linux-kernel - Re: r-o bind in nfsd

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Date:	Wed, 26 Mar 2008 08:04:02 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	NeilBrown <neilb@...e.de>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	miklos@...redi.hu, viro@...iv.linux.org.uk, haveblue@...ibm.com,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, hch@...radead.org,
	linux-security-module@...r.kernel.org, jmorris@...ei.org
Subject: Re: r-o bind in nfsd


On Wed, 2008-03-26 at 09:32 +1100, NeilBrown wrote:
> On Tue, March 25, 2008 10:45 pm, Tetsuo Handa wrote:
> > Hello.
> >
> >> Maybe some enhancement to the 'intent' structure with a similar
> >> effect could be done instead.
> >>
> >> Then you could, presumably, put a security hook somewhere in
> >> link_path_walk for those modules (like AppArmor) which want to do
> >> checks based on the namespace.
> >
> > I think link_path_walk() is not a good place to insert new LSM hooks
> > for pathname based access control (AppArmor and TOMOYO) purpose because
> >
> >   (1) The kernel don't know what operation (open/create/truncate etc.)
> > will be
> >       done at the moment of link_path_walk().
> 
> Though the 'indent' data structure could be used to carry this information.
> 
> >
> >   (2) Not all operations call link_path_walk() before these operations
> >       are done. For example, ftruncate() doesn't call link_path_walk().
> 
> But do you want to impose path-name based controls to ftruncate?
> Surely once you have a file open for write (not O_APPEND), then no
> other permission is required to truncate the file, is it?
> If it is, then maybe the 'struct file' should be tagged at open time
> to say whether 'truncate' is allowed.
> 
> >
> >   (3) The rename() and link() operations handle two pathnames.
> >       But, it is not possible to know both pathnames at the moment of
> >       link_path_walk().
> 
> Not an insolvable problem.
> One could imagine an implementation where a TYPE_RENAME_FROM security
> check produced a cookie that was consumed by a TYPE_RENAME_TO security
> check.  The cookie could then be used by the security module to
> make any connection between the two names that might be appropriate.
> 
> >
> > I think we need to introduce new LSM hooks outside link_path_walk().
> > http://kerneltrap.org/mailarchive/linux-fsdevel/2008/2/17/882024
> >
> <rant>
> I suspect we would be much better off removing all the security hooks.
> Security done at that level seems to be way too complex such that most
> people don't really understand it.  And people who don't understand
> security don't use it.
> We'd be much better off getting rid of the whole "micro-manage security"
> concept and provide isolation via some sort of high level container
> approach.
> </rant>

Containers can be useful, but they aren't a substitute for access
control, and they don't solve the same problem.

And SELinux does get used, and recent stats on Fedora 8 suggest that few
people disable it anymore.  Advances in the SELinux tools (loadable
modules, semanage, system-config-selinux, setroubleshoot, etc) have gone
a long way to enabling users to solve problems they encounter.

-- 
Stephen Smalley
National Security Agency

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