Messages in this thread | | | Subject | Re: [PATCH RFC 1/1] iommu: set the default iommu-dma mode as non-strict | From | Robin Murphy <> | Date | Mon, 4 Mar 2019 15:52:36 +0000 |
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On 02/03/2019 06:12, Leizhen (ThunderTown) wrote: > > > 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.
Aside from the problem that without massive implementation changes strict/non-strict is at best a per-domain property, not a per-device one, I can't see this being particularly practical - surely the whole point of a malicious endpoint is that it's going to pretend to be some common device for which a 'trusted' kernel driver already exists? If you've chosen to trust *any* external device, I think you may as well have just set non-strict globally anyway. The effort involved in trying to implement super-fine-grained control seems hard to justify.
Robin.
>> >> 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 >>>> >>>> >>>> . >>>> >>> >> >> >> . >> >
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