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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 CDDBEC2BCA1 for ; Fri, 7 Jun 2019 10:34:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AD24E212F5 for ; Fri, 7 Jun 2019 10:34:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727920AbfFGKd6 (ORCPT ); Fri, 7 Jun 2019 06:33:58 -0400 Received: from foss.arm.com ([217.140.110.172]:37534 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726584AbfFGKd5 (ORCPT ); Fri, 7 Jun 2019 06:33:57 -0400 X-Greylist: delayed 337 seconds by postgrey-1.27 at vger.kernel.org; Fri, 07 Jun 2019 06:33:57 EDT 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 622EF802; Fri, 7 Jun 2019 03:28:20 -0700 (PDT) Received: from [192.168.0.4] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A11773F246; Fri, 7 Jun 2019 03:29:56 -0700 (PDT) Subject: Re: [PATCH v8 26/29] vfio-pci: Register an iommu fault handler To: Jacob Pan Cc: Auger Eric , Alex Williamson , "eric.auger.pro@gmail.com" , "iommu@lists.linux-foundation.org" , "linux-kernel@vger.kernel.org" , "kvm@vger.kernel.org" , "kvmarm@lists.cs.columbia.edu" , "joro@8bytes.org" , "yi.l.liu@intel.com" , Will Deacon , Robin Murphy , "kevin.tian@intel.com" , "ashok.raj@intel.com" , Marc Zyngier , "peter.maydell@linaro.org" , Vincent Stehle References: <20190526161004.25232-1-eric.auger@redhat.com> <20190526161004.25232-27-eric.auger@redhat.com> <20190603163139.70fe8839@x1.home> <10dd60d9-4af0-c0eb-08c9-a0db7ee1925e@redhat.com> <20190605154553.0d00ad8d@jacob-builder> <2753d192-1c46-d78e-c425-0c828e48cde2@arm.com> <20190606132903.064f7ac4@jacob-builder> From: Jean-Philippe Brucker Message-ID: Date: Fri, 7 Jun 2019 11:28:13 +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: <20190606132903.064f7ac4@jacob-builder> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 06/06/2019 21:29, Jacob Pan wrote: >>>>>> iommu_unregister_device_fault_handler(&vdev->pdev->dev); >>>>> >>>>> >>>>> But this can fail if there are pending faults which leaves a >>>>> device reference and then the system is broken :( >>>> This series only features unrecoverable errors and for those the >>>> unregistration cannot fail. Now unrecoverable errors were added I >>>> admit this is confusing. We need to sort this out or clean the >>>> dependencies. >>> As Alex pointed out in 4/29, we can make >>> iommu_unregister_device_fault_handler() never fail and clean up all >>> the pending faults in the host IOMMU belong to that device. But the >>> problem is that if a fault, such as PRQ, has already been injected >>> into the guest, the page response may come back after handler is >>> unregistered and registered again. >> >> I'm trying to figure out if that would be harmful in any way. I guess >> it can be a bit nasty if we handle the page response right after >> having injected a new page request that uses the same PRGI. In any >> other case we discard the page response, but here we forward it to >> the endpoint and: >> >> * If the response status is success, endpoint retries the >> translation. The guest probably hasn't had time to handle the new >> page request and translation will fail, which may lead the endpoint >> to give up (two unsuccessful translation requests). Or send a new >> request >> > Good point, there shouldn't be any harm if the page response is a > "fake" success. In fact it could happen in the normal operation when > PRQs to two devices share the same non-leaf translation structure. The > worst case is just a retry. I am not aware of the retry limit, is it in > the PCIe spec? I cannot find it. I don't think so, it's the implementation's choice. In general I don't think devices will have a retry limit, but it doesn't seem like the PCI spec prevents them from implementing one either. It could be useful to stop retrying after a certain number of faults, for preventing livelocks when the OS doesn't fix up the page tables and the device would just repeat the fault indefinitely. > I think we should just document it, similar to having a spurious > interrupt. The PRQ trace event should capture that as well. > >> * otherwise the endpoint won't retry the access, and could also >> disable PRI if the status is failure. >> > That would be true regardless this race condition with handler > registration. So should be fine. We do give an invalid response for the old PRG (because of unregistering), but also for the new one, which has a different address that the guest might be able to page in and would normally return success. >>> We need a way to reject such page response belong >>> to the previous life of the handler. Perhaps a sync call to the >>> guest with your fault queue eventfd? I am not sure. >> >> We could simply expect the device driver not to send any page response >> after unregistering the fault handler. Is there any reason VFIO would >> need to unregister and re-register the fault handler on a live guest? >> > There is no reason for VFIO to unregister and register again, I was > just thinking from security perspective. Someone could write a VFIO app > do this attack. But I agree the damage is within the device, may get > PRI disabled as a result. Yes I think the damage would always be contained within the misbehaving software > So it seems we agree on the following: > - iommu_unregister_device_fault_handler() will never fail > - iommu driver cleans up all pending faults when handler is unregistered > - assume device driver or guest not sending more page response _after_ > handler is unregistered. > - system will tolerate rare spurious response > > Sounds right? Yes, I'll add that to the fault series Thanks, Jean