linux-kernel - Re: [RFC][PATCH 1/2] fs proc: make pagemap a privileged interface

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Date:	Mon, 09 Mar 2015 23:49:39 -0500
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>, Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [RFC][PATCH 1/2] fs proc: make pagemap a privileged interface

Dave Hansen <dave.hansen@...el.com> writes:

> On 03/09/2015 05:03 PM, Kees Cook wrote:
>> On Mon, Mar 9, 2015 at 4:43 PM, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>> A 1 to 1 blinding function like integer multiplication mudulo 2^32 by an
>>> appropriate random number ought to keep from revealing page numbers or
>>> page ajacencies while not requiring any changes in userspace.
>>>
>>> That way the revealed pfn and the physcial pfn would be different but
>>> you could still use pagemap for it's intended purpose.
>> 
>> If this could be done in a way where it was sufficiently hard to
>> expose the random number, we should absolutely do this.
>
> We would need something which is both reversible (so that the given
> offsets can still be used in /proc/kpagemap) and also hard to do a
> known-plaintext-type attack on it.
>
> Transparent huge pages are a place where userspace knows the
> relationship between 512 adjacent physical addresses.  That represents a
> good chunk of known data.  Surely there are more of these kinds of things.
>
> Right now, for instance, the ways in which a series of sequential
> allocations come out of the page allocator are fairly deterministic.  We
> would also need to do some kind of allocator randomization to ensure
> that userspace couldn't make good guesses about the physical addresses
> of things coming out of the allocator.
>
> Or, we just be sure and turn the darn thing off. :)

Yes.  If we are worried about something a big off switch is fine.

As for a one-to-one transform that is resitant to plain text attacks
I think that is the definition of a cypher.  That is we should just use
AES or something well know to encrypt the pafe frame numbers if we want
to hide them.  I don't know if the block mode of AES would be a problem
or not.

Eric


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