Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

From: Dustin Kirkland <kirkland@canonical.com>
To: "Ted Ts'o" <tytso@mit.edu>, "kees.cook" <kees.cook@canonical.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Michel Lespinasse <walken@google.com>,
	Hugh Dickins <hughd@google.com>,
	Christoph Hellwig <hch@infradead.org>,
	Dave Chinner <david@fromorbit.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Nick Piggin <npiggin@kernel.dk>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Rik van Riel <riel@redhat.com>,
	Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>,
	Theodore Tso <tytso@google.com>,
	Michael Rubin <mrubin@google.com>,
	Suleiman Souhlal <suleiman@google.com>
Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback
Date: Fri, 19 Nov 2010 18:29:49 -0600	[thread overview]
Message-ID: <1290212989.12760.87.camel@x201> (raw)
In-Reply-To: <20101119232254.GA28151@thunk.org>

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On Fri, 2010-11-19 at 18:22 -0500, Ted Ts'o wrote:
> On Fri, Nov 19, 2010 at 02:54:42PM -0800, Andrew Morton wrote:
> > 
> > Dirtying all that memory at mlock() time is pretty obnoxious.
> > ...
> > So all that leaves me thinking that we merge your patches as-is.  Then
> > work out why users can fairly trivially use mlock to hang the kernel on
> > ext2 and ext3 (and others?) 
> 
> So at least on RHEL 4 and 5 systems, pam_limits was configured so that
> unprivileged processes could only mlock() at most 16k.  This was
> deemed enough so that programs could protect crypto keys.  The
> thinking when we added the mlock() ulimit setting was that
> unprivileged users could very easily make a nuisance of themselves,
> and grab way too much system resources, by using mlock() in obnoxious
> ways.
> 
> I was just checking to see if my memory was correct, and to my
> surprise, I've just found that Ubuntu deliberately sets the memlock
> ulimit to be unlimited.  Which means that Ubuntu systems are
> completely wide open for this particular DOS attack.  So if you
> administer an Ubuntu-based server, it might be a good idea to make a
> tiny little change to /etc/security/limits.conf....
> 
> 							- Ted

Kees,

Copying you into this thread, in case you'd like to respond from the
Ubuntu side.  Thanks for the heads-up, Ted.

-- 
:-Dustin

Dustin Kirkland
Canonical, LTD
kirkland@canonical.com
GPG: 1024D/83A61194

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