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SubjectRe: Out of tree module using LSM
Al Viro <> wrote on 28/11/2007 18:30:40:

> On Wed, Nov 28, 2007 at 01:15:05PM -0500, wrote:
> > (Note that the concept has interesting implications in the other
> direction as
> > well - rather than stopping you from reading a file that has
> malware, you could
> > in theory write an anti-export package that would let you write
> onto external
> > memory or outbound e-mail, but prevent the write if it was
> corporate-sensitive
> > data, or whatever.
> You _can_ _not_ do that. If shared mapping gets dirtied, you have no
way to
> intercept that. At all. Especially since the page stays mapped while
it is
> written out, so the next modification can come when hardware had already
> started outbound DMA and there's no way to abort it, no matter what your
> external scanner would do.
> Folks, really, that doesn't work. At all. You can intercept all system
> calls you want and it will not be enough to prevent the "bad" contents
> from hitting the disk.

On this level we are not talking about stopping bad stuff from hitting the
disk. There is a lot simpler scenario how that can happen and that is via
a dropper at which point it can be detected and reported when the file is
closed. But more importantly further access to it can be blocked until
appropriate actions are taken which also applies with your example, no? Is
it possible to open for execute and have dirty mappings (or open for
write) on a file at the same time?

> And if we are talking about the situation when files are written to in
> controlled way (i.e. we are not concerned with malware running on the
> in question and just want to stop it from passing through mailsewer,
> then there's no damn need to play with LSM - just have e.g. coda with
> commit-on-close and run the scanner on commit. End of story. Mind you,
> in such setups one would be much better off just having the mail server
> the tests explicitly in the userland, along with the rest of anti-spam,
> filters.

That is true for that scenario.

Tvrtko August Ursulin
Senior Software Engineer, Sophos

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