On Tue, Jan 19, 2021 at 4:20 PM Greg Kroah-Hartman
[off-list ref] wrote:

On Tue, Jan 19, 2021 at 03:34:47PM +0100, Daniel Vetter wrote:

quoted

On Tue, Jan 19, 2021 at 3:32 PM Greg Kroah-Hartman
[off-list ref] wrote:

quoted

On Tue, Jan 19, 2021 at 09:17:55AM +0100, Daniel Vetter wrote:

quoted

On Fri, Nov 27, 2020 at 5:42 PM Daniel Vetter [off-list ref] wrote:

quoted

Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims
the region") /dev/kmem zaps ptes when the kernel requests exclusive
acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is
the default for all driver uses.

Except there's two more ways to access PCI BARs: sysfs and proc mmap
support. Let's plug that hole.

For revoke_devmem() to work we need to link our vma into the same
address_space, with consistent vma->vm_pgoff. ->pgoff is already
adjusted, because that's how (io_)remap_pfn_range works, but for the
mapping we need to adjust vma->vm_file->f_mapping. The cleanest way is
to adjust this at at ->open time:

- for sysfs this is easy, now that binary attributes support this. We
  just set bin_attr->mapping when mmap is supported
- for procfs it's a bit more tricky, since procfs pci access has only
  one file per device, and access to a specific resources first needs
  to be set up with some ioctl calls. But mmap is only supported for
  the same resources as sysfs exposes with mmap support, and otherwise
  rejected, so we can set the mapping unconditionally at open time
  without harm.

A special consideration is for arch_can_pci_mmap_io() - we need to
make sure that the ->f_mapping doesn't alias between ioport and iomem
space. There's only 2 ways in-tree to support mmap of ioports: generic
pci mmap (ARCH_GENERIC_PCI_MMAP_RESOURCE), and sparc as the single
architecture hand-rolling. Both approach support ioport mmap through a
special pfn range and not through magic pte attributes. Aliasing is
therefore not a problem.

The only difference in access checks left is that sysfs PCI mmap does
not check for CAP_RAWIO. I'm not really sure whether that should be
added or not.

Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Dan Williams <redacted>
Signed-off-by: Daniel Vetter <redacted>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kees Cook <redacted>
Cc: Dan Williams <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jérôme Glisse <redacted>
Cc: Jan Kara <jack@suse.cz>
Cc: Dan Williams <redacted>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-mm@kvack.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-samsung-soc@vger.kernel.org
Cc: linux-media@vger.kernel.org
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Signed-off-by: Daniel Vetter <redacted>
--
v2:
- Totally new approach: Adjust filp->f_mapping at open time. Note that
  this now works on all architectures, not just those support
  ARCH_GENERIC_PCI_MMAP_RESOURCE
---
 drivers/pci/pci-sysfs.c | 4 ++++
 drivers/pci/proc.c      | 1 +
 2 files changed, 5 insertions(+)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index d15c881e2e7e..3f1c31bc0b7c 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c

@@ -929,6 +929,7 @@ void pci_create_legacy_files(struct pci_bus *b)
        b->legacy_io->read = pci_read_legacy_io;
        b->legacy_io->write = pci_write_legacy_io;
        b->legacy_io->mmap = pci_mmap_legacy_io;
+       b->legacy_io->mapping = iomem_get_mapping();
        pci_adjust_legacy_attr(b, pci_mmap_io);
        error = device_create_bin_file(&b->dev, b->legacy_io);
        if (error)

@@ -941,6 +942,7 @@ void pci_create_legacy_files(struct pci_bus *b)
        b->legacy_mem->size = 1024*1024;
        b->legacy_mem->attr.mode = 0600;
        b->legacy_mem->mmap = pci_mmap_legacy_mem;
+       b->legacy_io->mapping = iomem_get_mapping();

Unlike the normal pci stuff below, the legacy files here go boom
because they're set up much earlier in the boot sequence. This only
affects HAVE_PCI_LEGACY architectures, which aren't that many. So what
should we do here now:
- drop the devmem revoke for these
- rework the init sequence somehow to set up these files a lot later
- redo the sysfs patch so that it doesn't take an address_space
pointer, but instead a callback to get at that (since at open time
everything is set up). Imo rather ugly
- ditch this part of the series (since there's not really any takers
for the latter parts it might just not make sense to push for this)
- something else?

Bjorn, Greg, thoughts?

What sysfs patch are you referring to here?

Currently in linux-next:

commit 74b30195395c406c787280a77ae55aed82dbbfc7 (HEAD ->
topic/iomem-mmap-vs-gup, drm/topic/iomem-mmap-vs-gup)
Author: Daniel Vetter [off-list ref]
Date:   Fri Nov 27 17:41:25 2020 +0100

   sysfs: Support zapping of binary attr mmaps

Or the patch right before this one in this submission here:

https://lore.kernel.org/dri-devel/20201127164131.2244124-12-daniel.vetter@ffwll.ch/ (local)

Ah.  Hm, a callback in the sysfs file logic seems really hairy, so I
would prefer that not happen.  If no one really needs this stuff, why
not just drop it like you mention?

Well it is needed, but just on architectures I don't care about much.
Most relevant is perhaps powerpc (that's where Stephen hit the issue).
I do wonder whether we could move the legacy pci files setup to where
the modern stuff is set up from pci_create_resource_files() or maybe
pci_create_sysfs_dev_files() even for HAVE_PCI_LEGACY. I think that
might work, but since it's legacy flow on some funny architectures
(alpha, itanium, that kind of stuff) I have no idea what kind of
monsters I'm going to anger :-)
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

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