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 X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC0D2C3A59B for ; Fri, 30 Aug 2019 14:28:02 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C60EB23427 for ; Fri, 30 Aug 2019 14:28:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728057AbfH3O2B (ORCPT ); Fri, 30 Aug 2019 10:28:01 -0400 Received: from foss.arm.com ([217.140.110.172]:33102 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727751AbfH3O2B (ORCPT ); Fri, 30 Aug 2019 10:28:01 -0400 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 743A0344; Fri, 30 Aug 2019 07:28:00 -0700 (PDT) Received: from [10.1.197.57] (e110467-lin.cambridge.arm.com [10.1.197.57]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A12BC3F703; Fri, 30 Aug 2019 07:27:56 -0700 (PDT) Subject: Re: [PATCH v8 7/7] iommu/vt-d: Use bounce buffer for untrusted devices To: David Laight , 'Lu Baolu' , David Woodhouse , Joerg Roedel , Bjorn Helgaas , Christoph Hellwig Cc: "ashok.raj@intel.com" , "jacob.jun.pan@intel.com" , "alan.cox@intel.com" , "kevin.tian@intel.com" , "mika.westerberg@linux.intel.com" , Ingo Molnar , Greg Kroah-Hartman , "pengfei.xu@intel.com" , Konrad Rzeszutek Wilk , Marek Szyprowski , Jonathan Corbet , Boris Ostrovsky , Juergen Gross , Stefano Stabellini , Steven Rostedt , "iommu@lists.linux-foundation.org" , "linux-kernel@vger.kernel.org" , Jacob Pan References: <20190830071718.16613-1-baolu.lu@linux.intel.com> <20190830071718.16613-8-baolu.lu@linux.intel.com> <4dee1bcef8474ebb95a7826a58bb72aa@AcuMS.aculab.com> From: Robin Murphy Message-ID: Date: Fri, 30 Aug 2019 15:27:54 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <4dee1bcef8474ebb95a7826a58bb72aa@AcuMS.aculab.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 30/08/2019 14:39, David Laight wrote: > From: Lu Baolu >> Sent: 30 August 2019 08:17 > >> The Intel VT-d hardware uses paging for DMA remapping. >> The minimum mapped window is a page size. The device >> drivers may map buffers not filling the whole IOMMU >> window. This allows the device to access to possibly >> unrelated memory and a malicious device could exploit >> this to perform DMA attacks. To address this, the >> Intel IOMMU driver will use bounce pages for those >> buffers which don't fill whole IOMMU pages. > > Won't this completely kill performance? Yes it will. Though hopefully by now we're all well aware that speed and security being inversely proportional is the universal truth of modern computing. > I'd expect to see something for dma_alloc_coherent() (etc) > that tries to give the driver page sized buffers. Coherent DMA already works in PAGE_SIZE units under the covers (at least in the DMA API implementations relevant here) - that's not an issue. The problem is streaming DMA, where we have to give the device access to, say, some pre-existing 64-byte data packet, from right in the middle of who knows what else. Since we do not necessarily have control over the who knows what else, the only universally-practical way to isolate the DMA data is to copy it away to some safe sanitised page which we *do* control, and make the actual DMA accesses target that. > Either that or the driver could allocate page sized buffers > even though it only passes fragments of these buffers to > the dma functions (to avoid excessive cache invalidates). Where, since we can't easily second-guess users' systems, "the driver" turns out to be every DMA-capable driver, every subsystem-level buffer manager, every userspace application which could possibly make use of some kind of zero-copy I/O call... Compared to a single effectively-transparent implementation in a single place at the lowest level with a single switch for the user to turn it on or off depending on how security-critical their particular system is, I know which approach I'd rather review, maintain and rely on. Robin. > Since you have to trust the driver, why not actually trust it? > > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK > Registration No: 1397386 (Wales) >