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Date: Sat, 22 Apr 2017 13:09:42 -0400
From: Matt Brown <>
To: "Serge E. Hallyn" <>
Subject: Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN

On 04/21/2017 01:24 AM, Serge E. Hallyn wrote:
> 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 (
>>>> 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.
>> 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.

I believe I have a working solution that I can show in the next version
of the patch later today, but I just want to run the logic by you first.

I added: "struct user_namespace *owner_user_ns;" as a field in
tty_struct (include/linux/tty.h) Note: I am totally open to suggestions
for a better name.

Then I added "tty->owner_user_ns = current_user_ns();" to the
alloc_tty_struct function. (drivers/tty/tty_io.c)

When testing with a docker container, running in a different user
namespace, I printed out current_user_ns()->level, which returned 1,
and tty->owner_user_ns->level, which returned 0. This seems to prove
that I am correctly storing the user namespace which opened the tty.

Please let me know if there are any edge cases that I am missing with
this approach.

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