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    SubjectRe: [malware-list] TALPA - a threat model? well sorta.
    On Thu, 14 Aug 2008, wrote:

    > Eric Paris wrote on 13/08/2008 19:57:44:
    >>> It's clear from the protection model that you described that on 'read'
    >>> you want to wait until the scan is done before you give the data to
    > the
    >>> process asking for it... and that's totally reasonable: "Do not give
    >>> out bad data" is a very clear line in terms of security.
    >>> for the "dirty" case it gets muddy. You clearly want to scan "some
    >>> time" after the write, from the principle of getting rid of malware
    >>> that's on the disk, but it's unclear if this HAS to be synchronous.
    >>> (obviously, synchronous behavior hurts performance bigtime so lets do
    >>> as little as we can of that without hurting the protection).
    >>> One advantage of doing the dirty case async (and a little time
    > delayed)
    >>> is that repeated writes will get lumped up into one scan in practice,
    >>> saving a ton of performance.
    >>> (scan-on-close is just another way of implementing "delay the dirty
    >>> scan").
    >>> Based on Alans comments, to me this sounds like we should have an
    >>> efficient mechanism to notify userspace of "dirty events"; this is not
    >>> virus scan specific in any way or form. And this mechanism likely will
    >>> need to allow multiple subscribers.
    >> I'm certainly willing to go down the inotify'ish path for async
    >> notification of 'dirty' inodes instead of implement my own async
    >> mechanism if I can find a way to do it.
    > Do I understand correctly that everyone agrees scanning whenever an inode
    > gets dirty would be a terrible thing for performance?
    > Another thing we have here is that malware could not be neccessariliy
    > identified until the very last write (one example where it will always be
    > the case are PDF files (I think)).
    > So the whole question is at which point should be performing an async
    > scan. Close seems like a natural point which should be ideal for majority
    > of applications, I don't see how any time-based lumping/delaying scheme
    > can be better than close?

    all you need is the ability to mark a file as 'dirty', and some way for
    programs that are interested in dirty files learning about it later and
    decideing to do a scan. if the file gets dirtied again after they do they
    scan they will need to do another one (this is a classic trade-off between
    the 'security' of looking for things quickly or 'efficiancy' of only
    looking when you don't think they will change again. in other words,
    policy -> userspace)

    David Lang

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