linux-kernel - Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

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Message-ID: <CAA9_cmdCmiu5wVs=Jo4c6VNw6N1raPTDXg4d9OuUj3w0+e5JHQ@mail.gmail.com>
Date:   Thu, 31 Oct 2019 14:52:46 -0700
From:   Dan Williams <dan.j.williams@...il.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user mappings

On Thu, Oct 31, 2019 at 12:17 PM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Wed, Oct 30, 2019 at 09:19:33AM +0100, David Hildenbrand wrote:
> > On 30.10.19 09:15, Mike Rapoport wrote:
> > >On Tue, Oct 29, 2019 at 12:02:34PM +0100, David Hildenbrand wrote:
> > >>On 27.10.19 11:17, Mike Rapoport wrote:
> > >>>From: Mike Rapoport <rppt@...ux.ibm.com>
> > >>>
> > >>>The mappings created with MAP_EXCLUSIVE are visible only in the context of
> > >>>the owning process and can be used by applications to store secret
> > >>>information that will not be visible not only to other processes but to the
> > >>>kernel as well.
> > >>>
> > >>>The pages in these mappings are removed from the kernel direct map and
> > >>>marked with PG_user_exclusive flag. When the exclusive area is unmapped,
> > >>>the pages are mapped back into the direct map.
> > >>>
> > >>
> > >>Just a thought, the kernel is still able to indirectly read the contents of
> > >>these pages by doing a kdump from kexec environment, right?
> > >
> > >Right.
> > >
> > >>Also, I wonder
> > >>what would happen if you map such pages via /dev/mem into another user space
> > >>application and e.g., use them along with kvm [1].
> > >
> > >Do you mean that one application creates MAP_EXCLUSIVE and another
> > >applications accesses the same physical pages via /dev/mem?
> >
> > Exactly.
> >
> > >
> > >With /dev/mem all physical memory is visible...
> >
> > Okay, so the statement "information that will not be visible not only to
> > other processes but to the kernel as well" is not correct. There are easy
> > ways to access that information if you really want to (might require root
> > permissions, though).
>
> Right, but /dev/mem is an easy way to extract any information in any
> environment if one has root permissions...
>

I don't understand this concern with /dev/mem. Just add these pages to
the growing list of the things /dev/mem is not allowed to touch.