linux-kernel - Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS

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Message-ID: <CAGXu5jLuqApE_yLNwHDZYfE7ujM2hVSr1dd_WHPfREPUiEwE-Q@mail.gmail.com>
Date:   Fri, 2 Jun 2017 12:25:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matt Brown <matt@...tt.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI
 ioctl require CAP_SYS_ADMIN

On Fri, Jun 2, 2017 at 12:22 PM, Matt Brown <matt@...tt.com> wrote:
> On 6/2/17 2:18 PM, Serge E. Hallyn wrote:
>> Quoting Matt Brown (matt@...tt.com):
>>> On 6/2/17 12:57 PM, Serge E. Hallyn wrote:
>>>> I'm not quite sure what you're asking for here.  Let me offer a precise
>>>> strawman design.  I'm sure there are problems with it, it's just a starting
>>>> point.
>>>>
>>>> system-wide whitelist (for now 'may_push_chars') is full by default.
>>>>
>>>
>>> So is may_push_chars just an alias for TIOCSTI? Or are there some
>>> potential whitelist members that would map to multiple ioctls?
>>
>> <shrug>  I'm seeing it as only TIOCSTI right now.
>>
>>>> By default, nothing changes - you can use those on your own tty, need
>>>> CAP_SYS_ADMIN against init_user_ns otherwise.
>>>>
>>>> Introduce a new CAP_TTY_PRIVILEGED.
>>>>
>>>
>>> I'm fine with this.
>>>
>>>> When may_push_chars is removed from the whitelist, you lose the ability
>>>> to use TIOCSTI on a tty - even your own - if you do not have CAP_TTY_PRIVILEGED
>>>> against the tty's user_ns.
>>>>
>>>
>>> How do you propose storing/updating the whitelist? sysctl?
>>>
>>> If it is a sysctl, would each whitelist member have a sysctl?
>>> e.g.: kernel.ioctlwhitelist.may_push_chars = 1
>>>
>>> Overall, I'm fine with this idea.
>>
>> That sounds reasonable.  Or a securityfs file - I guess not everyone
>> has securityfs, but if it were to become part of YAMA then that would
>> work.
>>
>
> Yama doesn't depend on securityfs does it?
>
> What do other people think? Should this be an addition to YAMA or its
> own thing?
>
> Alan Cox: what do you think of the above ioctl whitelisting scheme?

It's easy to stack LSMs, so since Yama is ptrace-focused, perhaps make
a separate one for TTY hardening?

-Kees

-- 
Kees Cook
Pixel Security