Re: Time to remove LSM (was Re: [RESEND][RFC][PATCH 2/7] implementation of LSM hooks)

[Stephen Smalley <sds@tycho.nsa.gov>]

On Tue, 2006-04-18 at 07:22 -0500, Serge E. Hallyn wrote:
> Quoting Valdis.Kletnieks@vt.edu (Valdis.Kletnieks@vt.edu):
> > On Mon, 17 Apr 2006 22:26:24 BST, Alan Cox said:
> > 
> > (Two replies to this paragraph, addressing 2 separate issues....)
> > 
> > > You can implement a BSD securelevel model in SELinux as far as I can see
> > > from looking at it, and do it better than the code today, so its not
> > > really a feature drop anyway just a migration away from some fossils
> > 
> > If we heave the LSM stuff overboard, there's one thing that *will* need
> > addressing - what to do with kernel support of Posix-y capabilities.  Currently
> > some of the heavy lifting is done by security/commoncap.c.
> > 
> > Frankly, that's *another* thing that we need to either *fix* so it works right,
> > or rip out of the kernel entirely.  As far as I know, there's no in-tree way
> > to make /usr/bin/ping be set-CAP_NET_RAW and have it DTRT.

As far as the capability module is concerned, ping can be setuid and can
drop all other capabilities at startup, so you can approximate the
above.  As far as SELinux is concerned, you can bound the capabilities
that ping can exercise based on its security context even if ping runs
as uid 0 and is granted full capabilities by the capability module,
regardless of whether ping ever drops capabilities itself.  So there are
in-tree ways to achieve what you want.

> Sigh...  it's such a cool idea, and yet such a dangerously easy thing to
> get wrong, ie dropping the ability for a root process to drop it's root
> privs.
> 
> If we were to drop posix caps, how would selinux change correspondingly?
> Would it just drop the capability class altogether, perhaps beef up the
> task or security class?  Just wondering whether anyone had thought about
> this.

I doubt you'd drop capability altogether.  You could incrementally
enable the direct granting of capabilities based on SELinux security
context by defining a new class in its policy (cap_override) that
mirrors the existing capability class, and modifying SELinux to
authoritatively grant the capability if it is allowed in that class for
the process' security context; otherwise, you fall back to the existing
combined behavior of requiring both SELinux+capability (or SELinux
+dummy) to grant the capability.  That is simple enough from a code
perspective.  You just need to be careful about the implications for
userspace and policy configuration, to avoid introducing security holes
in this manner.

> Alternatively, we could try yet again to get support for fs caps
> upstream...

Given the extensible nature of the security xattr namespace, it is
already possible to store the capability bits in the filesystem (just by
beginning to use security.effcap, security.inhcap, ...).  The code
modifications to the capability module should be simple.  But modifying
userspace to set and preserve those attributes would take some work,
work which has already been done for the SELinux attributes, and you'd
have to figure out the right set of capability bits for each program,
which doesn't scale very well (vs. using equivalence classes as in
SELinux types).  And the capability evolution logic has always been
problematic, vs. explicit transition definitions as in SELinux.  So fs
caps doesn't seem very promising to me as a path forward.

-- 
Stephen Smalley
National Security Agency

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