Re: Race between of_iommu_configure() and iommu_probe_device()

[Robin Murphy]

On 2023-11-03 12:48 pm, Konrad Dybcio wrote:
> On 3.11.2023 12:55, Hector Martin wrote:
> > I just hit a crash in of_iommu_xlate() -> apple_dart_of_xlate() because
> > dev->iommu was NULL. of_iommu_xlate() first calls iommu_fwspec_init
> > which calls dev_iommu_get(), which allocates that member if NULL. That
> > means it got freed in between, but the only thing that can do that is
> > dev_iommu_free(), which is called from __iommu_probe_device() in the
> > error path. That is serialized via a static lock, but not against the
> > xlate stuff.
> >
> > I think the specific sequence of events was as follows:
> >
> > - IOMMU driver has not probed yet
> > - Device driver tries to probe, and gets deferred via of_iommu_xlate()
> > -> driver_deferred_probe_check_state() because there are no IOMMU ops yet
> > - IOMMU driver probes
> > - IOMMU driver registration triggers device probes
> > - IOMMU device probe fails, because there is no fwnode/OF data yet (e.g.
> > apple_dart_probe_device returns ENODEV if dev_iommu_priv_get() returns
> > NULL, and that is set in apple_dart_of_xlate())
> > - __iommu_probe_device is in the error exit path, and at this exact
> > point a parallel device probe is running of_iommu_xlate()
> > - of_iommu_xlate() calls iommu_fwspec_init(), which ensures dev->iommu
> > is non-NULL, which at this point it is
> > - immediately after that, __iommu_probe_device() calls dev_iommu_free()
> > since it is in the process of erroring out. This frees and sets
> > dev->iommu to NULL.
> > - of_iommu_xlate() calls ops->of_xlate()
> > - apple_dart_of_xlate() calls dev_iommu_priv_set(), which crashes
> > because dev->iommu is now NULL.
> >
> > As far as I can tell it's not just the specific driver xlate call
> > setting priv that's the problem here, but there is one big race between
> > the entire fwspec codepath (accessing dev->iommu->fwspec) and
> > __iommu_probe_device() (allocating and freeing dev->iommu).
> >
> > Thinking about this whole thing is making my brain hurt. Thoughts? How
> > do we fix this?
> FWIW I've been getting inexplicable boot-time crashes that sometimes
> spew out a fraction of a log line like:
>
> [x.yyyyyyyy] addr.iommu
>
> on some Qualcomm devices every now and then for quite some time..
> Not very common though. Might be this, might be something else..

Sounds likely to all be the same thing as here:

https://lore.kernel.org/linux-iommu/1698825902-10685-1-git-send-email-quic_zhenhuah@xxxxxxxxxxx/

The true solution is to pull the of_xlate step into iommu_probe_device() 
itself, which I'm working towards, and finally get rid of the horrible 
"replay" logic which causes no end of problems.

Thanks,
Robin.