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 5B592ECAAD1 for ; Wed, 31 Aug 2022 08:02:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231255AbiHaICw (ORCPT ); Wed, 31 Aug 2022 04:02:52 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34914 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229449AbiHaICp (ORCPT ); Wed, 31 Aug 2022 04:02:45 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id C1146B99D3 for ; Wed, 31 Aug 2022 01:02:44 -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 B043CED1; Wed, 31 Aug 2022 01:02:50 -0700 (PDT) Received: from [10.57.15.237] (unknown [10.57.15.237]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 13CBF3F766; Wed, 31 Aug 2022 01:03:11 -0700 (PDT) Message-ID: <19a47bae-91ff-648c-c407-759468b8af6a@arm.com> Date: Wed, 31 Aug 2022 09:02:35 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:102.0) Gecko/20100101 Thunderbird/102.2.0 Subject: Re: [PATCH] swiotlb: fix a typo Content-Language: en-GB To: Chao Gao Cc: linux-kernel@vger.kernel.org, iommu@lists.linux.dev, hch@infradead.org, m.szyprowski@samsung.com References: <20220826095046.880626-1-chao.gao@intel.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-kernel@vger.kernel.org On 2022-08-31 05:22, Chao Gao wrote: > On Tue, Aug 30, 2022 at 10:23:51AM +0100, Robin Murphy wrote: >> On 2022-08-26 10:50, Chao Gao wrote: >>> "overwirte" isn't a word. It should be "overwrite". >>> >>> Signed-off-by: Chao Gao >>> --- >>> BTW, I am wondering if copying the original buffer to the tlb buffer >>> unconditionally will leak the original buffer to the VMM, especially >>> when VMM isn't trusted e.g., by confidential VMs. Would it be better >>> to zero the tlb buffer for dir == DMA_FROM_DEVICE? >> >> No, at the point of dma_map(), the buffer contents are owned by the caller, >> so if parts of that buffer are sensitive and shouldn't be exposed to DMA, >> then don't map the whole buffer for DMA. There are more DMA API >> implementations than SWIOTLB. >> > > I am not sure if all existing drivers ensure that all buffers allocated > for DMA_FROM_DEVICE are zeroed/poisoned so that they don't have sensitive > data before dma_map(). If that isn't the case, bouncing the original contents > (left by the previous user of the buffer) effectively makes the contents > visible to host/VMM. I am afraid it may be a concern for confidential VMs. Sure, and in a scheme where pages can be dynamically shared in-place instead of using SWIOTLB to bounce through a pre-shared buffer, then those same drivers will still expose the same stale data. Similarly, a driver could massively over-map with DMA_TO_DEVICE or DMA_BIDIRECTIONAL and expose all manner of potential secrets that way. It's a concern that cannot be addressed at the DMA API level. Anyone who wants to completely trust drivers not to do anything stupid in a confidential compute scenario is going to have to audit and possibly fix those drivers. Robin.