Re: xterm -C and TIOCCONS vs. PRIV_TTY_CONSOLE

On Fri, Jan 07, 2011 at 11:53:06AM +0100, Gary Jennejohn wrote:
> On Thu, 6 Jan 2011 21:09:05 -0800
> Garrett Cooper <gcooper_at_FreeBSD.org> wrote:
> 
> > On Thu, Jan 6, 2011 at 8:49 PM, Craig Leres <leres_at_ee.lbl.gov> wrote:
> > > On 01/06/11 20:05, Garrett Cooper wrote:
> > >> Just to make sure we're both on the same page:
> > >>
> > >> $ grep xterm /etc/ttys
> > >> ttyv0 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv1 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv2 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv3 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv4 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv5 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv6 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv7 "/usr/libexec/getty Pc" ? ? ? ? xterm ? on ?secure
> > >> ttyv8 "/usr/local/bin/xdm -nodaemon" ?xterm ? off secure
> > >
> > > No, that's not what mine looks like. I changed it to match and rebooted
> > > but it doesn't help with the TIOCCONS issue.
> > >
> > > When I run xinit, it starts up the xterm -C which does a TIOCCONS. The
> > > 8.1 kernel checks for PRIV_TTY_CONSOLE which isn't set and denies the
> > > request:
> > >
> > > ? ? ? ?case TIOCCONS:
> > > ? ? ? ? ? ? ? ?/* Set terminal as console TTY. */
> > > ? ? ? ? ? ? ? ?if (*(int *)data) {
> > > ? ? ? ? ? ? ? ? ? ? ? ?error = priv_check(td, PRIV_TTY_CONSOLE);
> > > ? ? ? ? ? ? ? ? ? ? ? ?if (error)
> > > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?return (error);
> > >
> > > ? ? ? ? ? ? ? ? ? ? ? ?/*
> > > ? ? ? ? ? ? ? ? ? ? ? ? * XXX: constty should really need to be locked!
> > > ? ? ? ? ? ? ? ? ? ? ? ? * XXX: allow disconnected constty's to be stolen!
> > > ? ? ? ? ? ? ? ? ? ? ? ? */
> > >
> > > ? ? ? ? ? ? ? ? ? ? ? ?if (constty == tp)
> > > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?return (0);
> > > ? ? ? ? ? ? ? ? ? ? ? ?if (constty != NULL)
> > > ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?return (EBUSY);
> > >
> > > ? ? ? ? ? ? ? ? ? ? ? ?tty_unlock(tp);
> > > ? ? ? ? ? ? ? ? ? ? ? ?constty_set(tp);
> > > ? ? ? ? ? ? ? ? ? ? ? ?tty_lock(tp);
> > > ? ? ? ? ? ? ? ?} else if (constty == tp) {
> > > ? ? ? ? ? ? ? ? ? ? ? ?constty_clear();
> > > ? ? ? ? ? ? ? ?}
> > > ? ? ? ? ? ? ? ?return (0);
> > >
> > >
> > > There's nothing I see in all of /usr/src that turns on PRIV_TTY_CONSOLE
> > > in any case. You could rewrite the above like this:
> > >
> > > ? ? ? ?case TIOCCONS:
> > > ? ? ? ? ? ? ? ?/* Set terminal as console TTY. */
> > > ? ? ? ? ? ? ? ?if (*(int *)data) {
> > > ? ? ? ? ? ? ? ? ? ? ? ?return (EPERM)
> > > ? ? ? ? ? ? ? ?} else if (constty == tp) {
> > > ? ? ? ? ? ? ? ? ? ? ? ?constty_clear();
> > > ? ? ? ? ? ? ? ?}
> > > ? ? ? ? ? ? ? ?return (0);
> > >
> > > and it won't change any behavior.
> > 
> >     Ok -- figured I would ask about the obvious. I wish I could help
> > you further right now, but unfortunately I have a lot on my plate.
> > I've CCed ed_at_ and the list again so that someone else might be able to
> > chime in and help you further.
> >
> 
> I'd say there are a few factors which come into play here:
> 1) the setting of security.bsd.suser_enabled, default 1
> 2) kern_tty.c checking for a cred which is never set
> 3) whether xterm is setuid root
> 
> a) suser_enabled is almost guaranteed to be 1 on OP's system since just
>    about nothing works when it is set to 0 (tried that)
> b) the kernel checking for the cred PRIV_TTY_CONSOLE is probably a bug
>    since it never gets set anywhere.  However, this usually isn't noticed
>    because
> c) xterm is generally setuid root and the logic in priv_check_cred() in
>    kern_priv.c doesn't even look at what cred is set to, except for a few
>    which can raise some limits, because cr_uid is 0 (super user)
> 
> So, the crux of the matter is whether OP's xterm is setuid root.  My
> xterm is and I can run 'xterm -C' without a problem.
> 

It seems I'm seeing this one on 8.2R. Of course, xterm is setuid
root. I hacked tty.c to return success for TIOCCONS so was able to
see kernel messages but messages passed through syslog still does
not work. :-(