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

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Message-ID: <65b09473-a96a-69cf-9e5d-4a7d7e763da0@nmatt.com>
Date:   Tue, 30 May 2017 19:59:12 -0400
From:   Matt Brown <matt@...tt.com>
To:     Daniel Micay <danielmicay@...il.com>,
        Nick Kralevich <nnk@...gle.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Kees Cook <keescook@...omium.org>
Cc:     Casey Schaufler <casey@...aufler-ca.com>,
        Boris Lukashev <blukashev@...pervictus.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        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 7:40 PM, Daniel Micay wrote:
> On Tue, 2017-05-30 at 19:00 -0400, Matt Brown wrote:
>> On 5/30/17 4:22 PM, Daniel Micay wrote:
>>>> 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.
>>>
>>> Since there's a capable LSM hook for ioctl already, it means it
>>> could go
>>> in Yama with ptrace_scope but core kernel code would still need to
>>> be
>>> changed to track the owning tty. I think Yama vs. core kernel
>>> shouldn't
>>> matter much anymore due to stackable LSMs.
>>>
>>
>> What does everyone think about a v8 that moves this feature under Yama
>> and uses
>> the file_ioctl LSM hook?
> 
> It would only make a difference if it could be fully contained there, as
> in not depending on tracking the tty owner.
> 

For the reasons discussed earlier (to allow for nested containers where one of
the containers is privileged) we want to track the user namespace that owns the tty.