gcxprq7ctv637fm7@GaryWorkstation> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171207075203.gcxprq7ctv637fm7@GaryWorkstation> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2972 Lines: 59 * Gary Lin wrote: > On Thu, Dec 07, 2017 at 07:09:27AM +0100, Ingo Molnar wrote: > > > > * Gary Lin wrote: > > > > > On Wed, Dec 06, 2017 at 07:37:34PM +0100, Ingo Molnar wrote: > > > > > > > > * Gary Lin wrote: > > > > > > > > > On Tue, Dec 05, 2017 at 04:14:26PM -0500, Josh Boyer wrote: > > > > > > On Tue, Dec 5, 2017 at 5:01 AM, Gary Lin wrote: > > > > > > > The series of patches introduce Security Version to EFI stub. > > > > > > > > > > > > > > Security Version is a monotonically increasing number and designed to > > > > > > > prevent the user from loading an insecure kernel accidentally. The > > > > > > > bootloader maintains a list of security versions corresponding to > > > > > > > different distributions. After fixing a critical vulnerability, the > > > > > > > distribution kernel maintainer bumps the "version", and the bootloader > > > > > > > updates the list automatically. When the user tries to load a kernel > > > > > > > with a lower security version, the bootloader shows a warning prompt > > > > > > > to notify the user the potential risk. > > > > > > > > > > > > If a distribution releases a kernel with a higher security version and > > > > > > that it automatically updated on boot, what happens if that kernel > > > > > > contains a different bug that causes it to fail to boot or break > > > > > > critical functionality? At that point, the user's machine would be in > > > > > > a state where the higher security version is enforced but the only > > > > > > kernel that provides that is broken. Wouldn't that make a bad > > > > > > situation even worse by now requiring manual acceptance of the older > > > > > > SV kernel boot physically at the machine? > > > > > > > > > > > > I feel like I'm missing a detail here or something. > > > > > > > > > > > If the new kernel fails to boot, then the user has to choose the kernel > > > > > manually anyway, and there will be an option in the warning prompt to > > > > > lower SV. > > > > > > > > And what if the firmware does not support a lowering of the SV? > > > > > > > The SV list is manipulated by the bootloader, and the firmware only > > > provides the interface to the storage, i.e. non-volatile flash. > > > > What about systems where the bootloader is part of the system and users only have > > the ability to provide kernel images, but no ability to change the boot loader? > > It depends on how the bootloader works. If the system uses my > implementation of shim loader, it surely has the ability to lower SV, > but it requires physical access on purpose. And that's my problem: if in practice the bootloader is 'part of the system', is signed and is updated like the firmware, then putting a "Security Version" into the kernel image and architecting a boot protocol for a monotonic method for the bootloader to restrict the loading of kernel images is an obviously bad idea. Thanks, Ingo