Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN

[Kees Cook <keescook@chromium.org>]

On Thu, Apr 20, 2017 at 10:24 PM, Serge E. Hallyn <serge@hallyn.com> 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 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.

Can we use file->cred->user_ns? Hm, but I see there isn't really a
single file associated with tty_struct.

-Kees

-- 
Kees Cook
Pixel Security