Date:	Thu, 14 Aug 2008 22:28:25 -0700 (PDT)
From:	david@...g.hm
To:	Johannes Weiner <hannes@...urebad.de>
cc:	Arjan van de Ven <arjan@...radead.org>,
	Theodore Tso <tytso@....edu>,
	Christoph Hellwig <hch@...radead.org>,
	Eric Paris <eparis@...hat.com>, tvrtko.ursulin@...hos.com,
	alan@...rguk.ukuu.org.uk, andi@...stfloor.org,
	linux-kernel@...r.kernel.org, malware-list@...ts.printk.net,
	malware-list-bounces@...sg.printk.net, peterz@...radead.org,
	viro@...IV.linux.org.uk
Subject: Re: [malware-list] TALPA - a threat model?  well sorta.

On Fri, 15 Aug 2008, Johannes Weiner wrote:

> Hi,
>
> david@...g.hm writes:
>
>> On Thu, 14 Aug 2008, Arjan van de Ven wrote:
>>
>>> On Thu, 14 Aug 2008 22:04:00 -0400
>>> Theodore Tso <tytso@....edu> wrote:
>>>
>>>> On Thu, Aug 14, 2008 at 06:44:33PM -0700, david@...g.hm wrote:
>>>>> could you do something like defining a namespace inside posix
>>>>> attributes and then setting up a mechanism in the kernel to alert
>>>>> if the attributes change (with the entire namespace getting cleared
>>>>> if the file gets dirtied)?
>>>>
>>>> According to Eric Paris the clean/dirty state is only stored in
>>>> memory.  We could use the extended attribute interface as a way of not
>>>> defining a new system call, or some other interface, but I'm not sure
>>>> it's such a great match given that the extended attributes interface
>>>> are designed for persistent data.
>>>>
>>>> I agree that doesn't actually work very well for the tracker use case,
>>>> where you the clean/dirty bit to be persistent (in case the tracker is
>>>> disabled due to the fact you are running on battery, for example, and
>>>> then you reboot).
>>>>
>>>
>>> but we need a "give me all dirty files" solution, not a "is this file
>>> dirty" solution.
>>>
>>> I do not want a virus scanner to constantly have to poll the whole fs
>>> for dirty files ;-)
>>
>> I'm not sure.
>>
>> there are two situations (with the transition between them)
>>
>> 1. unscanned system, we want to do everything. (this happens
>> immediatly after a new signature file is deployed)
>>
>> here you do just want to filter out the files that have been scanned
>> from the list of everything, and you probably want to check at the
>> time of scanning the file in case it was opened (and scanned) in the
>> meantime.
>>
>> 2. mostly scanned system, we only want to scan files that have been
>> dirtied.
>>
>> here you don't need to scan everything, you only need to scan in two cases
>>
>> 2a. the file was dirtied (you learn about it and add it to the queue
>> of files to scan when you get around to it)
>>
>> 2b. an unscanned file is opened (the library detects that the file was
>> not marked approved by all the current scanners, so it invokes the
>> scanners on this file before completing the open, or copy for mmap, or
>> whatever)
>
> 2b could also be used as a general lazy scanning mechanism, no?  If
> there is a new signature or the file is dirty or unknown, scan it.
> Always.
>
> So this should all just be one case, no?

this depends on the policy in userspace.

you could:

1. plan on doing all scanning as files are opened (this is going to be 
noticable to users)

2. run the background scanner continuously as a low priority (as 
soon as you finish one scan, do the next)

3. do one background scan when you get a new signature, and after you 
complete that only scan files that change.

and probably others

#3 has the advantage that the only time a user will hit a delay when 
opening a file is if the file was just recently modified and the scanner 
daemon(s) haven't had a chance to scan it yet.

David Lang
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