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

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Message-ID: <20250610164306.GJ543171@nvidia.com>
Date: Tue, 10 Jun 2025 13:43:06 -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 05:31:09PM +0100, Robin Murphy wrote:
> On 2025-06-10 4:36 pm, Jason Gunthorpe wrote:
> > 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.
> 
> Then you do iommu_group_get/put() around it as well. 

Same issue - can't use dev->iommu_group.kobj.kref to protect against
UAF. By the time you do a try_get you've already UAF'd the memory
holding the kref. It always needs some other enclosing protection.

> From a quick skim I suspect it's probably OK - at least device_del() gets to
> bus_remove_device()->device_remove_groups() well enough before the
> BUS_NOTIFY_REMOVED_DEVICE call that triggers iommu_release_device().

Make sense, Nicolin, a well placed comment explaining this would be
good
 
> And on an unrelated thought that's just come to mind, if we ever did FLR
> with PASIDs enabled, presumably that's going to wipe out the PASID
> configuration,

I've always been expecting the PCI FLR code to preserve the config
space that belongs the iommu subsystem. PASID, ATS, PRI, etc. Never
looked into it... Nicolin??

Otherwise we need a post-FLR callback to have the iommu driver reload
the right config values for its current config.. That's an existing
nasty bug :)

> so will the caller who requested the reset actually expect
> the attachments at the IOMMU end to be preserved, or would they assume to
> start over from scratch? Seems like there's not necessarily one right answer
> there :/

IMHO we have to preserve everything, and I think we should things back
to working normally overall, if that isn't happening already.

For something like VFIO preserving is desired. For DMA-API that is the
right thing too.

Something like mlx5, that has a robust RAS system, will unregister
itself from the rdma subsystem and that triggers a natural destruction
of the SVA/etc domains that might be there. We want the attachments to
be left undisturbed so there is no issue cleaning them up later.

Jason