linux-kernel - Re: [PATCH v4 09/10] iommu: Make iommu_queue

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Message-ID: <eca39154-bc45-3c7d-88a9-b377f4d248f9@linux.intel.com>
Date:   Mon, 11 Sep 2023 20:46:26 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>
Cc:     baolu.lu@...ux.intel.com, "Liu, Yi L" <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 09/10] iommu: Make iommu_queue_iopf() more generic

On 2023/9/11 14:57, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, September 5, 2023 1:24 PM
>>
>> Hi Kevin,
>>
>> I am trying to address this issue in below patch. Does it looks sane to
>> you?
>>
>> iommu: Consolidate per-device fault data management
>>
>> The per-device fault data is a data structure that is used to store
>> information about faults that occur on a device. This data is allocated
>> when IOPF is enabled on the device and freed when IOPF is disabled. The
>> data is used in the paths of iopf reporting, handling, responding, and
>> draining.
>>
>> The fault data is protected by two locks:
>>
>> - dev->iommu->lock: This lock is used to protect the allocation and
>>     freeing of the fault data.
>> - dev->iommu->fault_parameter->lock: This lock is used to protect the
>>     fault data itself.
>>
>> Improve the iopf code to enforce this lock mechanism and add a reference
>> counter in the fault data to avoid use-after-free issue.
>>
> 
> Can you elaborate the use-after-free issue and why a new user count
> is required?

I was concerned that when iommufd uses iopf, page fault report/response
may occur simultaneously with enable/disable PRI.

Currently, this is not an issue as the enable/disable PRI is in its own
path. In the future, we may discard this interface and enable PRI when
attaching the first PRI-capable domain, and disable it when detaching
the last PRI-capable domain.

> 
> btw a Fix tag is required given this mislocking issue has been there for
> quite some time...

I don't see any real issue fixed by this change. It's only a lock
refactoring after the code refactoring and preparing it for iommufd use.
Perhaps I missed anything?

Best regards,
baolu