Re: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx functions

[Robin Murphy <robin.murphy@arm.com>]

On 2021-02-12 17:28, Shameerali Kolothum Thodi wrote:
> 
> 
>> -----Original Message-----
>> From: Shameerali Kolothum Thodi
>> Sent: 12 February 2021 16:45
>> To: 'Robin Murphy' <robin.murphy@arm.com>; linux-kernel@vger.kernel.org;
>> iommu@lists.linux-foundation.org
>> Cc: joro@8bytes.org; jean-philippe@linaro.org; will@kernel.org; Zengtao (B)
>> <prime.zeng@hisilicon.com>; linuxarm@openeuler.org
>> Subject: RE: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx functions
>>
>>
>>
>>> -----Original Message-----
>>> From: Robin Murphy [mailto:robin.murphy@arm.com]
>>> Sent: 12 February 2021 16:39
>>> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>;
>>> linux-kernel@vger.kernel.org; iommu@lists.linux-foundation.org
>>> Cc: joro@8bytes.org; jean-philippe@linaro.org; will@kernel.org; Zengtao (B)
>>> <prime.zeng@hisilicon.com>; linuxarm@openeuler.org
>>> Subject: Re: [PATCH v2] iommu: Check dev->iommu in iommu_dev_xxx
>> functions
>>>
>>> On 2021-02-12 14:54, Shameerali Kolothum Thodi wrote:
>>>> Hi Robin/Joerg,
>>>>
>>>>> -----Original Message-----
>>>>> From: Shameer Kolothum
>> [mailto:shameerali.kolothum.thodi@huawei.com]
>>>>> Sent: 01 February 2021 12:41
>>>>> To: linux-kernel@vger.kernel.org; iommu@lists.linux-foundation.org
>>>>> Cc: joro@8bytes.org; robin.murphy@arm.com; jean-philippe@linaro.org;
>>>>> will@kernel.org; Zengtao (B) <prime.zeng@hisilicon.com>;
>>>>> linuxarm@openeuler.org
>>>>> Subject: [Linuxarm] [PATCH v2] iommu: Check dev->iommu in
>>> iommu_dev_xxx
>>>>> functions
>>>>>
>>>>> The device iommu probe/attach might have failed leaving dev->iommu
>>>>> to NULL and device drivers may still invoke these functions resulting
>>>>> in a crash in iommu vendor driver code. Hence make sure we check that.
>>>>>
>>>>> Also added iommu_ops to the "struct dev_iommu" and set it if the dev
>>>>> is successfully associated with an iommu.
>>>>>
>>>>> Fixes: a3a195929d40 ("iommu: Add APIs for multiple domains per
>> device")
>>>>> Signed-off-by: Shameer Kolothum
>>> <shameerali.kolothum.thodi@huawei.com>
>>>>> ---
>>>>> v1 --> v2:
>>>>>    -Added iommu_ops to struct dev_iommu based on the discussion with
>>> Robin.
>>>>>    -Rebased against iommu-tree core branch.
>>>>
>>>> A gentle ping on this...
>>>
>>> Is there a convincing justification for maintaining yet another copy of
>>> the ops pointer rather than simply dereferencing iommu_dev->ops at point
>>> of use?
>>>
>>
>> TBH, nothing I can think of now. That was mainly the way I interpreted your
>> suggestion
>> from the v1.  Now it looks like you didn’t mean it :). I am Ok to rework it to
>> dereference
>> it from iommu_dev. Please let me know.
> 
> So we can do something like this,
> 
> index fd76e2f579fe..5fd31a3cec18 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -2865,10 +2865,12 @@ EXPORT_SYMBOL_GPL(iommu_fwspec_add_ids);
>    */
>   int iommu_dev_enable_feature(struct device *dev, enum iommu_dev_features feat)
>   {
> -       const struct iommu_ops *ops = dev->bus->iommu_ops;
> +       if (dev->iommu && dev->iommu->iommu_dev && dev->iommu->iommu_dev->ops)
> +               struct iommu_ops  *ops = dev->iommu->iommu_dev->ops;
>   
> -       if (ops && ops->dev_enable_feat)
> -               return ops->dev_enable_feat(dev, feat);
> +               if (ops->dev_enable_feat)
> +                       return ops->dev_enable_feat(dev, feat);
> +       }
>   
>          return -ENODEV;
>   }
> 
> Again, not sure we need to do the checking for iommu->dev and ops here. If the
> dev->iommu is set, is it safe to assume that we have a valid iommu->iommu_dev
> and ops always? (May be it is safer to do the checking in case something
> else breaks this assumption in future). Please let me know your thoughts.

I think it *could* happen that dev->iommu is set by iommu_fwspec_init() 
but iommu_probe_device() later refuses the device for whatever reason, 
so we would still need to check iommu->iommu_dev to be completely safe. 
We can assume iommu_dev->ops is valid, since if the IOMMU driver has 
returned something bogus from .probe_device then it's a major bug in 
that driver and crashing is the best indicator :)

Robin.

> 
> Thanks,
> Shameer
> 
> 
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