Re: [PATCH v4] /dev/mem: Revoke mappings when a driver claims the region

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

On Thu, Jun 3, 2021 at 11:12 AM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Wed, Jun 02, 2021 at 09:15:35PM -0700, Dan Williams wrote:
> > On Wed, Jun 2, 2021 at 8:40 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > >
> > > [+cc Pali, Oliver]
> > >
> > > On Thu, May 27, 2021 at 02:30:31PM -0700, Dan Williams wrote:
> > > > On Thu, May 27, 2021 at 1:58 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > > > >
> > > > > [+cc Daniel, Krzysztof, Jason, Christoph, linux-pci]
> > > > >
> > > > > On Thu, May 21, 2020 at 02:06:17PM -0700, Dan Williams wrote:
> > > > > > Close the hole of holding a mapping over kernel driver takeover event of
> > > > > > a given address range.
> > > > > >
> > > > > > Commit 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> > > > > > introduced CONFIG_IO_STRICT_DEVMEM with the goal of protecting the
> > > > > > kernel against scenarios where a /dev/mem user tramples memory that a
> > > > > > kernel driver owns. However, this protection only prevents *new* read(),
> > > > > > write() and mmap() requests. Established mappings prior to the driver
> > > > > > calling request_mem_region() are left alone.
> > > > > >
> > > > > > Especially with persistent memory, and the core kernel metadata that is
> > > > > > stored there, there are plentiful scenarios for a /dev/mem user to
> > > > > > violate the expectations of the driver and cause amplified damage.
> > > > > >
> > > > > > Teach request_mem_region() to find and shoot down active /dev/mem
> > > > > > mappings that it believes it has successfully claimed for the exclusive
> > > > > > use of the driver. Effectively a driver call to request_mem_region()
> > > > > > becomes a hole-punch on the /dev/mem device.
> > > > >
> > > > > This idea of hole-punching /dev/mem has since been extended to PCI
> > > > > BARs via [1].
> > > > >
> > > > > Correct me if I'm wrong: I think this means that if a user process has
> > > > > mmapped a PCI BAR via sysfs, and a kernel driver subsequently requests
> > > > > that region via pci_request_region() or similar, we punch holes in the
> > > > > the user process mmap.  The driver might be happy, but my guess is the
> > > > > user starts seeing segmentation violations for no obvious reason and
> > > > > is not happy.
> > > > >
> > > > > Apart from the user process issue, the implementation of [1] is
> > > > > problematic for PCI because the mmappable sysfs attributes now depend
> > > > > on iomem_init_inode(), an fs_initcall, which means they can't be
> > > > > static attributes, which ultimately leads to races in creating them.
> > > >
> > > > See the comments in iomem_get_mapping(), and revoke_iomem():
> > > >
> > > >         /*
> > > >          * Check that the initialization has completed. Losing the race
> > > >          * is ok because it means drivers are claiming resources before
> > > >          * the fs_initcall level of init and prevent iomem_get_mapping users
> > > >          * from establishing mappings.
> > > >          */
> > > >
> > > > ...the observation being that it is ok for the revocation inode to
> > > > come on later in the boot process because userspace won't be able to
> > > > use the fs yet. So any missed calls to revoke_iomem() would fall back
> > > > to userspace just seeing the resource busy in the first instance. I.e.
> > > > through the normal devmem_is_allowed() exclusion.
> > >
> > > I did see that comment, but the race I meant is different.  Pali wrote
> > > up a nice analysis of it [3].
> > >
> > > Here's the typical enumeration flow for PCI:
> > >
> > >   acpi_pci_root_add                 <-- subsys_initcall (4)
> > >     pci_acpi_scan_root
> > >       ...
> > >         pci_device_add
> > >           device_initialize
> > >           device_add
> > >             device_add_attrs        <-- static sysfs attributes created
> > >     ...
> > >     pci_bus_add_devices
> > >       pci_bus_add_device
> > >         pci_create_sysfs_dev_files
> > >           if (!sysfs_initialized) return;    <-- Ugh :)
> > >           ...
> > >             attr->mmap = pci_mmap_resource_uc
> > >             attr->mapping = iomem_get_mapping()  <-- new dependency
> > >               return iomem_inode->i_mapping
> > >             sysfs_create_bin_file   <-- dynamic sysfs attributes created
> > >
> > >   iomem_init_inode                  <-- fs_initcall (5)
> > >     iomem_inode = ...               <-- now iomem_get_mapping() works
> > >
> > >   pci_sysfs_init                    <-- late_initcall (7)
> > >     sysfs_initialized = 1           <-- Ugh (see above)
> > >     for_each_pci_dev(dev)           <-- Ugh
> > >       pci_create_sysfs_dev_files(dev)
> > >
> > > The race is between the pci_sysfs_init() initcall (intended for
> > > boot-time devices) and the pci_bus_add_device() path (used for all
> > > devices including hot-added ones).  Pali outlined cases where we call
> > > pci_create_sysfs_dev_files() from both paths for the same device.
> > >
> > > "sysfs_initialized" is a gross hack that prevents this most of the
> > > time, but not always.  I want to get rid of it and pci_sysfs_init().
> > >
> > > Oliver had the excellent idea of using static sysfs attributes to do
> > > this cleanly [4].  If we can convert things to static attributes, the
> > > device core creates them in device_add(), so we don't have to create
> > > them in pci_create_sysfs_dev_files().
> > >
> > > Krzysztof recently did some very nice work to convert most things to
> > > static attributes, e.g., [5].  But we can't do this for the PCI BAR
> > > attributes because they support ->mmap(), which now depends on
> > > iomem_get_mapping(), which IIUC doesn't work until after fs_initcalls.
> >
> > Ah, sorry, yes, I see the race now. And yes, anything that gets in the
> > way of the static attribute conversion needs fixing. How about
> > something like this?
>
> That looks like it would solve our problem, thanks a lot!  Obvious in
> retrospect, like all good ideas :)
>
> Krzysztof noticed a couple other users of iomem_get_mapping()
> added by:
>
>   71a1d8ed900f ("resource: Move devmem revoke code to resource framework")
>   636b21b50152 ("PCI: Revoke mappings like devmem")
>
> I *could* extend your patch below to cover all these, but it's kind of
> outside my comfort zone, so I'd feel better if Daniel V (who wrote the
> commits above) could take a look and do a follow-up.
>
> If I could take the resulting patch via PCI, we might even be able to
> get the last static attribute conversions in this cycle.

Sounds good, I'll circle back and give it a try if Daniel does not get
a chance to chime in in the next few days.