[kernel-hardening] Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN

["Serge E. Hallyn" <serge@hallyn.com>]

On Fri, Apr 21, 2017 at 01:09:59AM -0400, Matt Brown wrote:
> On 04/20/2017 01:41 PM, Serge E. Hallyn wrote:
> >Quoting matt@nmatt.com (matt@nmatt.com):
> >>On 2017-04-20 11:19, Serge E. Hallyn wrote:
> >>>Quoting Matt Brown (matt@nmatt.com):
> >>>>On 04/19/2017 07:53 PM, Serge E. Hallyn wrote:
> >>>>>Quoting Matt Brown (matt@nmatt.com):
> >>>>>>On 04/19/2017 12:58 AM, Serge E. Hallyn wrote:
> >>>>>>>On Tue, Apr 18, 2017 at 11:45:26PM -0400, Matt Brown wrote:
> >>>>>>>>This patch reproduces GRKERNSEC_HARDEN_TTY functionality from the grsecurity
> >>>>>>>>project in-kernel.
> >>>>>>>>
> >>>>>>>>This will create the Kconfig SECURITY_TIOCSTI_RESTRICT and the corresponding
> >>>>>>>>sysctl kernel.tiocsti_restrict that, when activated, restrict all TIOCSTI
> >>>>>>>>ioctl calls from non CAP_SYS_ADMIN users.
> >>>>>>>>
> >>>>>>>>Possible effects on userland:
> >>>>>>>>
> >>>>>>>>There could be a few user programs that would be effected by this
> >>>>>>>>change.
> >>>>>>>>See: <https://codesearch.debian.net/search?q=ioctl%5C%28.*TIOCSTI>
> >>>>>>>>notable programs are: agetty, csh, xemacs and tcsh
> >>>>>>>>
> >>>>>>>>However, I still believe that this change is worth it given that the
> >>>>>>>>Kconfig defaults to n. This will be a feature that is turned on for the
> >>>>>>>
> >>>>>>>It's not worthless, but note that for instance before this was fixed
> >>>>>>>in lxc, this patch would not have helped with escapes from privileged
> >>>>>>>containers.
> >>>>>>>
> >>>>>>
> >>>>>>I assume you are talking about this CVE:
> >>>>>>https://bugzilla.redhat.com/show_bug.cgi?id=1411256
> >>>>>>
> >>>>>>In retrospect, is there any way that an escape from a privileged
> >>>>>>container with the this bug could have been prevented?
> >>>>>
> >>>>>I don't know, that's what I was probing for.  Detecting that the pgrp
> >>>>>or session - heck, the pid namespace - has changed would seem like a
> >>>>>good indicator that it shouldn't be able to push.
> >>>>>
> >>>>
> >>>>pgrp and session won't do because in the case we are discussing
> >>>>current->signal->tty is the same as tty.
> >>>>
> >>>>This is the current check that is already in place:
> >>>>| if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN))
> >>>>| 	return -EPERM;
> >>>
> >>>Yeah...
> >>>
> >>>>The only thing I could find to detect the tty message coming from a
> >>>>container is as follows:
> >>>>| task_active_pid_ns(current)->level
> >>>>
> >>>>This will be zero when run on the host, but 1 when run inside a
> >>>>container. However this is very much a hack and could probably break
> >>>>some userland stuff where there are multiple levels of namespaces.
> >>>
> >>>Yes.  This is also however why I don't like the current patch, because
> >>>capable() will never be true in a container, so nested containers
> >>>break.
> >>>
> >>
> >>What do you mean by "capable() will never be true in a container"?
> >>My understanding
> >>is that if a container is given CAP_SYS_ADMIN then
> >>capable(CAP_SYS_ADMIN) will return
> >>true?
> >
> >No, capable(X) checks for X with respect to the initial user namespace.
> >So for root-owned containers it will be true, but containers running in
> >non-initial user namespaces cannot pass that check.
> >
> >To check for privilege with respect to another user namespace, you need
> >to use ns_capable.  But for that you need a user_ns to target.
> >
> 
> How about: ns_capable(current_user_ns(),CAP_SYS_ADMIN) ?
> 
> current_user_ns() was found in include/linux/cred.h

Any user can create a new user namespace and pass the above check.  What we
want is to find the user namespace which opened the tty.