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Date:   Thu, 21 May 2020 20:01:53 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     gregkh@...uxfoundation.org, Arnd Bergmann <arnd@...db.de>,
        Ingo Molnar <mingo@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Russell King <linux@....linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v4] /dev/mem: Revoke mappings when a driver claims the
 region

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@...omium.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

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