Re: [PATCH] iommu: Fallback to default setting when def_domain_type() callback returns 0

[Kai-Heng Feng <kai.heng.feng@canonical.com>]

On Tue, Jul 6, 2021 at 5:27 PM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 2021-07-06 07:51, Kai-Heng Feng wrote:
> > Commit 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted
> > device into core") not only moved the check for untrusted device to
> > IOMMU core, it also introduced a behavioral change by returning
> > def_domain_type() directly without checking its return value. That makes
> > many devices no longer use the default IOMMU setting.
> >
> > So revert back to the old behavior which defaults to
> > iommu_def_domain_type when driver callback returns 0.
> >
> > Fixes: 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted device into core")
>
> Are you sure about that? From that same commit:
>
> @@ -1507,7 +1509,7 @@ static int iommu_alloc_default_domain(struct
> iommu_group *group,
>          if (group->default_domain)
>                  return 0;
>
> -       type = iommu_get_def_domain_type(dev);
> +       type = iommu_get_def_domain_type(dev) ? : iommu_def_domain_type;
>
>          return iommu_group_alloc_default_domain(dev->bus, group, type);
>   }
>
> AFAICS the other two callers should also handle 0 correctly. Have you
> seen a problem in practice?

Thanks for pointing out how the return value is being handled by the callers.
However, the same check is missing in probe_get_default_domain_type():
static int probe_get_default_domain_type(struct device *dev, void *data)
{
        struct __group_domain_type *gtype = data;
        unsigned int type = iommu_get_def_domain_type(dev);
...
}

I personally prefer the old way instead of open coding with ternary
operator, so I'll do that in v2.

In practice, this causes a kernel panic when probing Realtek WiFi.
Because of the bug, dma_ops isn't set by probe_finalize(),
dma_map_single() falls back to swiotlb which isn't set and caused a
kernel panic.
I didn't attach the panic log because the system simply is frozen at
that point so the message is not logged to the storage.
I'll see if I can find another way to collect the log and attach it in v2.

Kai-Heng

>
> Robin.
>
> > Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
> > ---
> >   drivers/iommu/iommu.c | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 5419c4b9f27a..faac4f795025 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -1507,14 +1507,15 @@ EXPORT_SYMBOL_GPL(fsl_mc_device_group);
> >   static int iommu_get_def_domain_type(struct device *dev)
> >   {
> >       const struct iommu_ops *ops = dev->bus->iommu_ops;
> > +     unsigned int type = 0;
> >
> >       if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
> >               return IOMMU_DOMAIN_DMA;
> >
> >       if (ops->def_domain_type)
> > -             return ops->def_domain_type(dev);
> > +             type = ops->def_domain_type(dev);
> >
> > -     return 0;
> > +     return (type == 0) ? iommu_def_domain_type : type;
> >   }
> >
> >   static int iommu_group_alloc_default_domain(struct bus_type *bus,
> >