Subject: Re: [PATCH v3 3/4] lib: logic_pio: Reject accesses to unregistered CPU MMIO regions

From: Bjorn Helgaas <[email-masked]>
Date: Thu, 4 Apr 2019 13:58:15 -0500

On Thu, Apr 04, 2019 at 10:43:36AM -0700, Guenter Roeck wrote:
> On Thu, Apr 04, 2019 at 05:52:35PM +0100, John Garry wrote:
> > >>Note that the f71805f driver does not call
> > >>request_{muxed_}region(), as it should.
> > 
> > >... which is the real problem, one that is not solved by this
> > >patch. This may result in parallel and descructive accesses if
> > >there is another device on the LPC bus, and another driver
> > >accessing that device. Personally I'd rather have
> > >request_muxed_region() added to the f71805f driver.
> > 
> > Right, we should and will still fix f71805f. If you recall, I did
> > have the f71805f fix in the v1 series, but you committed that it
> > was orthogonal, so I decided to take it out of this work for now.
> > 
> > And even if we fix up f71805f and other known drivers which don't
> > call request_muxed_region(), we still need to police against these
> > rogue accesses, which is what this patch attempts to do.
> > 
> Do we ? I am personally not convinced that LPC accesses _have_ to
> occur through PCI on any given system.

On current systems, I suspect ISA/LPC devices are typically connected
via a PCI-to-ISA/LPC bridge.  But AFAIK there's no actual requirement
for that bridge, and there certainly *were* systems with ISA devices
but no PCI at all.

IMO, if you want to build ISA drivers on your arch, you need to make
sure the inb() probing done by those drivers works like it does on
x86.  If there's no device there, the inb() should return 0xff with no
fuss and no crash.

Bjorn

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