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

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Date:   Wed, 6 Mar 2019 12:07:50 +0000
From:   John Garry <john.garry@...wei.com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Hanjun Guo <guohanjun@...wei.com>,
        "Will Deacon" <will.deacon@....com>,
        Joerg Roedel <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
CC:     Yunsheng Lin <linyunsheng@...wei.com>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH RFC 1/1] iommu: set the default iommu-dma mode as
 non-strict


>>>
>>>> (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.
>

+1

On another topic, we did also see a use case for selectively 
passthrough'ing devices.

Typically, having the kernel use the identity mapping for when driving a 
device is fine. In fact, having the IOMMU translating puts a big 
performance burden on the system. However sometimes we may require the 
IOMMU involved for certain devices, like for when the kernel device 
driver has big contiguous DMA requirements, which is the case for some 
RDMA NIC cards.

John


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