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Date: Thu, 20 Apr 2017 12:41:00 -0500
From: "Serge E. Hallyn" <>
Cc: "Serge E. Hallyn" <>,,,,,,,,,
Subject: Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN

Quoting (
> On 2017-04-20 11:19, Serge E. Hallyn wrote:
> >Quoting Matt Brown (
> >>On 04/19/2017 07:53 PM, Serge E. Hallyn wrote:
> >>>Quoting Matt Brown (
> >>>>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: <*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:
> >>>>
> >>>>
> >>>>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.

>  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
> 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.

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