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

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Message-ID: <20191031191651.GA26165@rapoport-lnx>
Date:   Thu, 31 Oct 2019 20:16:52 +0100
From:   Mike Rapoport <rppt@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     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@...r.kernel.org,
        linux-mm@...ck.org, x86@...nel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings

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...
 
> -- 
> 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.