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Thu, 21 Oct 2021 08:10:52 +0000 (UTC) Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 457B560FDA; Thu, 21 Oct 2021 08:10:52 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1mdTA2-000eLF-32; Thu, 21 Oct 2021 09:10:50 +0100 Date: Thu, 21 Oct 2021 09:10:49 +0100 Message-ID: <87wnm6bxx2.wl-maz@kernel.org> From: Marc Zyngier To: Lu Baolu Subject: Re: [PATCH v3 4/6] iommu: Move IOMMU pagesize check to attach_device In-Reply-To: <6a886030-cbc6-9e92-bf79-77b659da2915@linux.intel.com> References: <20211019163737.46269-1-sven@svenpeter.dev> <20211019163737.46269-5-sven@svenpeter.dev> <9e25f2c0-d9d3-475d-e973-63be1891f9a5@linux.intel.com> <8735ovdbcv.wl-maz@kernel.org> <6a886030-cbc6-9e92-bf79-77b659da2915@linux.intel.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: baolu.lu@linux.intel.com, sven@svenpeter.dev, iommu@lists.linux-foundation.org, robin.murphy@arm.com, arnd@kernel.org, marcan@marcan.st, linux-kernel@vger.kernel.org, graf@amazon.com, mohamed.mediouni@caramail.com, will@kernel.org, alyssa@rosenzweig.io X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Cc: Arnd Bergmann , Will Deacon , Hector Martin , linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org, Alexander Graf , Mohamed Mediouni , Robin Murphy , Alyssa Rosenzweig X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Thu, 21 Oct 2021 03:22:30 +0100, Lu Baolu wrote: > > On 10/20/21 10:22 PM, Marc Zyngier wrote: > > On Wed, 20 Oct 2021 06:21:44 +0100, > > Lu Baolu wrote: > >> > >> On 2021/10/20 0:37, Sven Peter via iommu wrote: > >>> + /* > >>> + * Check that CPU pages can be represented by the IOVA granularity. > >>> + * This has to be done after ops->attach_dev since many IOMMU drivers > >>> + * only limit domain->pgsize_bitmap after having attached the first > >>> + * device. > >>> + */ > >>> + ret = iommu_check_page_size(domain); > >>> + if (ret) { > >>> + __iommu_detach_device(domain, dev); > >>> + return ret; > >>> + } > >> > >> It looks odd. __iommu_attach_device() attaches an I/O page table for a > >> device. How does it relate to CPU pages? Why is it a failure case if CPU > >> page size is not covered? > > > > If you allocate a CPU PAGE_SIZE'd region, and point it at a device > > that now can DMA to more than what you have allocated because the > > IOMMU's own page size is larger, the device has now access to data it > > shouldn't see. In my book, that's a pretty bad thing. > > But even you enforce the CPU page size check here, this problem still > exists unless all DMA buffers are PAGE_SIZE aligned and sized, right? Let me take a CPU analogy: you have a page that contains some user data *and* a kernel secret. How do you map this page into userspace without leaking the kernel secret? PAGE_SIZE allocations are the unit of isolation, and this applies to both CPU and IOMMU. If you have allocated a DMA buffer that is less than a page, you then have to resort to bounce buffering, or accept that your data isn't safe. M. -- Without deviation from the norm, progress is not possible. _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu