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

[Leizhen (ThunderTown)]

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