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

[david]

On Fri, 15 Aug 2008, Johannes Weiner wrote:

> Hi,
>
> david@xxxxxxx writes:
>
> > On Thu, 14 Aug 2008, Arjan van de Ven wrote:
> >
> > > On Thu, 14 Aug 2008 22:04:00 -0400
> > > Theodore Tso <tytso@xxxxxxx> wrote:
> > >
> > > > On Thu, Aug 14, 2008 at 06:44:33PM -0700, david@xxxxxxx 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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