Re: [PATCH v2] /dev/mem: Revoke mappings when a driver claims the region

[Greg KH]

On Tue, May 19, 2020 at 12:03:06AM -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.
> 
> The typical usage of unmap_mapping_range() is part of
> truncate_pagecache() to punch a hole in a file, but in this case the
> implementation is only doing the "first half" of a hole punch. Namely it
> is just evacuating current established mappings of the "hole", and it
> relies on the fact that /dev/mem establishes mappings in terms of
> absolute physical address offsets. Once existing mmap users are
> invalidated they can attempt to re-establish the mapping, or attempt to
> continue issuing read(2) / write(2) to the invalidated extent, but they
> will then be subject to the CONFIG_IO_STRICT_DEVMEM checking that can
> block those subsequent accesses.
> 
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Fixes: 90a545e98126 ("restrict /dev/mem to idle io memory ranges")
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
> Changes since v1 [1]:
> 
> - updated the changelog to describe the usage of unmap_mapping_range().
>   No other logic changes:
> 
> [1]: http://lore.kernel.org/r/158662721802.1893045.12301414116114602646.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> 
> Greg, Andrew,
> 
> I have a regression test for this case now. This was found by an
> intermittent data corruption scenario on pmem from a test tool using
> /dev/mem.

Ick, why are test tools messing around in /dev/mem :)

Anyway, this seems sane to me, want me to take it through my tree?

thanks,

greg k-h