linux-kernel - Re: Q: selinux_bprm_committed_creds() && signals/do

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Date:	Wed, 29 Apr 2009 09:43:03 -0400
From:	Stephen Smalley <sds@...ho.nsa.gov>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...isplace.org>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Q: selinux_bprm_committed_creds() && signals/do_wait

On Wed, 2009-04-29 at 15:42 +0200, Oleg Nesterov wrote:
> On 04/29, Stephen Smalley wrote:
> >
> > On Wed, 2009-04-29 at 14:56 +0200, Oleg Nesterov wrote:
> > > On 04/29, Stephen Smalley wrote:
> > > >
> > > > On Wed, 2009-04-29 at 08:58 +0200, Oleg Nesterov wrote:
> > > > >
> > > > > Why do we need to s/IGN/DFL/ and why do we clear ->blocked ? How this can
> > > > > help from the security pov?
> > > >
> > > > We don't want the caller to be able to arrange conditions that prevent
> > > > correct handling of signals (e.g. SIGHUP) by the callee.  That was
> > > > motivated by a specific attack against newrole, but was a general issue
> > > > for any program that runs in a more trusted domain than its caller.
> > >
> > > Still can't understand...
> > >
> > > If the new image runs in a more trusted domain, then we should not change
> > > SIG_IGN to SIG_DFL ?
> > >
> > > For example, a user does "nohup setuid_app". Now, why should we change
> > > SIG_IGN to SIG_DFL for SIGHUP? This makes setuid_app more "vulnerable"
> > > to SIGHUP, not more "protected". Confused.
> >
> > Not if the app was depending on the default handler for SIGHUP to
> > correctly handle vhangup().  The point is that we don't necessarily
> > trust the caller to define the handling behavior for signals in the
> > callee.  If we trust the caller to do so, then we can grant the
> > corresponding permission.
> >
> > newrole scenario was to run it nohup, logout, wait for other user to
> > login on same tty, trigger termination of newrole'd child shell, and
> > have newrole relabel other user's tty to attacker's sid.
> >
> > > OK. Since I don't understand the security magic, you can just ignore me.
> > > But I will appreciate any explanation for dummies ;)
> > >
> > > > As I recall, I based the logic in part on existing logic in
> > > > call_usermodehelper().
> > >
> > > ____call_usermodehelper() does this because we should not exec a user-space
> > > application with SIGKILL/SIGSTOP ignored/blocked. We don't have this problem
> > > when user-space execs.
> >
> > But we still have the problem of the caller setting up the signal
> > handlers or blocked signal mask prior to exec'ing the privileged
> > program, right?
> 
> The callee can never setup the signal handler. Note that flush_old_exec()
> does flush_signal_handlers() too. But it uses force_default == F.

Right, but it can set it to SIG_IGN, which was the problem in the
situation above.

> OK, please forget this. I trust you even if can't understand ;)
> 
> My real concerns were SIGKILL and do_wait(), they were addressed.
> 
> Thanks!
> 
> Oleg.
-- 
Stephen Smalley
National Security Agency

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