Re: [PATCH v5 11/15] PCI: Obey iomem restrictions for procfs mmap

[Dan Williams <dan.j.williams@intel.com>]

On Wed, Nov 4, 2020 at 8:50 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Wed, Nov 04, 2020 at 09:44:04AM +0100, Daniel Vetter wrote:
> > On Tue, Nov 3, 2020 at 11:09 PM Dan Williams <dan.j.williams@intel.com> wrote:
> > > On Tue, Nov 3, 2020 at 1:28 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > > On Fri, Oct 30, 2020 at 11:08:11AM +0100, Daniel Vetter wrote:
> > > > > There's three ways to access PCI BARs from userspace: /dev/mem, sysfs
> > > > > files, and the old proc interface. Two check against
> > > > > iomem_is_exclusive, proc never did. And with CONFIG_IO_STRICT_DEVMEM,
> > > > > this starts to matter, since we don't want random userspace having
> > > > > access to PCI BARs while a driver is loaded and using it.
> > > > >
> > > > > Fix this by adding the same iomem_is_exclusive() check we already have
> > > > > on the sysfs side in pci_mmap_resource().
> > > > >
> > > > > References: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > > > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > > >
> > > > This is OK with me but it looks like IORESOURCE_EXCLUSIVE is currently
> > > > only used in a few places:
> > > >
> > > >   e1000_probe() calls pci_request_selected_regions_exclusive(),
> > > >   ne_pci_probe() calls pci_request_regions_exclusive(),
> > > >   vmbus_allocate_mmio() calls request_mem_region_exclusive()
> > > >
> > > > which raises the question of whether it's worth keeping
> > > > IORESOURCE_EXCLUSIVE at all.  I'm totally fine with removing it
> > > > completely.
> > >
> > > Now that CONFIG_IO_STRICT_DEVMEM upgrades IORESOURCE_BUSY to
> > > IORESOURCE_EXCLUSIVE semantics the latter has lost its meaning so I'd
> > > be in favor of removing it as well.
> >
> > Still has some value since it enforces exclusive access even if the
> > config isn't enabled, and iirc e1000 had some fun with userspace tools
> > clobbering the firmware and bricking the chip.
>
> There's *some* value; I'm just skeptical since only three drivers use
> it.
>
> IORESOURCE_EXCLUSIVE is from e8de1481fd71 ("resource: allow MMIO
> exclusivity for device drivers"), and the commit message says this is
> only active when CONFIG_STRICT_DEVMEM is set.  I didn't check to see
> whether that's still true.
>
> That commit adds a bunch of wrappers and "__"-prefixed functions to
> pass the IORESOURCE_EXCLUSIVE flag around.  That's a fair bit of
> uglification for three drivers.
>
> > Another thing I kinda wondered, since pci maintainer is here: At least
> > in drivers/gpu I see very few drivers explicitly requestion regions
> > (this might be a historical artifact due to the shadow attach stuff
> > before we had real modesetting drivers). And pci core doesn't do that
> > either, even when a driver is bound. Is this intentional, or
> > should/could we do better? Since drivers work happily without
> > reserving regions I don't think "the drivers need to remember to do
> > this" will ever really work out well.
>
> You're right, many drivers don't call pci_request_regions().  Maybe we
> could do better, but I haven't looked into that recently.  There is a
> related note in Documentation/PCI/pci.rst that's been there for a long
> time (it refers to "pci_request_resources()", which has never existed
> AFAICT).  I'm certainly open to proposals.

It seems a bug that the kernel permits MMIO regions with side effects
to be ioremap()'ed without request_mem_region() on the resource. I
wonder how much log spam would happen if ioremap() reported whenever a
non-IORESOURE_BUSY range was passed to it? The current state of
affairs to trust *remap users to have claimed their remap target seems
too ingrained to unwind now.