From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8B304C433F5 for ; Thu, 17 Mar 2022 13:43:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234155AbiCQNoZ (ORCPT ); Thu, 17 Mar 2022 09:44:25 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40206 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234339AbiCQNoX (ORCPT ); Thu, 17 Mar 2022 09:44:23 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 85F5C1263C; Thu, 17 Mar 2022 06:43:06 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 44A181576; Thu, 17 Mar 2022 06:43:06 -0700 (PDT) Received: from [10.57.42.204] (unknown [10.57.42.204]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B6DA83F766; Thu, 17 Mar 2022 06:43:04 -0700 (PDT) Message-ID: <23f232a1-f511-d2fe-b1f8-5fd32b3a1a8f@arm.com> Date: Thu, 17 Mar 2022 13:42:56 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH] thunderbolt: Stop using iommu_present() Content-Language: en-GB To: Mika Westerberg Cc: "michael.jamet@intel.com" , "linux-usb@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "andreas.noever@gmail.com" , "iommu@lists.linux-foundation.org" , "Limonciello, Mario" , "YehezkelShB@gmail.com" , "hch@lst.de" References: <16852eb2-98bb-6337-741f-8c2f06418b08@arm.com> <3bb6a2f8-005b-587a-7d7a-7a9a5391ec05@arm.com> <5ef1c30a-1740-00cc-ad16-4b1c1b02fca4@arm.com> <0709e994-1c8b-56fe-7743-8fdbf3ba748b@arm.com> From: Robin Murphy In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-usb@vger.kernel.org On 2022-03-17 08:08, Mika Westerberg wrote: > Hi Robin, > > On Wed, Mar 16, 2022 at 07:17:57PM +0000, Robin Murphy wrote: >> The feeling I'm getting from all this is that if we've got as far as >> iommu_dma_protection_show() then it's really too late to meaningfully >> mitigate bad firmware. > > Note, these are requirements from Microsoft in order for the system to > use the "Kernel DMA protection". Because of this, likelyhood of "bad > firmware" should be quite low since these systems ship with Windows > installed so they should get at least some soft of validation that this > actually works. > >> We should be able to detect missing >> untrusted/external-facing properties as early as nhi_probe(), and if we >> could go into "continue at your own risk" mode right then *before* anything >> else happens, it all becomes a lot easier to reason about. > > I think what we want is that the DMAR opt-in bit is set in the ACPI > tables and that we know the full IOMMU translation is happening for the > devices behind "external facing ports". If that's not the case the > iommu_dma_protection_show() should return 0 meaning the userspace can > ask the user whether the connected device is allowed to use DMA (e.g > PCIe is tunneled or not). Ah, if it's safe to just say "no protection" in the case that we don't know for sure, that's even better. Clearly I hadn't quite grasped that aspect of the usage model, thanks for the nudge! > We do check for the DMAR bit in the Intel IOMMU code and we also do > check that there actually are PCIe ports marked external facing but we > could issue warning there if that's not the case. Similarly if the user > explicitly disabled the IOMMU translation. This can be done inside a new > IOMMU API that does something like the below pseudo-code: > > #if IOMMU_ENABLED > bool iommu_dma_protected(struct device *dev) > { > if (dmar_platform_optin() /* or the AMD equivalent */) { > if (!iommu_present(...)) /* whatever is needed to check that the full translation is enabled */ > dev_warn(dev, "IOMMU protection disabled!"); > /* > * Look for the external facing ports. Should be at > * least 1 or issue warning. > */ > ... > > return true; > } > > return false; > } > #else > static inline bool iommu_dma_protected(struct device *dev) > { > return false; > } > #endif > > Then we can make iommu_dma_protection_show() to call this function. The problem that I've been trying to nail down here is that dmar_platform_optin() really doesn't mean much for us - I don't know how Windows' IOMMU drivers work, but there's every chance it's not the same way as ours. The only material effect that dmar_platform_optin() has for us is to prevent the user from disabling the IOMMU driver altogether, and thus ensure that iommu_present() is true. Whether or not we can actually trust the IOMMU driver to provide reliable protection depends entirely on whether it knows the PCIe ports are external-facing. If not, we can only *definitely* know what the IOMMU driver will do for a given endpoint once that endpoint has appeared behind the port and iommu_probe_device() has decided what its default domain should be, and as far as I now understand, that's not an option for Thunderbolt since it can only happen *after* the tunnel has been authorised and created. Much as I'm tempted to de-scope back to my IOMMU API cleanup and run away from the rest of the issue, I think I can crib enough from the existing code to attempt a reasonable complete fix, so let me give that a go... Thanks, Robin.