linux-kernel - Re: [PATCH 0/3] Taming execve, setuid, and LSMs

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Date:	Wed, 21 Apr 2010 08:34:26 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Andrew Lutomirski <luto@....edu>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	"Serge E. Hallyn" <serue@...ibm.com>, linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Eric Biederman <ebiederm@...ssion.com>,
	"Andrew G. Morgan" <morgan@...nel.org>
Subject: Re: [PATCH 0/3] Taming execve, setuid, and LSMs

On Tue, 2010-04-20 at 11:53 -0400, Andrew Lutomirski wrote:
> On Tue, Apr 20, 2010 at 11:34 AM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
> >>
> >> True,  but I think it's still asking for trouble -- other LSMs could
> >> (and almost certainly will, especially the out-of-tree ones) do
> >> something, and I think that any action at all that an LSM takes in the
> >> bprm_set_creds hook for a nosuid (or whatever it's called) process is
> >> wrong or at best misguided.
> >>
> >> Can you think of anything that an LSM should do (or even should be
> >> able to do) when a nosuid process calls exec, other than denying the
> >> request outright?  With my patch, LSMs can still reject the open_exec
> >> call.
> >
> > In the case where the context transition would shed permissions rather
> > than gain permissions, it has been suggested that we shouldn't disable
> > the transition even in the presence of nosuid.  But automatically
> > computing that for a domain transition is non-trivial, so we have the
> > present behavior for SELinux.
> >
> > There also can be state updates even in the non-suid exec case, e.g.
> > saved uids, clearing capabilities, etc.
> 
> Ah, right.
> 
> In my patch, execve_nosecurity is (or will be, anyway) documented to
> skip all of this, and it's a new syscall, so nothing should need to be
> done.  It doesn't allow anything that a userland ELF loader couldn't
> already do.  (I'm not thrilled with changing the behavior of the
> original execve syscall, but one way or another, any nosuid mechanism
> will probably allow programs to exec other things without losing
> permissions that the admin might have expected.  I don't see this is a
> real problem, though.)

The further you deviate from existing execve semantics, the less likely
your solution will work cleanly as a transparent replacement for execve
for userland running in this nosuid state, and the less compelling the
case for implementing execve_nosecurity in the kernel vs. just userspace
emulation of it.

> Is it even possible to purely drop permissions in SELinux?  If your
> original type was orig_t and your new type is new_t, and if the rights
> granted to orig_t and new_t overlap nontrivially, then what are you
> supposed to do?  Check both types for each hook?  (Some annoying admin
> could even *change* the rights for orig_t or new_t after execve
> finishes.)

It has always been possible to configure policy such that one type is
less privileged than its caller, and the typebounds construct introduced
in more recent SELinux provides a kernel-enforced mechanism for ensuring
that one type is strictly bounded by the permissions of another type.

-- 
Stephen Smalley
National Security Agency

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