linux-kernel - Re: [PATCH v4 2/9] iommu: Replace sva_iommu with iommu_attach

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Message-ID: <20240429202458.GR231144@ziepe.ca>
Date: Mon, 29 Apr 2024 17:24:58 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Kevin Tian <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
	Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Nicolin Chen <nicolinc@...dia.com>, Yi Liu <yi.l.liu@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Joel Granados <j.granados@...sung.com>, iommu@...ts.linux.dev,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/9] iommu: Replace sva_iommu with iommu_attach_handle

On Sun, Apr 28, 2024 at 06:22:28PM +0800, Baolu Lu wrote:

>         /* A bond already exists, just take a reference`. */
>         handle = iommu_attach_handle_get(group, iommu_mm->pasid);
>         if (handle) {
>                 if (handle->domain->iopf_handler != iommu_sva_iopf_handler)
> {
>                         ret = -EBUSY;
>                         goto out_unlock;
>                 }
> 
>                 refcount_inc(&handle->users);
>                 mutex_unlock(&iommu_sva_lock);
>                 return handle;
>         }
> 
> But it appears that this code is not lock safe. If the domain on the
> PASID is not a SVA domain, the check of "handle->domain->iopf_handler !=
> iommu_sva_iopf_handler" could result in a use-after-free issue as the
> other thread might detach the domain in between the fetch and check
> lines.

For the above you just need to pass in the iommu_sva_iopf_handler as
an argument to attach_handle_get() and have it check it under the
xa_lock.

The whole thing is already protected under the ugly sva_lock.

Ideally it would be protected by the group mutex..

Jason