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

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Date:   Tue, 30 May 2017 14:57:17 -0400
From:   Matt Brown <matt@...tt.com>
To:     Nick Kralevich <nnk@...gle.com>,
        Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI
 ioctl require CAP_SYS_ADMIN

On 5/30/17 2:44 PM, Nick Kralevich wrote:
> On Tue, May 30, 2017 at 11:32 AM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
>>> Seccomp requires the program in question to "opt-in" so to speak and
>>> set
>>> certain restrictions on itself. However as you state above, any
>>> TIOCSTI
>>> protection doesn't matter if the program correctly allocates a
>>> tty/pty pair.
>>> This protections seeks to protect users from programs that don't do
>>> things
>>> correctly. Rather than killing bugs, this feature attempts to kill an
>>> entire
>>> bug class that shows little sign of slowing down in the world of
>>> containers and
>>> sandboxes.
>>
>> Just FYI, you can also restrict TIOCSTI (or any other ioctl command)
>> via SELinux ioctl whitelisting, and Android is using that feature to
>> restrict TIOCSTI usage in Android O (at least based on the developer
>> previews to date, also in AOSP master).
> 
> For reference, this is https://android-review.googlesource.com/306278
> , where we moved to a whitelist for handling ioctls for ptys.
> 
> -- Nick
> 

Thanks, I didn't know that android was doing this. I still think this feature
is worthwhile for people to be able to harden their systems against this attack
vector without having to implement a MAC.

Matt