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Subject: Re: [PATCH RFC] mm: add MAP_EXCLUSIVE to create exclusive user
 mappings
To:     Mike Rapoport <rppt@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        James Bottomley <jejb@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, linux-api@vger.kernel.org,
        linux-mm@kvack.org, x86@kernel.org,
        Mike Rapoport <rppt@linux.ibm.com>
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 <085ed07e-e646-f7a4-0370-06f33a2a4e4a@redhat.com>
 <20191030081529.GB20624@rapoport-lnx>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <9eae3941-64cf-4ea1-0287-0e64bab192c6@redhat.com>
Date:   Wed, 30 Oct 2019 09:19:33 +0100
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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@linux.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?
>=20
> Right.
>=20
>> Also, I wonder
>> what would happen if you map such pages via /dev/mem into another user s=
pace
>> application and e.g., use them along with kvm [1].
>=20
> Do you mean that one application creates MAP_EXCLUSIVE and another
> applications accesses the same physical pages via /dev/mem?

Exactly.

>=20
> With /dev/mem all physical memory is visible...

Okay, so the statement "information that will not be visible not only to=20
other processes but to the kernel as well" is not correct. There are=20
easy ways to access that information if you really want to (might=20
require root permissions, though).

--=20

Thanks,

David / dhildenb