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Re: need rules help



On Sun, 2009-08-09 at 09:37 -0400, Steve Grubb wrote:
> On Saturday 08 August 2009 01:59:53 pm LC Bruzenak wrote:
> > > The issue is that SE Linux AVCs travel a different path. When an AVC
> > > denial occurs and there is not a dontaudit associated with it, it sends
> > > the event straight to the netlink queue. To suppress an AVC, you would
> > > need to make a change to SE Linux policy. The SE Linux folks wanted to
> > > make sure there was no way to suppress an AVC without explicitly stating
> > > so in policy.
> >
> > Bummer. But thanks for the explanation; that makes sense...sort of.
> > Does the "exclude" rule then work for msgtype=AVC (as the manpage says)?
> > If so, seems like a broad stroke is allowed whereas detailed exclusion
> > isn't.
> 
> Did some more digging on this and found I missed a line of code.
> 
> http://lxr.linux.no/linux+v2.6.30.4/kernel/audit.c#L1167
> 
> When audit_log_start is called to create an AVC, it calls audit_filter_type() 
> which is the exclude filter.
> 
> http://lxr.linux.no/linux+v2.6.30.4/kernel/auditfilter.c#L1743
> 
> At line 1757, you can see that it only cares about the event type field. It 
> does not check any other fields that you might have in the rule such as 
> subjects. Originally there was some discussion about not allowing the audit 
> system to suppress AVC's since correcting policy is really the best way to go.

It may be the best way to go in theory, but in practice, IIUC, the
policy rules are not granular enough to specify what the audit rules
potentially can. 
Also I still think my aggregation-filter ideas have merit eventually and
policy won't help this. 

> 
> So, I think yes you can suppress AVC's. But its all AVC's and not any 
> particular one. It seems like it would be trivial to add some more checking to 
> the type filter to better tune what is being thrown away.

This would be a huge help to me; I think others would find it useful
also. I don't see it as a lesser security stance; there is precedence in
legacy systems in the field where this behavior is SOP (although not as
elegant as the linux audit rules).

Ideally the complete scontext/tcontext fields (incl. level) in AVC lines
are would be what should be filterable.

Thanks for the time/effort to look into this Steve; I really appreciate
it!
LCB.

-- 
LC (Lenny) Bruzenak
lenny magitekltd com


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