Re: concept of a permissive domain

[Eric Paris <eparis@redhat.com>]

On Thu, 2007-09-13 at 15:38 -0400, Stephen Smalley wrote:
> On Thu, 2007-09-13 at 15:25 -0400, Daniel J Walsh wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> > 
> > Stephen Smalley wrote:
> > > On Thu, 2007-09-13 at 10:46 -0400, Karl MacMillan wrote:
> > >> On Thu, 2007-09-13 at 10:08 -0400, Stephen Smalley wrote:
> > >>> On Tue, 2007-09-11 at 17:26 -0400, Karl MacMillan wrote:
> > >>>> On Tue, 2007-09-11 at 16:31 -0400, Daniel J Walsh wrote:
> > >>>> [...]
> > >>>>> One other feature/requirement would be to not override dontaudit rules.
> > >>>>> So if I have a domain in permissive mode and I have a dontaudit rule  on
> > >>>>> reading /etc/shadow.  The app should still be denied reading
> > >>>>> /etc/shadow.  (This is not a show stopper, but would allow us to force
> > >>>>> apps to take the code paths they will take in enforcing mode.)
> > >>>> This isn't specific to per-domain permissive, right? It would be useful
> > >>>> in general for permissive.
> > >>> I would be opposed to such a change, as it is a semantic change to what
> > >>> dontaudit means.
> > >>>
> > >>> Keep in mind that allow, auditallow, and dontaudit/auditdeny are all
> > >>> independent of one another today and none of them imply the other, e.g.
> > >>> 	allow a b:file read;
> > >>> 	auditallow a b:file *;
> > >>> 	dontaudit a b:file *;
> > >>> is perfectly valid and means:
> > >>> - Let a read files labeled b,
> > >>> - Audit all permission grantings from a to files labeled b,
> > >>> - Don't audit any permission denials from a to files labeled b.
> > >>>
> > >> I agree that changing the dontaudit semantic has problems - however the
> > >> reason Dan suggested is still valid. Currently, generating policy in
> > >> permissive mode can lead to bogus or overly permissive policy. It would
> > >> be nice to have some solution to that problem.
> > > 
> > > Can't you handle that in the tool, by giving matching interfaces with
> > > dontaudit rules precedence over ones with allow rules and asking the
> > > user?
> > > 
> > > I'd actually rather see an improved capability for (more easily)
> > > generating policy incrementally in enforcing mode.  That would make it
> > > more suitable for production use and avoid the problem above.
> > > 
> > Said large financial institution rols out policy for managing huge money
> > transactions in permissive mode.   Policy build with a pam for verifying
> > users.
> 
> Note that I said it would be better to provide a capability to let
> people develop policy incrementally while in enforcing mode, not
> permissive mode.

A fine goal, but not really feasible ATM.  It also doesn't allow for
going live and testing in production at the same time.  This is still a
'must have perfect coverage before usage' model.  Just because you
develop you way means we don't have to worry about the permissive-denial
issue but it doesn't solve the 'you are fired' issue.  Great in theory,
but when talking about some ginormous app that noone fully understands I
wouldn't want to be the one who sticks his neck out.

> 
> > Users selected app_uses_pam in policy design tool.
> > 
> > Tool adds
> > 
> > dontaudit financeapp_t shadow_t:file r_file_perms;
> > 
> > App in permissive mode reads shadow perms.  Dontaudit covers it up.
> 
> So don't do that.  Don't include new dontaudit rules while generating
> policy in permissive mode, and let the user decide whether to select the
> dontaudit branch or the allow branch in the final form of the generated
> policy.  For that matter, we should really minimize use of dontaudit
> rules whenever possible - if we can fix the code to _default_ to the
> less privileged code path and only try the more privileged code path if
> it truly needs it, then we should do that.

You rule out the possibility of running in production without enforcing.
Obviously your customers would never do that, but a customer going from
no MAC to some non-enforcing MAC to enforcing MAC, would like to
actually test things along the way.  Its a different customer base.  And
we don't have to harm one to help the other.

Fixing the code is always the better way to go than dontaudit,
thankfully they exist because that isn't always possible.  Good thing we
were thinking about this issue before.

> > App runs for three months.  policy writer sees no avc messages for a
> > long time,  Thinks everything is fine.
> > 
> > Turns on enforcing mode, app tries to authenticate on mysql, gets
> > denials apps blow up, millions lost, people say selinux sucks, policy
> > writer is fired.
> > 
> > If I want the current behavior to see full permissive mode, I can
> > semodule -DB or build my policy without dontaudit.
> > 
> > permissive mode not following code path of dontaudit would causes major
> > problems.
> 
> Let me say it again - dontaudit rules don't affect whether or not
> something is allowed in SELinux today; 

correct

> they are NOT deny rules.  If you
> want deny rules, add those (gasp).  Permissive mode is simply following
> the code path of allowing everything, as requested.

it's not following ANY code path now.  enforcing=0 is following the path
you describe.

Start looking at this as a whole new type of domain, not just some
extension of enforcing=X (although I do plan to tie them into each other
at some level in the code since they provide similar features).

We need denial rules, we just don't have to call them that.  We just
need to define how dontaudit and auditallow rules work in this new type
of domain.  Then decide if that somehow interferes if the domain is not
a permissive domain.  Aside from Karl's little flub, noone is arguing we
should change anything having to do with enforcing=0 or enforcing=1 and
a non-permissive-domain.  We don't need to rewrite how dontaudit rules
affect enforcing domains or non-enforcing-systems, we just need an
extended definition for permissive domains.  Seems dontaudit and
auditallow can work very nicely to solve real practical problems
constraining large applications in environments which may not be friends
to problems introduced by SELinux.  Everyone agrees running an
application and just allowing whatever it asks for isn't the best way to
write policy, but when it's all you have, and all you are going to
reasonably have I'm not hearing a argument why we can't do what Dan
wants, other than 'that's not what it means today.'

dontaudit as an 'implicit denial in permissive domains' isn't what we
have today, but then again, we don't have permissive domains today.

-Eric


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