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


> 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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