Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752347AbdHRTIh (ORCPT ); Fri, 18 Aug 2017 15:08:37 -0400 Received: from mail-io0-f180.google.com ([209.85.223.180]:36284 "EHLO mail-io0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752291AbdHRTIg (ORCPT ); Fri, 18 Aug 2017 15:08:36 -0400 MIME-Version: 1.0 In-Reply-To: References: <20170804212010.15064-1-mjg59@google.com> From: Matthew Garrett Date: Fri, 18 Aug 2017 12:08:34 -0700 Message-ID: Subject: Re: [PATCH] Enable reset attack mitigation To: Ard Biesheuvel Cc: "linux-kernel@vger.kernel.org" , "linux-efi@vger.kernel.org" , Matt Fleming Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1928 Lines: 40 On Fri, Aug 18, 2017 at 11:52 AM, Ard Biesheuvel wrote: > On 4 August 2017 at 22:20, Matthew Garrett wrote: >> + * Enable reboot attack mitigation. This requests that the firmware clear the >> + * RAM on next reboot before proceeding with boot, ensuring that any secrets >> + * are cleared. If userland has ensured that all secrets have bene removed > > s/bene/been/ Thanks. >> + set_efi_var(efi_MemoryOverWriteRequest_name, &var_guid, >> + EFI_VARIABLE_NON_VOLATILE | >> + EFI_VARIABLE_BOOTSERVICE_ACCESS | >> + EFI_VARIABLE_RUNTIME_ACCESS, sizeof(val), val); > > Shouldn't this be &val? Ooh, good catch - not sure how that got eaten. >> What's the threat model? If there's no way for userland to ask the >> kernel to drop any secrets it still holds, that seems like a problem >> in any case. If the concern is that someone may be able to clear the >> flag and then reboot in order to deliberately attempt to obtain kernel >> secrets then there's no hugely easy way around this without special >> casing the variable and preventing userland from being able to modify >> it. There's a MemoryOverwriteRequestLock spec from Microsoft that >> provides a mechanism for this (the firmware and the OS configure a >> shared secret that controls access to MemoryOverwriteRequestControl, >> so we'd keep that in the kernel and clear it on reboot), but that's >> not implemented everywhere and we'd still want to base on top of this. > > So how would that work with, e.g., the keys for your encrypted file > system? Surely, you can't expect the kernel to drop that secret when > userland asks it to, so there will always be a window where userland > has set the variable but the kernel is not ready to drop its secrets > yet. If the kernel doesn't synchronously zero the key when dm-crypt is torn down, that feels like a bug?