[+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 [off-list ref] wrote:

quoted

[+cc Daniel, Krzysztof, Jason, Christoph, linux-pci]

On Thu, May 21, 2020 at 02:06:17PM -0700, Dan Williams wrote:

quoted

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.

quoted

So I'm raising the question of whether this hole-punch is the right
strategy.

  - Prior to revoke_iomem(), __request_region() was very
    self-contained and really only depended on the resource tree.  Now
    it depends on a lot of higher-level MM machinery to shoot down
    mappings of other tasks.  This adds quite a bit of complexity and
    some new ordering constraints.

  - Punching holes in the address space of an existing process seems
    unfriendly.  Maybe the driver's __request_region() should fail
    instead, since the driver should be prepared to handle failure
    there anyway.

It's prepared to handle failure, but in this case it is dealing with a
root user of 2 minds.

quoted

  - [2] suggests that the hole punch protects drivers from /dev/mem
    writers, especially with persistent memory.  I'm not really
    convinced.  The hole punch does nothing to prevent a user process
    from mmapping and corrupting something before the driver loads.

The motivation for this was a case that was swapping between /dev/mem
access and /dev/pmem0 access and they forgot to stop using /dev/mem
when they switched to /dev/pmem0. If root wants to use /dev/mem it can
use it, if root wants to stop the driver from loading it can set
mopdrobe policy or manually unbind, and if root asks the kernel to
load the driver while it is actively using /dev/mem something has to
give. Given root has other options to stop a driver the decision to
revoke userspace access when root messes up and causes a collision
seems prudent to me.

[3] https://lore.kernel.org/linux-pci/20200716110423.xtfyb3n6tn5ixedh@pali/ (local)
[4] https://lore.kernel.org/linux-pci/CAOSf1CHss03DBSDO4PmTtMp0tCEu5kScn704ZEwLKGXQzBfqaA@mail.gmail.com/ (local)
[5] https://git.kernel.org/linus/e1d3f3268b0e