linux-kernel - Re: [malware-list] TALPA - a threat model? well sorta.

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Date:	Thu, 14 Aug 2008 22:36:12 -0700 (PDT)
From:	david@...g.hm
To:	Arjan van de Ven <arjan@...radead.org>
cc:	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 Thu, 14 Aug 2008, david@...g.hm wrote:

> 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)
>
>
>
> In the first case the attributes work "don't bother scanning me".
>
> In the second case they also work (becouse you aren't trying to scan 
> everything)
>
>
> the only time there is a headache is in the transition between them when you 
> have scanned a lot of the system, but not all of it, and the machine was 
> rebooted so you lost track of what you had scanned.
>
> it shouldn't be too hard to deal with this. even if you never resume the scan 
> you are still safe (becouse of the scan-on-open), but it's also possible to 
> either do a find f(or equivalent) or files without the attribute and store 
> the results (similar to updatedb) and then updating the file to mark the 
> files as being scanned (update in place, change the first character or 
> something to be fairly crash safe). after the full list of files has been 
> scanned shift to the second mode.
>
> the sweep scan should be a background task, possibly disabled when on battery 
> power.
>
> why would this not satisfy the requirements?
>
> David Lang

one way this approach would be a pain is that as signature files got 
updated the attributes would accumulate.

a couple ways of dealing with this.

1. scanners clean up after themselves (when they go to add a new one the 
remove the old one)

2. background sweep through the system removing all tags that aren't in 
the current 'blessed' set

David Lang
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