Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN

[Matt Brown]

On 04/20/2017 01:41 PM, Serge E. Hallyn wrote:
> Quoting matt@xxxxxxxxx (matt@xxxxxxxxx):
> > On 2017-04-20 11:19, Serge E. Hallyn wrote:
> > > Quoting Matt Brown (matt@xxxxxxxxx):
> > > > On 04/19/2017 07:53 PM, Serge E. Hallyn wrote:
> > > > > Quoting Matt Brown (matt@xxxxxxxxx):
> > > > > > 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

> >  I agree the hack I mentioned above would be a bad idea because
> > it would break
> > nested containers, but the current patch would not IMO.
> >
> > A better version of the hack could involve a config
> > CONFIG_TIOCSTI_MAX_NS_LEVEL where
> > a check would be performed to ensure that
> > task_active_pid_ns(current)->level is not
> > greater than the config value(an integer that is >= 0) .
>
> Yeah.  That would break a different set of cases than the capable
> check, I assume.  A smaller set, I think.
>
> > Again, I think we both would agree that this is not the best
> > solution. The clear
> > downside is that you could have multiple container layers where the
> > desired security
> > boundaries happened to fall at different levels. Just throwing ideas
> > around.
>
> Yup, appreciated.