From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Leizhen (ThunderTown)" Subject: Re: [PATCH RFC 1/1] iommu: set the default iommu-dma mode as non-strict Date: Sat, 2 Mar 2019 14:12:49 +0800 Message-ID: <5C7A1EE1.6020200@huawei.com> References: <20190131135211.6732-1-thunder.leizhen@huawei.com> <94b9b0c9-1a24-63ba-5abe-5f6d79fed415@arm.com> <5C78B89C.7040100@huawei.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Jean-Philippe Brucker , Hanjun Guo , John Garry , Robin Murphy , Will Deacon , Joerg Roedel , linux-arm-kernel , iommu , linux-kernel Cc: Yunsheng Lin , Linuxarm List-Id: iommu@lists.linux-foundation.org On 2019/3/1 19:07, Jean-Philippe Brucker wrote: > Hi Leizhen, > > On 01/03/2019 04:44, Leizhen (ThunderTown) wrote: >> >> >> On 2019/2/26 20:36, Hanjun Guo wrote: >>> Hi Jean, >>> >>> On 2019/1/31 22:55, Jean-Philippe Brucker wrote: >>>> Hi, >>>> >>>> On 31/01/2019 13:52, Zhen Lei wrote: >>>>> Currently, many peripherals are faster than before. For example, the top >>>>> speed of the older netcard is 10Gb/s, and now it's more than 25Gb/s. But >>>>> when iommu page-table mapping enabled, it's hard to reach the top speed >>>>> in strict mode, because of frequently map and unmap operations. In order >>>>> to keep abreast of the times, I think it's better to set non-strict as >>>>> default. >>>> >>>> Most users won't be aware of this relaxation and will have their system >>>> vulnerable to e.g. thunderbolt hotplug. See for example 4.3 Deferred >>>> Invalidation in >>>> http://www.cs.technion.ac.il/users/wwwb/cgi-bin/tr-get.cgi/2018/MSC/MSC-2018-21.pdf >> Hi Jean, >> >> In fact, we have discussed the vulnerable of deferred invalidation before upstream >> the non-strict patches. The attacks maybe possible because of an untrusted device or >> the mistake of the device driver. And we limited the VFIO to still use strict mode. >> As mentioned in the pdf, limit the freed memory with deferred invalidation only to >> be reused by the device, can mitigate the vulnerability. But it's too hard to implement >> it now. >> A compromise maybe we only apply non-strict to (1) dma_free_coherent, because the >> memory is controlled by DMA common module, so we can make the memory to be freed after >> the global invalidation in the timer handler. (2) And provide some new APIs related to >> iommu_unmap_page/sg, these new APIs deferred invalidation. And the candiate device >> drivers update the APIs if they want to improve performance. (3) Make sure that only >> the trusted devices and trusted drivers can apply (1) and (2). For example, the driver >> must be built into kernel Image. > > Do we have a notion of untrusted kernel drivers? A userspace driver It seems impossible to have such driver. The modules insmod by root users should be guaranteed by themselves. > (VFIO) is untrusted, ok. But a malicious driver loaded into the kernel > address space would have much easier ways to corrupt the system than to > exploit lazy mode... Yes, so that we have no need to consider untrusted drivers. > > For (3), I agree that we should at least disallow lazy mode if > pci_dev->untrusted is set. At the moment it means that we require the > strictest IOMMU configuration for external-facing PCI ports, but it can > be extended to blacklist other vulnerable devices or locations. I plan to add an attribute file for each device, espcially for hotplug devices. And let the root users to decide which mode should be used, strict or non-strict. Becasue they should known whether the hot-plug divice is trusted or not. > > If you do (3) then maybe we don't need (1) and (2), which require a > tonne of work in the DMA and IOMMU layers (but would certainly be nice > to see, since it would also help handle ATS invalidation timeouts) > > Thanks, > Jean > >> So that some high-end trusted devices use non-strict mode, and keep others still using >> strict mode. The drivers who want to use non-strict mode, should change to use new APIs >> by themselves. >> >> >>>> >>>> Why not keep the policy to secure by default, as we do for >>>> iommu.passthrough? And maybe add something similar to >>>> CONFIG_IOMMU_DEFAULT_PASSTRHOUGH? It's easy enough for experts to pass a >>>> command-line argument or change the default config. >>> >>> Sorry for the late reply, it was Chinese new year, and we had a long discussion >>> internally, we are fine to add a Kconfig but not sure OS vendors will set it >>> to default y. >>> >>> OS vendors seems not happy to pass a command-line argument, to be honest, >>> this is our motivation to enable non-strict as default. Hope OS vendors >>> can see this email thread, and give some input here. >>> >>> Thanks >>> Hanjun >>> >>> >>> . >>> >> > > > . > -- Thanks! BestRegards