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

[David Hildenbrand]

On 27.05.21 23:30, Dan Williams wrote:
> On Thu, May 27, 2021 at 1:58 PM Bjorn Helgaas <helgaas@xxxxxxxxxx> 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.
>
> > 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.
>
> >    - [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.

Is there a real use case for mapping pmem via /dev/mem or could we just 
prohibit the access to these areas completely?

What's the use case for "swapping between /dev/mem access and /dev/pmem0 
access" ?

--
Thanks,

David / dhildenb