Re: [PATCH RESEND] intel/pinctrl: check capability offset is between MMIO region

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Mar 24, 2021 at 06:57:12PM +0200, Andy Shevchenko wrote:
> On Wed, Mar 24, 2021 at 04:13:59PM +0100, Roger Pau Monné wrote:
> > On Wed, Mar 24, 2021 at 04:22:44PM +0200, Andy Shevchenko wrote:
> > > On Wed, Mar 24, 2021 at 02:55:15PM +0100, Roger Pau Monné wrote:
> > > > On Wed, Mar 24, 2021 at 02:58:07PM +0200, Andy Shevchenko wrote:
> > > > > On Wed, Mar 24, 2021 at 01:31:18PM +0100, Roger Pau Monne wrote:
> > > > > Moreover, it seems you are bailing out and basically denying driver to load.
> > > > > This does look that capability is simply the first register that blows the setup.
> > > > > I think you have to fix something into Xen to avoid loading these drivers or
> > > > > check with something like pci_device_is_present() approach.
> > > > 
> > > > Is there a backing PCI device BAR for those MMIO regions that the
> > > > pinctrl driver is trying to access? AFAICT those regions are only
> > > > reported in the ACPI DSDT table on the _CRS method of the object (at
> > > > least on my system).
> > > 
> > > Unfortunately it does not expose PCI configuration space.
> > 
> > Are those regions supposed to be marked as reserved in the memory map,
> > or that's left to the discretion of the hardware vendor?
> 
> I didn't get. The OS doesn't see them and an internal backbone simply drops any
> IO access to that region.

I'm not sure I understand the above reply. My question was whether the
MMIO regions used by the pinctrl device (as fetched from the ACPI DSDT
table) are supposed belong to regions marked as RESERVED in the
firmware memory map (ie: either the e820 or the EFI one).

> > > > Doing something like pci_device_is_present would require a register
> > > > that we know will never return ~0 unless the device is not present. As
> > > > said above, maybe we could use REVID to that end?
> > > 
> > > Yes, that's good, see above.
> > > 
> > > WRT capabilities, if we crash we will see the report immediately on the
> > > hardware which has such an issue. (It's quite unlikely we will ever have one,
> > > that's why I consider it's not critical)
> > 
> > I would rather prefer to not crash, because I think the kernel should
> > only resort to crashing when there's no alternative, and here it's
> > perfectly fine to just print an error message and don't load the
> > driver.
> 
> Are we speaking about real hardware that has an issue? I eagerly want to know
> what is that beast.

OK, I'm not going to resend this anymore. I'm happy with just getting
the first patch in.

I think you trust the hardware more that I would do, and I also think
the check added here is very minimal an unintrusive and serves as a
way to sanitize the data fetched from the hardware in order to prevent
a kernel page fault if such data turns out to be wrong.

Taking a reactive approach of requiring a broken piece of hardware to
exist in order to sanitize a fetched value seems too risky. I could
add a WARN_ON or similar if you want some kind of splat that's very
noticeable when this goes wrong but that doesn't end up in a fatal
kernel page fault.

Thanks, Roger.