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

[Leizhen (ThunderTown)]

On 2019/3/4 23:52, Robin Murphy wrote:
> 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? 
Yes, It should be assumed that all kernel drivers and all hard-wired devices are trusted. There is
no reason to doubt that the open source drivers or the drivers and devices provided by legitimate
suppliers are malicious.


> 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.
The default mode of external devices is strict, it can be obviously changed to non-strict mode. But as
you said, it maybe hard to be implemented. In addition, bring a malicious device into computer room,
attach and export data it's not easy also. Maybe I should follow Jean'suggestion first, add a config item.

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

-- 
Thanks!
BestRegards