linux-kernel - Re: [PATCH 3/6] ksm: change the KSM_REMOVE_MEMORY

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Date:	Wed, 6 May 2009 18:54:22 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Chris Wright <chrisw@...hat.com>
cc:	Izik Eidus <ieidus@...hat.com>, Rik van Riel <riel@...hat.com>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	aarcange@...hat.com, alan@...rguk.ukuu.org.uk, device@...ana.org,
	linux-mm@...ck.org, nickpiggin@...oo.com.au
Subject: Re: [PATCH 3/6] ksm: change the KSM_REMOVE_MEMORY_REGION ioctl.

On Wed, 6 May 2009, Chris Wright wrote:
> * Hugh Dickins (hugh@...itas.com) wrote:
> > 
> > Is the phrase "covert channel" going to come up somehow?
> 
> There's two (still hand wavy) conerns I see there.  First is the security
> implication: timing writes to see cow and guess the shared data for
> another apps VM_LOCKED region,

Mmm, yes, there's fun to be had there; though I don't see it as having
anything to do with VM_LOCKED, beyond that the paranoid have reason to
place their most anxious data in VM_LOCKED areas.

I'm thinking of an app which prepares pages full of scurrilous rumour,
then waits around looking at its /proc/self/smaps to see if anyone else
is writing stories like that!

> second is just plain old complaints of
> those rt latency sensitive apps that somehow have VM_LOCKED|VM_MERGE
> and complain of COW fault time, probably just "don't do that".

Right.  There are sensitive sites which ought not to configure such
merging on; but I don't think we should disallow merging locked.

Hugh
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