linux-kernel - Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

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Date:   Thu, 16 Feb 2023 08:42:53 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Robin Murphy <robin.murphy@....com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

On 2/15/23 7:09 PM, Robin Murphy wrote:
> On 2023-02-15 07:28, Baolu Lu wrote:
>> On 2023/2/15 14:56, Tian, Kevin wrote:
>>>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>>>> Sent: Wednesday, February 15, 2023 1:51 PM
>>>>
>>>> On 2/13/23 10:19 PM, Jason Gunthorpe wrote:
>>>>> On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
>>>>>> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct
>>>> iommu_group *group,
>>>>>>        else
>>>>>>            return -EINVAL;
>>>>>>
>>>>>> +    if (req_type != IOMMU_DOMAIN_DMA_FQ ||
>>>>>> +        group->default_domain->type != IOMMU_DOMAIN_DMA) {
>>>>>> +        ret = iommu_group_claim_dma_owner(group, (void *)buf);
>>>>>> +        if (ret)
>>>>>> +            return ret;
>>>>>> +        group_owner_claimed = true;
>>>>>> +    }
>>>>> I don't get it, this should be done unconditionally. If we couldn't
>>>>> take ownership then we simply can't progress.
>>>> The existing code allows the user to switch the default domain from
>>>> strict to lazy invalidation mode. The default domain is not changed,
>>>> hence it should be seamless and transparent to the device driver.
>>> Is there real usage relying on this transition for a bound device?
>>>
>>> In concept strict->lazy transition implies relaxed DMA security. It's 
>>> hard
>>> to think of a motivation of doing so while the device might be doing
>>> in-fly DMAs.
>>>
>>> Presumably such perf/security tradeoff should be planned way before
>>> binding device/driver together.
>>>
>>> btw if strict->lazy is allowed why lazy->strict is prohibited?
>>>
>>
>> We all know, strict vs. lazy is a tradeoff between performance and
>> security.
>>
>> strict -> lazy: driver works in secure mode. This transition trades off
>> security for better performance.
>>
>> lazy->strict: The driver is already working in non-safety mode. This
>> transition only results in worse performance. It makes no sense. If user
>> want to put the driver in a secure mode, they need to unbind the driver,
>> reset the device and do the lazy->strict transition.
>>
>> Robin might have better insights.
> 
> Yes, this was added for a definite use-case in ChromeOS, where 
> strict->lazy needs to support being done "live" since the device in 
> question is the storage controller for the already-mounted root 
> filesystem.

Thanks for letting us know this.

> Your reasoning seems to match what I summarised in the 
> original commit message 😄

Haha, it seems that my memory is till good. :-)

Best regards,
baolu