Re: [PATCH RFC 1/1] iommu: set the default iommu-dma mode as non-strict

[Robin Murphy]

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
> > > >
> > > >
> > > > .
> >
> >
> > .