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Date:   Mon, 4 Mar 2019 15:52:36 +0000
From:   Robin Murphy <>
To:     "Leizhen (ThunderTown)" <>,
        Jean-Philippe Brucker <>,
        Hanjun Guo <>,
        John Garry <>,
        Will Deacon <>,
        Joerg Roedel <>,
        linux-arm-kernel <>,
        iommu <>,
        linux-kernel <>
Cc:     Yunsheng Lin <>,
        Linuxarm <>
Subject: Re: [PATCH RFC 1/1] iommu: set the default iommu-dma mode as

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
>>> 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.


>> 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
>>>> .
>> .

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