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 (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... 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. 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 >> >> >> . >> > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
