[PATCH v6 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

[Peter Dolding]

On Tue, May 16, 2017 at 6:57 AM, Alan Cox <gnomes at lxorguk.ukuu.org.uk> wrote:
> O> I'm not implying that my patch is supposed to provide safety for
>> "hundreds of other" issues. I'm looking to provide a way to lock down a
>> single TTY ioctl that has caused real security issues to arise. For
>
> In other words you are not actually fixing anything.
>
>> this reason, it's completely incorrect to say that this feature is
>> snake oil. My patch does exactly what it claims to do. No more no less.
>>
>> > In addition your change to allow it to be used by root in the guest
>> > completely invalidates any protection you have because I can push
>> >
>> > "rm -rf /\n"
>> >
>> > as root in my namespace and exit
>> >
>> > The tty buffers are not flushed across the context change so the shell
>> > you return to gets the input and oh dear....
>>
>> This is precisely what my patch prevents! With my protection enabled, a
>> container will only be able to use the TIOCSTI ioctl on a tty if that
>> container has CAP_SYS_ADMIN in the user namespace in which the tty was
>> created.
>
> Which is not necessarily the namespace of the process that next issues a
> read().
>
> This is snake oil. There is a correct and proper process for this use
> case. That proper process is to create a new pty/tty pair. There are two
> cases
>
> - processes that do it right in which case the attacker is screwed and we
>   don't need to mess up TIOCSTI handling for no reason.
>
> - processes that do it wrong. If they do it wrong then they'll also use
>   all the other holes and attacks available via the same path which are
>   completely unfixable without making your system basically unusable.
>
>
> So can we please apply the minimum of common sense reasoning to this and
> drop the patch.
>
> Alan
You missed some important.

From: http://man7.org/linux/man-pages/man7/capabilities.7.html
Don't choose CAP_SYS_ADMIN if you can possibly avoid it!
A vast  proportion of existing capability checks are associated with this
capability (see the partial list above).  It can plausibly be
called "the new root", since on the one hand, it confers a wide
range of powers, and on the other hand, its broad scope means that
 this is the capability that is required by many privileged
programs.  Don't make the problem worse.  The only new features
that should be associated with CAP_SYS_ADMIN are ones that closely
match existing uses in that silo.

This not only a improper fix the attempted fix is breach do
documentation.   CAP_SYS_ADMIN is that far overloaded it does not
require any more thrown in it direction.

This is one of the grsecurity patches mistakes.   GRKERNSEC_HARDEN_TTY
 is from 18 Feb 2016 this documentation as in place at the time they
wrote this.  Yes GRKERNSEC_HARDEN_TTY does exactly the same thing.
Yes Grsecurity guys did the same error and the grsecurity patches are
filled with this error.

The result is from the TIOCSTI patch done this way is you have to use
CAP_SYS_ADMIN to use TIOSCTI so opening up every exploit that
Grsecurity has added and every exploit CAP_SYS_ADMIN can do what is
quite a few.

Now I don't know if CAP_SYS_TTY_CONFIG what is an existing capability
might be what TIOCSTI should own under.

The reality here is CAP_SYS_ADMIN as become the Linux kernel security
equal what big kernel lock was for multi threading.

 In a ideal world CAP_SYS_ADMIN would not be used directly in most
cases.  Instead CAP_SYS_ADMIN would have a stack of sub capabilities
groups under it.

The excuse for doing it wrong grsecurity is
https://forums.grsecurity.net/viewtopic.php?f=7&t=2522

Yes most capabilities open up possibility of exploiting the system.
They are not in fact designed to prevent this.   They are designed to
limit the damage in case of malfunction so that a program/user has
only limited methods of damaging the system.  Like a program
malfunctioning with only limit capabilities if it does an action those
capabilities don't allow no damage will happen.   Now CAP_SYS_ADMIN is
for sure not limited.

But since grsecurity developers took the point of view these are False
Boundaries they then proceed to stack item after item under
CAP_SYS_ADMIN because the boundary made no sense to them.    Also some
mainline Linux Kernel developers are guilty of the same sin of
overloading CAP_SYS_ADMIN.

>From my point of view any new patching containing CAP_SYS_ADMIN
directly used should be bounced just for that.   If features need to
be added to CAP_SYS_ADMIN now they should have to go into another
capability that is enabled when  CAP_SYS_ADMIN is and hopeful if we do
this over time we will be able to clean up CAP_SYS_ADMIN into sanity.


Peter
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