linux-kernel - Re: [PATCH RFC v1 1/2] iommu: Introduce iommu_dev_reset_prepare() and iommu_dev_reset

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Message-ID: <20250610153646.GH543171@nvidia.com>
Date: Tue, 10 Jun 2025 12:36:46 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Robin Murphy <robin.murphy@....com>
Cc: Nicolin Chen <nicolinc@...dia.com>, Baolu Lu <baolu.lu@...ux.intel.com>,
	joro@...tes.org, will@...nel.org, bhelgaas@...gle.com,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org, patches@...ts.linux.dev,
	pjaroszynski@...dia.com, vsethi@...dia.com
Subject: Re: [PATCH RFC v1 1/2] iommu: Introduce iommu_dev_reset_prepare()
 and iommu_dev_reset_done()

On Tue, Jun 10, 2025 at 03:40:40PM +0100, Robin Murphy wrote:
> On 2025-06-10 2:04 pm, Jason Gunthorpe wrote:
> > On Tue, Jun 10, 2025 at 12:07:00AM -0700, Nicolin Chen wrote:
> > > On Tue, Jun 10, 2025 at 12:26:07PM +0800, Baolu Lu wrote:
> > > > On 6/10/25 02:45, Nicolin Chen wrote:
> > > > > +	ops = dev_iommu_ops(dev);
> > > > 
> > > > Should this be protected by group->mutext?
> > > 
> > > Not seemingly, but should require the iommu_probe_device_lock I
> > > think.
> > 
> > group and ops are not permitted to change while a driver is attached..
> > 
> > IIRC the FLR code in PCI doesn't always ensure that (due to the sysfs
> > paths), so yeah, this looks troubled. iommu_probe_device_lock perhaps
> > would fix it.
> 
> No, iommu_probe_device_lock is for protecting access to dev->iommu in the
> probe path until the device is definitively assigned to a group (or not).
> Fundamentally it defends against multiple sources triggering a probe of the
> same device in parallel - once the device *is* probed it is no longer
> relevant, and the group mutex is the right thing to protect all subsequent
> operations.

Yes, adding iommu_probe_device_lock to iommu_deinit_device() would be
gross.

but something is required to protect the load of
dev->iommu_group.. dev->iommu_group->mutex can't protect itself
against a race UAF on deinit.

READ_ONCE is good enough to protect from races with the probe path, no
need for iommu_probe_device_lock there.

In this case need to look at the PCI sysfs for races against the
iommu_release_device()/etc that is freeing the dev->iommu_group.

Maybe the sysfs is always removed before we get to release. Or maybe
the PCI FLR sysfs code should hold the device_lock..

Jason