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

[Kees Cook <keescook@chromium.org>]

*thread necromancy*

Hi Dan,

I'm doing a KSPP bug scrub and am reviewing
https://github.com/KSPP/linux/issues/74 again.

Do you have a chance to look at this? I'd love a way to make mmap()
behave the same way as read() for the first meg of /dev/mem.

-Kees

On Thu, May 21, 2020 at 08:01:53PM -0700, Kees Cook wrote:
> On Thu, May 21, 2020 at 02:06:17PM -0700, Dan Williams wrote:
> > 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.
> 
> Nice!
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> 
> And a thread hijack...   ;)
> 
> I think this is very close to providing a way to solve another issue
> I've had with /dev/mem, which is to zero the view of the first 1MB of
> /dev/mem via mmap. I only fixed the read/write accesses:
> a4866aa81251 ("mm: Tighten x86 /dev/mem with zeroing reads")
> I.e. the low 1MB range should be considered allowed, but any reads will see
> zeros.
> 
> > +	unmap_mapping_range(inode->i_mapping, res->start, resource_size(res), 1);
> 
> Is unmap_mapping_range() sufficient for this? Would it need to happen
> once during open_port() or something more special during mmap_mem()?
> 
> -- 
> Kees Cook

-- 
Kees Cook