linux-kernel - Re: [RFC] transcendent memory for Linux

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Date:	Tue, 30 Jun 2009 15:46:48 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com, npiggin@...e.de,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, Avi Kivity <avi@...hat.com>,
	Rik van Riel <riel@...hat.com>, alan@...rguk.ukuu.org.uk,
	Rusty Russell <rusty@...tcorp.com.au>,
	Martin Schwidefsky <schwidefsky@...ibm.com>, akpm@...l.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	tmem-devel@....oracle.com, sunil.mushran@...cle.com,
	linux-mm@...ck.org, Himanshu Raj <rhim@...rosoft.com>,
	Keir Fraser <keir.fraser@...citrix.com>
Subject: Re: [RFC] transcendent memory for Linux

On 06/30/09 14:21, Dan Magenheimer wrote:
> No, the uuid can't be verified.  Tmem gives no indication
> as to whether a newly-created pool is already in use (shared)
> by another guest.  So without both the 128-bit uuid and an
> already-in-use 64-bit object id and 32-bit page index, no data
> is readable or writable by the attacker.
>   

You have to consider things like timing attacks as well (for example, a
tmem hypercall might return faster if the uuid already exists).

Besides, you can tell whether a uuid exists, by at least a couple of
mechanisms (from a quick read of the source, so I might have overlooked
something):

   1. You can create new shared pools until it starts failing as a
      result of hitting the MAX_GLOBAL_SHARED_POOLS limit with junk
      uuids.  If you then successfully "create" a shared pool while
      searching, you know it already existed.
   2. The returned pool id will increase unless the pool already exists,
      in which case you'll get a smaller id back (ignoring wraparound).


> Hmmm... that is definitely a thornier problem.  I guess the
> security angle definitely deserves more design.  But, again,
> this affects only shared precache which is not intended
> to part of the proposed initial tmem patchset, so this is a futures
> issue.)

Yeah, a shared namespace of accessible objects is an entirely new thing
in the Xen universe.  I would also drop Xen support until there's a good
security story about how they can be used.

    J

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