linux-kernel - Re: [PATCH v2 6/9] Revert "iommu/arm-smmu: Make arm-smmu-v3 explicitly non-modular"

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Message-ID: <5a7df825-ca69-a2ad-e0b8-94a971528b89@huawei.com>
Date:   Fri, 8 Nov 2019 17:49:13 +0000
From:   John Garry <john.garry@...wei.com>
To:     Will Deacon <will@...nel.org>
CC:     <iommu@...ts.linux-foundation.org>, <linux-kernel@...r.kernel.org>,
        "Isaac J. Manjarres" <isaacm@...eaurora.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Saravana Kannan <saravanak@...gle.com>,
        "Bjorn Helgaas" <bhelgaas@...gle.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v2 6/9] Revert "iommu/arm-smmu: Make arm-smmu-v3
 explicitly non-modular"

On 08/11/2019 17:32, Will Deacon wrote:
> On Fri, Nov 08, 2019 at 05:25:09PM +0000, John Garry wrote:
>> On 08/11/2019 16:47, Will Deacon wrote:
>>> On Fri, Nov 08, 2019 at 04:44:25PM +0000, John Garry wrote:
>>>> BTW, it now looks like it was your v1 series I was testing there, on your
>>>> branch iommu/module. It would be helpful to update for ease of testing.
>>>
>>> Yes, sorry about that. I'll update it now (although I'm not sure it will
>>> help with this -- I was going to see what happens with other devices such
>>> as the intel-iommu or storage controllers)
>>
>> So I tried your v2 series for this - it has the same issue, as I
>> anticipated.
> 
> Right, I'm just not sure how resilient drivers are expected to be to force
> unbinding like this. You can break lots of stuff with root...

For sure, but it is good practice to limit that.

I had to fix something like this recently, so know about it... another 
potential problem is use-after-frees, where your device managed memory 
is freed at removal but still registered somewhere.

> 
>> It seems that some iommu drivers do call iommu_device_register(), so maybe a
>> decent reference. Or simply stop the driver being unbound.
> 
> I'm not sure what you mean about iommu_device_register() (we call that
> already), 

Sorry, I meant to say iommu_device_unregister().

but I guess we can keep the '.suppress_bind_attrs = true' if
> necessary. 

It may be good to add it to older stable kernels also, pre c07b6426df92.

I'll have a play on my laptop and see how well that works if
> you start unbinding stuff.

Cheers,
John