Re: [malware-list] scanner interface proposal was: [TALPA] Intro to a linux interface for on access scanning

From: tvrtko.ursulin@sophos.com
To: david@lang.hm
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@infradead.org>,
	Adrian Bunk <bunk@kernel.org>,
	capibara@xs4all.nl, Casey Schaufler <casey@schaufler-ca.com>,
	davecb@sun.com, linux-kernel <linux-kernel@vger.kernel.org>,
	linux-security-module@vger.kernel.org,
	malware-list@lists.printk.net,
	malware-list-bounces@dmesg.printk.net,
	Mihai Don??u <mdontu@bitdefender.com>,
	Peter Dolding <oiaohm@gmail.com>, Pavel Machek <pavel@suse.cz>,
	rmeijer@xs4all.nl, Theodore Tso <tytso@mit.edu>
Subject: Re: [malware-list] scanner interface proposal was: [TALPA] Intro	to a linux interface for on access scanning
Date: Tue, 19 Aug 2008 09:40:57 +0100	[thread overview]
Message-ID: <20080819084200.EA2822FE863@pmx1.sophos.com> (raw)
In-Reply-To: <alpine.DEB.1.10.0808181004380.15109@asgard.lang.hm>

david@lang.hm wrote on 18/08/2008 18:07:30:

> On Mon, 18 Aug 2008, tvrtko.ursulin@sophos.com wrote:
> 
> > Theodore Tso <tytso@mit.edu> wrote on 18/08/2008 15:25:11:
> >
> >> On Mon, Aug 18, 2008 at 02:15:24PM +0100, tvrtko.ursulin@sophos.com
> > wrote:
> >>> Then there is still a question of who allows some binary to declare
> > itself
> >>> exempt. If that decision was a mistake, or it gets compromised
> > security
> >>> will be off. A very powerful mechanism which must not be easily
> >>> accessible.  With a good cache your worries go away even without a
> > scheme
> >>> like this.
> >>
> >> I have one word for you --- bittorrent.  If you are downloading a 
very
> >> large torrent (say approximately a gigabyte), and it contains many
> >> pdf's that are say a few megabytes a piece, and things are coming in
> >> tribbles, having either a indexing scanner or an AV scanner wake up
> >> and rescan the file from scratch each time a tiny piece of the pdf
> >> comes in is going to eat your machine alive....
> >
> > Huh? I was never advocating re-scan after each modification and I even
> > explicitly said it does not make sense for AV not only for performance 
but
> > because it will be useless most of the time. I thought sending out
> > modified notification on close makes sense because it is a natural 
point,
> > unless someone is trying to subvert which is out of scope. Other have
> > suggested time delay and lumping up.
> >
> > Also, just to double-check, you don't think AV scanning would read the
> > whole file on every write?
> 
> if it doesn't read the entire file and only reads the parts that change, 

> out-of-order writes (which bittorrent does a _lot_ of) can assemble a 
> virus from pieces and the scanner will never see it.

No, it would catch it once it gets assembled. It doesn't read the parts 
that change but parts which it finds interesting.

> as for Ted's issue, the scanner(s) would get notified when the file was 
> dirtied, they would then get notified if something scanned the file and 
it 
> was marked dirty again after that. If nothing got around to scanning the 

> file then all the following writes would not send any notification 
becouse 
> the file would already be dirty.

This sound like a good strategy.

--
Tvrtko A. Ursulin
Senior Software Engineer, Sophos

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